Biology for Premedical Students PDF

Benghazi University, Zoology Department- Biology – for Premedical Students 1. Introduction To Biology 1.1. The Science of Biology Biology is the science that studies living organisms and their interactions with one another and their environments. Science attempts to describe and understand the natu...

Benghazi University, Zoology Department- Biology – for Premedical Students 1. Introduction To Biology 1.1. The Science of Biology Biology is the science that studies living organisms and their interactions with one another and their environments. Science attempts to describe and understand the nature of the universe in whole or in part by rational means. Science has many fields; those fields related to the physical world and its phenomena are considered natural sciences. 1.2. Themes and Concepts of Biology Biology is the science of life. All living organisms share several key properties such as order, sensitivity or response to stimuli, reproduction, growth and development, regulation, homeostasis, and energy processing. Biology is very broad and includes many branches and sub disciplines. Examples include molecular biology, microbiology, neurobiology, zoology, and botany, among others. 1.3. The Chemical Foundation Of Life Elements in various combinations comprise all matter, including living things. Some of the most abundant elements in living organisms include carbon, hydrogen, nitrogen, oxygen, sulfur, and phosphorus. These form the nucleic acids, proteins, carbohydrates, and lipids that are the fundamental components of living matter. Biologists must understand these important building blocks and the unique structures of the atoms that make up molecules, allowing for the formation of cells, tissues, organ systems, and entire organisms. The four elements common to all living organisms are oxygen (O), carbon (C), hydrogen (H), and nitrogen (N). O. Alturshani\K. Mohamed\W. Owgie page 1  Benghazi University, Zoology Department- Biology – for Premedical Students 1.4. Properties of Life All living organisms share several key characteristics or functions: order, sensitivity or response to the environment, reproduction, adaptation, growth and development, regulation, homeostasis, energy processing, and evolution. When viewed together, these nine characteristics serve to define life. 1.4.1. Order Organisms are highly organized, coordinated structures that consist of one or more cells. Even very simple, single-celled organisms are remarkably complex: inside each cell, atoms make up molecules; these in turn make up cell organelles and other cellular inclusions. In multicellular organisms (Figure 1.1), similar cells form tissues. Tissues, in turn, collaborate to make organs (body structures with a distinct function). Organs work together to form organ systems. Figure 1.1 1.4.2. Sensitivity or Response to Stimuli Organisms respond to diverse stimuli. For example, plants can bend toward a source of light, climb on fences and walls, or respond to touch (Figure 1.2). Even tiny bacteria can move toward or away from chemicals (a process called chemotaxis) or light (phototaxis). Movement toward a stimulus is considered a positive response, while movement away from a stimulus is considered a negative response. Figure 1.2 O. Alturshani\K. Mohamed\W. Owgie page 2  Benghazi University, Zoology Department- Biology – for Premedical Students 1.4.3. Reproduction Single-celled organisms reproduce by first duplicating their DNA, and then dividing it equally as the cell prepares to divide to form two new cells. Multicellular organisms often produce specialized reproductive germline cells that will form new individuals. When reproduction occurs, genes containing DNA are passed along to an organism‘s offspring. These genes ensure that the offspring will belong to the same species and will have similar characteristics, such as size and shape. 1.4.4. Growth and Development Organisms grow and develop following specific instructions coded for by their genes. These genes provide instructions that will direct cellular growth and development, ensuring that a species‘ young (Figure 1.3) will grow up to exhibit many of the same characteristics as its parents. 1.4.5. Regulation Even the smallest organisms are complex and require multiple regulatory mechanisms to coordinate internal functions, respond to stimuli, and cope with environmental stresses. Two examples of internal functions regulated in an organism are nutrient transport and blood flow. Organs (groups of tissues working together) perform specific functions, such as carrying oxygen throughout the body, removing wastes, delivering nutrients to every cell, and cooling the body. 1.4.6. Homeostasis In order to function properly, cells need to have appropriate conditions such as proper temperature, pH, and appropriate concentration of diverse chemicals. These conditions may, however, change from one moment to the next. Organisms are able to maintain internal conditions within a narrow range almost constantly, despite environmental changes, through homeostasis (literally, ―steady state‖)—the ability of an organism to maintain constant internal conditions. For example, an organism needs to regulate body temperature through a process known as thermoregulation. Organisms that live in cold climates, such as the polar bear (Figure 1.4), have body structures that help them withstand low temperatures and conserve body heat. Structures that aid in this type of insulation include fur, feathers, blubber, and fat. In hot climates, organisms have methods (such as perspiration in humans or panting in dogs) that help them to shed excess body heat. O. Alturshani\K. Mohamed\W. Owgie page 3  Benghazi University, Zoology Department- Biology – for Premedical Students 1.4.7. Energy Processing All organisms use a source of energy for their metabolic activities. Some organisms capture energy from the sun and convert it into chemical energy in food; others use chemical energy in molecules they take in as food (Figure 1.5). 1.5. Levels of Organization of Living Things Living things are highly organized parts of a hierarchy that includes atoms, molecules, organelles, cells, tissues, organs, and organ systems. Organisms, in turn, are grouped as populations, communities, ecosystems, and the biosphere. 1.6. BIOLOGICAL MACROMOLECULES Food provides the body with the nutrients it needs to survive. Many of these critical nutrients are biological macromolecules, or large molecules, necessary for life. These macromolecules (polymers) are built from different combinations of smaller organic molecules (monomers). There are four major classes of biological macromolecules (carbohydrates, lipids, proteins, and nucleic acids); each is an important cell component and performs a wide array of functions. O. Alturshani\K. Mohamed\W. Owgie page 4  Benghazi University, Zoology Department- Biology – for Premedical Students Combined, these molecules make up the majority of a cell‘s dry mass ( recall that water makes up the majority of its complete mass) (Figure 1.6). Biological macromolecules are organic, meaning they contain carbon. In addition, they may contain hydrogen, oxygen, nitrogen, and additional minor elements. O. Alturshani\K. Mohamed\W. Owgie page 5  Benghazi University, Zoology Department- Biology – for Premedical Students 1.6.1. Dehydration Synthesis Most macromolecules are made from single subunits, or building blocks, called monomers. The monomers combine with each other using covalent bonds to form larger molecules known as polymers (Figure 1.7). In doing so, monomers release water molecules as byproducts. This type of reaction is known as dehydration synthesis, which means ―to put together while losing water.‖ 1.6.2. Hydrolysis Polymers are broken down into monomers in a process known as hydrolysis, which means ―to split water,‖ a reaction in which a water molecule is used during the breakdown (Figure 1.8). During these reactions, the polymer is broken into two components: one part gains a hydrogen atom (H+) and the other gains a hydroxyl molecule (OH–) from a split water molecule. O. Alturshani\K. Mohamed\W. Owgie page 6  Benghazi University, Zoology Department- Biology – for Premedical Students 2. Cytology We all know that the Animal cell is a unit of composition and function in living organisms, and the cell is linked to the discovery of the microscope , which invented by (Levinhock in 1591). Anthony Levinhock became more involved in science and with his new improved microscope was able to see things that no man had ever seen before. He saw bacteria, yeast, blood cells and many tiny animals swimming about in a drop of water. 2.1. CELL STRUCTURE A cell is the smallest unit of a living thing. A living thing, whether made of one cell (like bacteria) or many cells (like a human), is called an organism. Thus, cells are the basic building blocks of all organisms. Cells vary in size. With few exceptions, individual cells cannot be seen with the naked eye, so scientists use microscopes (micro- = ―small‖; -scope = ―to look at‖) to study them. A microscope is an instrument that magnifies an object. Most photographs of cells are taken with a microscope, and these images can also be called micrographs. 2.2. Cell Theory botanist Matthias Schleiden and zoologist Theodor Schwann were studying tissues and proposed the unified cell theory, which states that all living things are composed of one or more cells, the cell is the basic unit of life, and new cells arise from existing cells. 2.3. The basic structure of the cell All cells share four common components: 1- A plasma membrane, an outer covering that separates the cell‘s interior from its surrounding environment 2- Cytoplasm, consisting of a jelly-like cytosol within the cell in which other cellular components are found 3- DNA, the genetic material of the cell 4- Ribosomes, which synthesize proteins. However, prokaryotes differ from eukaryotic cells in several ways. Cells fall into one of two broad categories: prokaryotic and eukaryotic. Only the predominantly single-celled organisms of the domains Bacteria and Archaea are classified as prokaryotes (pro- = ―before‖; -kary- = ―nucleus‖). Cells of animals, plants, fungi, and protists are all eukaryotes (ceu- = ―true‖) and are made up of eukaryotic cells. O. Alturshani\K. Mohamed\W. Owgie page 7  Benghazi University, Zoology Department- Biology – for Premedical Students 2.4. Main types of cells: 1 - Eukaryotic cells 2 - Prokaryotic cells 2.4.1. Prokaryotic Cells A prokaryote is a simple, mostly single-celled (unicellular) organism that lacks a nucleus, or any other membrane-bound organelle. We will shortly come to see that this is significantly different in eukaryotes. Prokaryotic DNA is found in a central part of the cell: the nucleoid. Most prokaryotes have a peptidoglycan cell wall and many have a polysaccharide capsule (Figure 2.1). The cell wall acts as an extra layer of protection, helps the cell maintain its shape, and prevents dehydration. The capsule enables the cell to attach to surfaces in its environment. Some prokaryotes have flagella, pili. Flagella are used for locomotion. Pili are used to exchange genetic material during a type of reproduction called conjugation. 2.4.2. Eukaryotic Cells Unlike prokaryotic cells, eukaryotic cells have: 1) a membrane-bound nucleus; 2) numerous membrane-bound organelles such as the endoplasmic reticulum, Golgi apparatus, chloroplasts, mitochondria, and others; and 3) several, rod-shaped chromosomes. Because a eukaryotic cell‘s nucleus is surrounded by a membrane, it is often said to have a ―true nucleus.‖ (Figure 2.2) The word ―organelle‖ means ―little organ,‖ and, as already mentioned, organelles have specialized cellular functions, just as the organs of your body have specialized functions. At this point, it should be clear to you that eukaryotic cells have a more complex structure than prokaryotic cells. Organelles allow different functions to be compartmentalized in different areas of the cell. O. Alturshani\K. Mohamed\W. Owgie page 8  Benghazi University, Zoology Department- Biology – for Premedical Students The following table shows the different between prokaryotic and eukaryotic cell Eukaryotic cells Prokaryotic cells True nucleus surrounded No true nucleus Endoplasmic reticulum present No Endoplasmic reticulum or associated organelles such as Golgi apparatus Membrane bounded organelles such as No Membrane bounded organelles mitochondria Large (80S) ribosome attached to Small(70S) ribosome scattered in endoplasmic reticulum cytoplasma If present flagella have (9+2) If present flagella are made of single arrangement of microtubules microtubule Cell wall if present made of cellulose Cell wall containing peptidoglycan Cells are large typically 10-100 µm Cells are small typically 0.3-5µm O. Alturshani\K. Mohamed\W. Owgie page 9  Benghazi University, Zoology Department- Biology – for Premedical Students 2.5 Cell components 2.5.1 Nucleus Usually spherical in shape, large-sized. Surrounded by a nuclear envelope and it is composed of two layers of membranes separated by a distance of 20-40 nm. The Merges of the two layers in many places to form a nuclear pores. The genetic material DNA exists in the form of filaments. 2.5.1.1 Functions of nucleus 1 - Contains the genetic material inherited from one cell to another by division. 2 - Regulate the activity of the cell. 2.5.2 Cytoplasm Is part of the cell material, which is located between the cell membrane and the nucleus. Consists of about 80% water and 15% as proteins, it also contains fats, sugars , and mineral salts 5%. Cytoplasm is the medium in which the chemical reactions occur within several structures surrounded by membranes called organelles, each of which is specific functions. Cytoplasm has various functions in the cell : 1- Most of the important activities of the cell occur in the cytoplasm. Cytoplasm contains molecules such as enzymes which are responsible for breaking down waste and also aid in metabolic activity. 2- Cytoplasm is responsible for giving a cell its shape. It helps to fill out the cell and keeps organelles in their place. Without cytoplasm, the cell would be deflated and materials would not be able to pass easily from one organelle to another. 3- Cytosol is the part of the cytoplasm that does not contain organelles. O. Alturshani\K. Mohamed\W. Owgie page 10  Benghazi University, Zoology Department- Biology – for Premedical Students 2.5.2.1 Mitochondria Considered among the largest cell organelles in size, It is a rod organelles, or spherical surrounded by a double membrane bends inward forming folds called criste. The cavity of the mitochondria fill by thick liquid called matrix, contains some enzymes that are involved in the chemical processes in the Krebs cycle and cellular respiration. functions: 1-Is the place that it is made up the energy ( ATP). 2-The greater the need for the cell to power the greater the number and size of mitochondria. 3- Contain mitochondrial DNA on its own which is capable of self-cleavage. 2.5.2.2 Endoplasmic reticulum The endoplasmic reticulum is a multifold membranous structure within eukaryotic cells which plays a major role in the synthesis of the complex molecules required by the cell and the organism. There are two types of endoplasmic reticulum, rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER) ,both types present in animal and plant cells, the two types of ER separate entities and are not joined together. Cells specializing in the production of proteins will tend to have larger amount of rough ER whilst cells producing lipids (fat) and steroids hormones will have a greater amount of smooth ER. O. Alturshani\K. Mohamed\W. Owgie page 11  Benghazi University, Zoology Department- Biology – for Premedical Students Often the membranes of these structures are lined with ribosomes on their outer surfaces, giving them a rough appearance. These parts are called the rough endoplasmic reticulum to contrast them with the smooth endoplasmic reticulum where there are no attached ribosomes. The main functions of the (Rough and Smooth) A- Rough ER synthesis of the proteins and modification of some proteins that are produced by the ribosomes. B-The smooth endoplasmic reticulum plays a major role in synthesizing lipids such as phospholipids and cholesterol. In the reproductive organs, smooth ER in the cells produces the steroid hormones testosterone and estrogen 2.5.2.3 Ribosome : Ribosomes are consists of two units, one large and the other small and do not unite only when the synthesis of numerous peptides (protein). There are large numbers of the ribosomes in the cells that create proteins because it is the only place where the amino acids is created. 2.5.2.4 Golgi apparatus : A Golgi complex is composed of flat sacs known as cisternae, and associated small hollow spheres of membrane called vesicles.The Golgi complex or Golgi apparatus is responsible for manufacturing, warehousing, and shipping certain cellular products, particularly those from the endoplasmic reticulum (ER). Depending on the type of cell, there can be just a few complexes or there can be hundreds. Cells that specialize in secreting various substances typically have a high number of Golgi complexes. O. Alturshani\K. Mohamed\W. Owgie page 12  Benghazi University, Zoology Department- Biology – for Premedical Students 2.5.2.5 The lysosomes Small vesicles are formed in the Golgi apparatus, contain enzymes analyst of carbohydrates and protein and fatty acids. Lysosomes are also found in most plant and animal cells, especially in the animals cells which are phagocytic, these are cells which carry out the process of phagocytosis. Lysosomes are formed by inclusion of digestive enzymes such as proteases and lipases. It is very important that enzymes contained within lysosomes are isolated from the rest of the cell inside the lysosomes membrane, otherwise their release would result in self digestion of the cell for instance disease called (rheumatoid arthritis) where the cartilage of joints is attacked by lysosmes enzymes. O. Alturshani\K. Mohamed\W. Owgie page 13  Benghazi University, Zoology Department- Biology – for Premedical Students 2.5.2.6 Peroxisomes These vesicles contain enzymes crash organic compounds such as hydrogen peroxide and then the decomposition of this toxic compound to water and oxygen. 2.5.2.7 The cytoskeleton The cytoskeleton has three different types of protein elements. From narrowest to widest, they are the microfilaments (actin filaments), intermediate filaments, and microtubules. Microfilaments are often associated with myosin. They provide rigidity and shape to the cell and facilitate cellular movements. Intermediate filaments bear tension and anchor the nucleus and other organelles in place. Microtubules help the cell resist compression, serve as tracks for motor proteins that move vesicles through the cell, and pull replicated chromosomes to opposite ends of a dividing cell. They are also the structural element of centrioles, flagella, and cilia. O. Alturshani\K. Mohamed\W. Owgie page 14  Benghazi University, Zoology Department- Biology – for Premedical Students 2.5.2.8 Centrioles : Centrioles are small hollow cylindrical organelles, present in pairs in animal cells, they are about 0.5µm long and 0.2µm in diameter. Each centriole is made up of nine triplets of microtubules. During cell division the centrioles replicate themselves and migrate to the opposite poles of the cell. They are thought to have a role in the formation of the spindle fibers which are also made of microtubules. Recently ,experiments have shown that centrioles may be the site of formation of the whole cytoskeleton network, not just spindle.This has led them being renamed microtubule organizing center. 2.5.3 The Endomembrane System and Proteins The endomembrane system (endo = ―within‖) is a group of membranes and organelles (Figure) in eukaryotic cells that works together to modify, package, and transport lipids and proteins. It includes the nuclear envelope, lysosomes, and vesicles, which we‘ve already mentioned, and the endoplasmic reticulum and Golgi apparatus. Although not technically within the cell, the plasma membrane is included in the endomembrane system because, as you will see, it interacts with the other endomembranous organelles. The endomembrane system does not include the membranes of either mitochondria or chloroplasts O. Alturshani\K. Mohamed\W. Owgie page 15  Benghazi University, Zoology Department- Biology – for Premedical Students 2.5.4 Cilia and flagella : Cilia and flagella are organelles belonging to some animal cells they are thin cytoplasmic threads projecting from the surface of the cell also containing microtubules they are similar in structure ,but flagella are longer about 100µm compared with 5-10µm for cilia and fewer in number then the cilia. Cilia are present in large numbers on the surface of some cells such as epithelia lining the trachea, their function is to beat backswords and forwards in one direction. The flagella normally move the whole cell or organism as in case of the tail of the sperm cells which is a single flagellum. Cilia and flagella both contain a characteristic arrangement of nine outer pairs of microtubules and two central ones this is called (9+2) arrangement and probably responsible for producing the beating movements, although the exact mechanism is not clear. O. Alturshani\K. Mohamed\W. Owgie page 16  Benghazi University, Zoology Department- Biology – for Premedical Students 2.5.5 Microvilli : Microvilli are the finger like projections present on the outer surface of the cell, the function of microvillia is to greatly increase the surface area of cells allowing the increased absorption of materials for example in the small intestine, the microvilli of the epithelium allow a faster uptake of the products of digestion. 2.6 Plasma Membrane All membranes have a similar structure ,including the outer cell membrane or the cell surface membrane of both prokaryotic and eukaryotic cells and membranes around organelles in eukaryotic. Some organelles have single membrane (Golgi apparatus) ,but the others have a double membrane (nucleus, mitochondria). Membranes are composed of phospholipids with proteins scattered amongst them. The plasma membrane is selectively permeable. This means that the membrane allows some materials to freely enter or leave the cell, while other materials cannot move freely, but require the use of a specialized structure, and occasionally, even energy investment for crossing. O. Alturshani\K. Mohamed\W. Owgie page 17  Benghazi University, Zoology Department- Biology – for Premedical Students 2.6.1 Phospholipids in the cell membrane Phospholipids are molecules which are made up of phosphate (heads) and fatty acid (tail) ,the phospholipid in the cell membrane are constantly moving from side to side and swapping places. This gives the name fluid mosaic model as the structure moves and changes. Phosphate is attracted to water and described as hydrophilic(water loving). The phosphate heads turn toward solution (water). fatty acid are repelled by water and are called hydrophobic (water hating). The hydrophobic fatty acids tails turn away from solutions (water). The phospholipids molecules automatically orientate themselves so that the hydrophobic fatty acid tails avoid contact with water and hydrophilic phosphate heads attracted to water this maintains the membrane structure with the phosphate heads on the outer part facing water and the fatty acid tails pointing inwards. The figure above shows the phospholipid molecules naturally from a bilayer ,two layers Phospholipids with the phosphates on the outside and fatty acids pointing inwards. O. Alturshani\K. Mohamed\W. Owgie page 18  Benghazi University, Zoology Department- Biology – for Premedical Students The main purpose of the hydrophobic center of the membrane is to provide the free diffusion of water and polar molecules through the membrane. It acts as a barrier to free movement. The membrane exerts control over what passes through via protein carrier and channel proteins. It is selectively permeable only allowing specific molecules to pass through. If the structure is disrupted by molecules passing through the membrane, it is easy springs back into the original position with phosphates on the outside and fatty acids on the inside. 2.6.2 Proteins in cell membrane : There are proteins floating in the cell membrane between the phospholipids, the two main function of the proteins is to provide support and stability in the fluid structural ,and transport of molecules across membrane. Apart from some structural proteins the membrane proteins are mainly globular. Globular proteins in the cell membrane form specific shaped protein carries the specific shape determines which molecules can cross the membrane only those of matching shape will be transported. 2.6.2.1 Types of the proteins in cell membrane : a. Transport proteins b. Channel proteins c. Receptors d. Carrier proteins e. Enzymatic f. Recognition proteins 2.6.2.2 Glycoprotein and Glycolipids : The cell surface membrane contains protein molecules and phospholipids. A polysaccharide chain may be attached to a protein, forms a glycoprotein, polysaccharide attached to phospholipid, forms a glycolipid. The polysaccharide is always on the outside of the cell surface membrane. Function of glycoprotein and glycolipid: Both form hydrogen bonds with water molecules outside the cell, helping to stabiles the membrane , glycoprotein and glycolipid are involved in cell to cell recognition ,enabling cells of similar type to group together to form a tissue. The varying carbohydrate chains emerging from the cell surface membrane of the red blood cells are responsible for producing the different type blood group A ,B, AB ,and O. Some glycoprotein and glycolipid act as receptor sites. 2.6.3 The way in which the materials transferred cross the cell membrane: There are two basic types of transport, passive which use no energy and active transport that is generally employ ATP to drive the transport. See the drawing below O. Alturshani\K. Mohamed\W. Owgie page 19  Benghazi University, Zoology Department- Biology – for Premedical Students 2.6.3.1 Passive transport : Diffusion : Is the movement of substance from where it is in high concentration to an area where it is in lower concentration. Property plays an important role in spreading the exchange of materials between the cell and its surrounding medium, and these materials are oxygen ,carbon dioxide, and substances that dissolve in lipids. Osmosis : Is the net movement of water from a high water concentrate to lower water concentrate through a partially permeable membrane. Types of solution 1. Hypertonic solution: the solution surrounding a cell have a higher solute concentration. 2. Hypotonic solution: the solution surrounding a cell have a lower solute concentration. 3. Isotonic solution: the solution surrounding a cell have same solute concentration. Solution Solution concentration Direction of osmosis Hypotonic Lower than cell Into cell Hypertonic Higher than cell Out of cell Isotonic Same as cell No net movement O. Alturshani\K. Mohamed\W. Owgie page 20  Benghazi University, Zoology Department- Biology – for Premedical Students Facilited diffusion : Proteins of specific shape called proteins carriers have binding sites which carry matching shaped substances across the membrane.Molecules are carried from high to low concentration assisted by protein carriers. 2.6.3.2 Active transport : Active transport mechanisms require the use of the cell‘s energy, usually in the form of adenosine triphosphate (ATP). a substance must move into the cell against its concentration gradient and needs the presence of specific carrier proteins or pumps. A. Sodium and potassium pump : the sodium-potassium pump, also known as the Na,K- ATPase. It functions in the active transport of sodium and potassium ions across the cell membrane against their concentration gradients. For each ATP the pump breaks down, two potassium ions are transported into the cell and three sodium ions out of the cell. O. Alturshani\K. Mohamed\W. Owgie page 21  Benghazi University, Zoology Department- Biology – for Premedical Students Purple Balls = Sodium Ions Blue Balls =Potassium Ions. B. Bulk Transport Active transport methods require the direct use of ATP to fuel the transport. Large particles, such as macromolecules, parts of cells, or whole cells, can be engulfed by other cells in a process called phagocytosis. In phagocytosis, a portion of the membrane invaginates and flows around the particle, eventually pinching off and leaving the particle entirely enclosed by an envelope of plasma membrane. Vesicle contents are broken down by the cell, with the particles either used as food or dispatched. Pinocytosis is a similar process on a smaller scale. The plasma membrane invaginates and pinches off, producing a small envelope of fluid from outside the cell. Pinocytosis imports substances that the cell needs from the extracellular fluid. The cell expels waste in a similar but reverse manner: it pushes a membranous vacuole to the plasma membrane, allowing the vacuole to fuse with the membrane and incorporate itself into the membrane structure, releasing its contents to the exterior. O. Alturshani\K. Mohamed\W. Owgie page 22  Benghazi University, Zoology Department- Biology – for Premedical Students 2.7 Connections between Cells and Cellular Activities 2.7.1 Extracellular Matrix of Animal Cells Most animal cells release materials into the extracellular space. The primary components of these materials are proteins, and the most abundant protein is collagen. Collagen fibers are interwoven with carbohydrate-containing protein molecules called proteoglycans. Collectively, these materials are called the extracellular matrix (Figure 4.27). Not only does the extracellular matrix hold the cells together to form a tissue, but it also allows the cells within the tissue to communicate with each other. O. Alturshani\K. Mohamed\W. Owgie page 23  Benghazi University, Zoology Department- Biology – for Premedical Students 2.7.2 Intercellular Junctions Cells can also communicate with each other via direct contact, referred to as intercellular junctions. There are some differences in the ways that plant and animal cells do this. Plasmodesmata are junctions between plant cells, whereas animal cell contacts include tight junctions, gap junctions, and desmosomes. 2.7.3 Gap junctions Gap junctions in animal cells are channels between adjacent cells that allow for the transport of ions, nutrients, and other substances that enable cells to communicate (Figure 4.31) Gap junctions are particularly important in cardiac muscle: The electrical signal for the muscle to contract is passed efficiently through gap junctions, allowing the heart muscle cells to contract in sequence. O. Alturshani\K. Mohamed\W. Owgie page 24  Benghazi University, Zoology Department- Biology – for Premedical Students 3. The cell cycle Multiplication of cells takes place by division of pre-existing cells. Such multiplication constitutes an essential feature of embryonic development. Cell multiplication is equally necessary after birth of the individual for growth and for replacement of dead cells. The chromosomes within the nuclei of cells carry genetic information that controls the development and functioning of various cells and tissues and , therefore, of the body as a whole. When a cell divides it is essential that the whole of the genetic information within it be passed on to both the daughter cells resulting from the division. In other words the daughter cells must have chromosomes identical in number and in genetic content to those in the mother cell, this type of cell division is called mitosis. A different kind of cell division called meiosis occurs during the formation of gametes. This consists of two successive divisions called first and second meiotic divisions. The cells resulting from these divisions (i.e. the gametes) differ from other cells in the body in that: (a) The number of chromosomes is reduced to half the normal number. (b) The genetic information in the various gametes produced is not identical. The cell cycle is a series of events within the cell that prepare the cell for dividing into two daughter cells. The cell cycle is divided into two major events : interphase, a long period of time during which the cell increase its size and content and replicates its genetic material, and mitosis, a shorter period of time during which the cell divides its nucleus and cytoplasm, giving rise to two daughter cells. Cells that become highly differentiated after the last mitotic event may cease to undergo mitosis permanently (e.g. neurons, muscle cells) and return to the cell cycle at a later time. Cells that have left the cell cycle are said to be in a resting stage, the G0 (outside) phase, or the stable phase. 3.1. Interphase : Interphase is subdivided into three phases: (a) G1 (gap) phase, when the synthesis of macromolecules essential for DNA duplication begins. (b) S (synthetic) phase, when the DNA is duplicated. (c) G2 (gap) phase, when the cell undergoes preparations for mitosis. Gap 1: daughter cells formed during mitosis enter the G 1 phase. During this phase, the cells synthesize RNA, regulatory proteins essential to DNA replication, and enzymes necessary to carry out these synthetic activities. Thus the cell volume, reduced by dividing the cell in half during mitosis, is restored to normal. Additionally, the nucleoli are reestablished during the G1 phase. It is during this time that the centrioles begin to duplicate themselves, a process that is completed by the G2 phase. S phase : during the S phase, the synthetic phase of the cell cycle, the genome is duplicated. All of the requisite nucleoproteins, including the histones, are imported and incorporated into the DNA molecule, forming the chromatin material. The cell now contains twice the normal complement of its DNA. The amount of DNA present in autosomal and germ cells also varies. O. Alturshani\K. Mohamed\W. Owgie page 25  Benghazi University, Zoology Department- Biology – for Premedical Students Autosomal cells contain the diploid (2n) amount of DNA before the synthetic (S) phase of the cell cycle when the diploid (2n) amount of DNA is doubled (4n) in preparation for cell division. In contrast, germ cells produced by meiosis possess the haploid (1n) number of chromosomes and also the haploid (1n) amount of DNA. G2 phase : the gap 2 phase (G2 phase) is the period between the end of DNA synthesis and the beginning of mitosis. During the G2 phase, the RNA and proteins essential to cell division are synthesized, the energy for mitosis is stored, tubulin is synthesized for assembly into microtubules required for mitosis. 3.2. Mitosis Mitosis is the process of cell division that results in the formation of two identical daughter cells. Mitosis (M) occurs at the conclusion of the G2 phase and thus completes the cell cycle. Mitosis is the process whereby the cytoplasm and the nucleus of the cell are divided equally into two identical daughter cells. First, the nuclear material is divided in a process called Karyokinesis, followed by division of the cytoplasm, called Cytokinesis. The process of mitosis divided into four distinct stages : prophase, metaphase, anaphase, and telophase. 3.2.1 Prophase: At the beginning of prophase, the chromosomes are condensing, and thus becoming visible microscopically. Each chromosome consists of two parallel sister chromatids, joined together at one point along their length, the centromere. As chromosomes condense, the nucleolus disappears. The centrosome also divides into two regions, each half containing a pair of centrioles, which migrate away from each other to opposite poles of the cell. In this phase begins as the nuclear envelope disappears, and the chromosomes are arranged randomly throughout the cytoplasm. 3.2.2 Metaphase: During metaphase , the chromosomes become maximally condensed and are lined up at the equator of the mitotic spindle (metaphase plate configuration). Sister chromatids must be O. Alturshani\K. Mohamed\W. Owgie page 26  Benghazi University, Zoology Department- Biology – for Premedical Students maintained in close proximity as the chromosome condenses and aligns on the metaphase mitotic spindle. 3.2.3 Anaphase: Anaphase begins when sister chromatids, located at the equator of the metaphase plate, pull apart and begin their migration toward the opposite poles of the mitotic spindle. In the late of this phase, a cleavage furrow begins to form at the plasmalemma, indicating the region where the cell will be divided during cytokinesis. 3.2.4 Telophase: It the terminal phase of mitosis, is characterized by cytokinesis, reconstitution of the nucleus and nuclear envelope, disappearance of the mitotic spindle, and unwinding of the chromosomes into chromatin. Each daughter cell resulting from mitosis is identical in every respect, including the entire genome, and each daughter cell possesses a diploid (2n) number of chromosomes. 3.3. Meiosis Whereas mitosis is cell division of somatic cells into two identical daughter cells, meiosis is a special type of cell division resulting in formation of gametes (spermatozoa or ova) whose chromosome number has been reduced from the diploid (2n) to the haploid (1n) number. Meiosis begins at the conclusion of interphase in the cell cycle. It produces the germ cells-the ova and the spermatozoa. This process has two crucial results: 1. Reduction in the number of chromosomes from the diploid (2n) to the haploid (1n) number, ensuring that each gamete carries the haploid amount of DNA and the haploid number of chromosomes. 2. Recombination of genes, ensuring genetic variability and diversity of the gene pool. O. Alturshani\K. Mohamed\W. Owgie page 27  Benghazi University, Zoology Department- Biology – for Premedical Students Meiosis is divided into two separate events: Meiosis I, or reductional division (first event), homologous pairs of chromosomes line up, members of each pair separate and go to opposite poles, and the cell divides; thus, each daughter cell receives half the number of chromosomes (haploid number). Meiosis II, or reductional division (second event), the two chromatids of each chromosome are separated, as in mitosis, followed by migration of the chromatids to opposite poles and the formation of two daughter cells. These two events produce four cells (gametes), each with the haploid number of chromosomes and haploid DNA content. 3.3.1. Meiosis I : Reductional division separates the homologous pairs of chromosomes, thus reducing the number from diploid (2n) to haploid (1n). In gametogenesis, when the germ cells are in the S phase of the cell cycle preceding meiosis, the amount of DNA is doubled to 4n but the chromosome number remains at 2n (46 chromosomes). O. Alturshani\K. Mohamed\W. Owgie page 28  Benghazi University, Zoology Department- Biology – for Premedical Students 3.3.1.1. Prophase I Prophase of meiosis I lasts a long time and is subdivided into the following five phases: homologous pairs of chromosomes approximate each other, lining up and make synapses , forming a tetrad. chiasmata (crossing over sites) are formed random exchange of genetic material occurs between homologous chromosomes. 3.3.1.2. Metaphase I: It is characterized by homologous pairs of chromosomes, each composed of two chromatids, lining up on the equatorial plate of the meiotic spindle. 3.3.1.3. Anaphase I: In anaphase I, homologous pairs of chromosomes migrate away from each other, going to opposite poles. Each chromosome still consists of two chromatids. 3.3.1.4. Telophase I: Telophase I is similar to telophase of mitosis. The chromosomes reach the opposing poles, nuclei are reformed and cytokinesis occurs, giving rise to two daughter cells. Each cell possesses 23 chromosomes, the haploid (1n) number, but because each chromosome is composed of two chromatids, the DNA content is still diploid. Each of the two newly formed daughter cells enters meiosis II. 3.3.2. Meiosis II Meiosis II (equatorial division) is not preceded by S phase (without DNA synthesis). It is very similar to mitosis and is subdivided into prophase II, metaphase II, anaphase II, telophase II, and cytokinesis. The chromosomes line up on the equator, the kinetochores attach to spindle fibers, followed by the chromatids migrating to opposite poles, and cytokinesis divides each of the two cells, resulting in a total of four daughter cells from the original diploid germ cell. Each of the four cells contains a haploid amount of DNA and a haploid chromosome number. O. Alturshani\K. Mohamed\W. Owgie page 29  Benghazi University, Zoology Department- Biology – for Premedical Students Unlike the daughter cells resulting from mitosis, each of which contains the diploid number of chromosomes and is an identical copy of the other, the four cells resulting from meiosis contain the haploid number of chromosome and are genetically distinct because of reshuffling of the chromosomes and crossing over. Thus, every gametes contains its own unique genetic complement. O. Alturshani\K. Mohamed\W. Owgie page 30  Benghazi University, Zoology Department- Biology – for Premedical Students Embryology Embryology : is the branch of biology that studies the prenatal development of gametes ( sex cells ), fertilization, and development of embryos and fetuses. Embryology has a long history. Aristotle proposed the currently accepted theory of epigenesis, that organisms develop from seed or egg in a sequence of steps. The alternative theory, preformationism, that organisms develop from pre-existing miniature versions of themselves, however held sway until the 18th century. Modern embryology developed from the work of von Baer, though accurate observations had been made in Italy by anatomists such as Aldrovandi and Leonardo da Vinci in the Renaissance. Sexual Differentiation During embryonic development there is a sexually indifferent stage in which the embryo has the potential to develop either male or female structures. Internally, adjacent to each developing gonad, are two primitive ducts that can give rise to either the male or the female reproductive tracts. The Wolffian (mesonephric) ducts are more medial. The Müllerian (paramesonephric) ducts are more lateral, but then fuse in the midline more caudally. Sexual differentiation begins with sexual determination, which depends upon the sex chromosomes, X and Y. Sexual determination involves the specification of the gonads as either testes or ovaries.If the embryo is XY, the SRY gene (for sex determining region of the Y chromosome ) will be present. The protein produced by SRY activates a gene network that directs the gonads to develop as testes. In the absence of a Y chromosome and SRY, the gonads develop as ovaries. Once the gonad begins to develop as a testis, the two support cells in the testis differentiate and begin to generate important regulatory molecules that direct sexual differentiation. The Leydig cells produce testosterone, which promotes development of the Wolffian ducts. The Wolffian ducts then differentiate to form the epididymis, vas deferens, seminal vesicles, and ejaculatory ducts. The Sertoli cells produce Müllerian inhibiting substance (MIS; also known as Anti-Müllerian hormone, AMH), a peptide hormone that causes the Müllerian ducts to regress. Female development proceeds when there is an absence of the SRY gene. No testosterone or MIS is made. The Wolffian ducts regress, and the Müllerian ducts persist, developing into the fallopian tubes, the uterus and the upper part of the vagina. Müllerian inhibiting substance (MIS) is actually produced in the ovary (after it differentiates) by granulosa cells. MIS is expressed mainly by small growing follicles. The level of MIS is thus a good indicator of the size of the ovarian reserve (ability to produce eggs capable of being fertilized). A test for MIS level may be used in the context of in vitro fertilization treatment, as a means to predict how the woman will respond to controlled ovarian stimulation. O. Alturshani\K. Mohamed\W. Owgie page 31  Benghazi University, Zoology Department- Biology – for Premedical Students Gametogenesis Gametogenesis is arbitrarily designated as the first stage of embryonic development. In embryology, the gametes are usually discussed first, as they provide both the blue print and the raw material from which the embryo is formed. Gametogenesis is defined as the process of formation of respective gametes( sperm and ova) in respective gonads. It involves Spermatogenesis and Oogenesis. Gametogenesis i.e. formation of gametes in the sexes is tailored to their future roles in reproduction. The female gamete is usually non-mobile, larger and nutrient filled cell, the ovum or egg. The female gamete must be competent to be fertilized, which means that it must develop a number of specialized properties to enable it to interact with the sperm. The male gamete is usually small and mobile sex cell, the spermatozoon or sperm. The formation of female gamete, ova or egg is known as oogenesis, whereas formation of male gamete, sperm is termed as spermatogenesis. Both classes of gametes, spermatozoon and ova make an equal contribution to the nucleus of the zygote. It is said that egg and sperm possess the ‗information‘ that is needed to build a new organism in the encoded form. During the development of egg, the encoded information is decoded. The decoding or reading of the information is equivalent to the process of ontogenetic development (i.e. transformation of zygote into new adult individual). Another aspect of gametogenesis requires that chromosome number be reduced from the diploid number to the haploid condition. Sexual reproduction involves union of gametes from two different individuals, so either gamete possesses one half the number of chromosomes of the parents. The reduction of chromosome number is accomplished by meiotic (Z-44/D.B.) division. In both the sexes of initial cells (germinal cells) giving rise to the gametes are very similar, and the steps in the production of gametes include (i) proliferation of cells by mitosis (ii) growth and (iii) maturation. Spermatogenesis- It is the process of formation of sperm in testis. Testis: it is the primary male reproductive organ. Shape and size: pinkish oval bodies occurring in pair, size is about 4.5cm long, 2.5 cm wide and 3 cm thick. Location: situated outside of abdominal cavity in scrotal sac. (Temperature of 2-3°C below body temperature is required for spermatogenesis) Each testis is surrounded by three layers. 1. Tunical vaginalis: double membrane outer covering, made up of fibrous connective tissue 2. Tunica albuginea: it is the middle layer below the tunica vaginalis. 3. Tunica vasculosa: it is the innermost highly vascular layer with network of blood capillaries. O. Alturshani\K. Mohamed\W. Owgie page 32  Benghazi University, Zoology Department- Biology – for Premedical Students Histology of testis: Each testis consists of 200-300 lobules, and each lobule contains 1-4 convulated loops called Seminiferious tubules. In between the seminiferous tubules, there is a group of interstitial cell called Leydig cell, which secrete testosterone, a male sex hormone. Each seminiferous tubules lined with germinal epithelium produce sperm by the process Spermatogenesis Spermatogenesis occur in three phase i) Multiplication phase: the germinal epithelium of seminiferous tubules produce primodial germ cell. These cell multiplies repeatedly by mitosis to produce large number of spermatogonia. ii) Growth or Maturation phase: The spermatogonia undergoes maturation. It is a diploid cell. After maturation spermatogonia is known as Sperm mother cell because it will eventually develop into the mature sperm. iii) Meiotic phase: Duplication of homologous chromosome in sperm mother cell occur and become ready for meiosis. First meiotic division produce two Primary spermatocyte with haploid number of chromosome. The first meiotic division separates the homologous chromosomes from each parent. The second meiotic division of each primary spermatocytes occur resulting altogether of 4 haploid secondary spermatocytes. The secondary spermatocytes after maturation is known as spermatids. Each Spermatids goes on metamorphosis into sperm by the process of Spermiogenesis. Spermiogenesis: Sperm is a motile male gamete with head, neck and tail. During Metamorphosis of spermatids into sperm, following changes occurs  Spermatids elongates and its Nucleus  Cytoplasm extended to develop Flagella  Golgi body produces Acrosome  Mitochondria aggregate to form super mitochondria around base of flagella, providing energy for sperm motility  By tubulobular process, sertoli cell phagocytose the sheded cytoplasm O. Alturshani\K. Mohamed\W. Owgie page 33  Benghazi University, Zoology Department- Biology – for Premedical Students Figure show cross section through mammalian testis Figure: stages of spermatogenesis O. Alturshani\K. Mohamed\W. Owgie page 34  Benghazi University, Zoology Department- Biology – for Premedical Students Oogenesis- It is the process of formation of ova or egg in ovary. Ovary: it is the primary female reproductive organ. Shape and size: grayish pink almond shaped structure, size is 2.5-3.5 cm long, 2 cm wide and 1 cm thick Location: in the abdominal cavity, one on either side of vertebral column behind kidney. Each ovary can be differentiated into 3 parts 1. Outer germinal epithelium 2. Tunica albuginea: it is middlelayer of delicate connective tissue 3. Stroma: it is the inner mass of connective tissue. It is further differentiated into 2 layer- outer cortex and inner medulla. It is lined with germinal epithelium which form ovarian follicle. Each ovary is composed of about 400000 ovarian follicle. Ovaries are inactive before puberty, but stroma already contain immature follicle; Primordial follicle. Priomrdial follicle mature in about 28 days, rapture and release ova; process known as ovulation. The germinal epithelium of ovarian follicle give ova germ cell called Oogoia. Oogenesis occur in 3 phages O. Alturshani\K. Mohamed\W. Owgie page 35  Benghazi University, Zoology Department- Biology – for Premedical Students Figure: stages of Oogenesis i) Multiplication: The initial phase of Oogenesis starts during fetal stage. The primary germ cell, Oogonia develop from stem cell by mitosis cell division. In adult ovaries, primordial follicle contains a primary oocyte. Primary oocyte is also known as ova mother cell, which eventually produce ova. ii) Growth or maturation phase: the Oogonia undergoes maturation. It is a diploid cell. Mature Oogonia is knownas primary oocytes, which undergoes meiosis, howerer, meiosis stopped at Prophase-I. iii) Meiotic phase: Completion of meiosis-I produces a secondary oocyte and a polar body. The second meiosis division os Secondary Oocyte occur with unequal distribution of cytoplaswm producing large egg and a small second polar body. Eventually 1 egg and 3 polar bodies are produced. O. Alturshani\K. Mohamed\W. Owgie page 36  Benghazi University, Zoology Department- Biology – for Premedical Students DIFFERENCES BETWEEN SPERMATOGENESIS AND OOGENESIS Spermatogenesis Oogenesis 1. Occurs in testes. 1. Occurs in ovaries. 2 Spermatogonia are not found in follicles. 2. Oogonia are found in ovarian follicles. 3. Spermatogonia store some food during 3. The food reserve accumulated in oogonia growth phase, but this is never yolk. during growth phase is mostly yolk. 4. Both maturation divisions are completed in 4. First maturation division may be completed in testes themselves. the ovaries, but the second one is completed outside ovaries after fertilization begins. 5. Daughter cells formed in each maturation 5. Daughter cells formed in each maturation division are similar, so that four spermatids are division are dissimilar; each time a small. ultimately formed from each spermatogonium. nonfunctional polar cell ( = polar body) is formed together with functional oocyte cell. Thus, a single ootid is formed from an oogonium. 6. To take part in reproduction, the spermatids 6. Ootids as such take part in reproduction. have to transform into thread-like sperms by O. Alturshani\K. Mohamed\W. Owgie page 37  Benghazi University, Zoology Department- Biology – for Premedical Students Hormonal Control of Gametogenesis The human male and female reproductive cycles are controlled by the interaction of hormones from the hypothalamus and anterior pituitary with hormones from reproductive tissues and organs. When the reproductive hormone is required, the hypothalamus sends a gonadotropin- releasing hormone (GnRH) to the anterior pituitary. This causes the release of follicle stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary into the blood. Although FSH and LH are named after their functions in female reproduction, they are produced and play important roles in controlling reproduction in both sexes. Reproductive hormones in males Spermatogenesis is controlled by FSH, LH, and testosterone:  FSH enters the testes and stimulates spermatogenesis  LH enters the testes and stimulates production of testosterone  Testosterone further stimulates spermatogenesis. It is also the hormone responsible for the secondary sexual characteristics that develop in the male during adolescence, including a deepening of the voice, the growth of facial, axillary, and pubic hair, and the beginnings of the sex drive. Reproductive hormones in females The control of reproduction in females is more complex. Oogenesis is controlled by FSH, LH, estrogen, and progesterone.  FSH stimulates development of egg cells, called ova, which develop in structures called follicles. Follicle cells produce the hormone inhibin, which inhibits FSH production. Follicles also produce estradiol and progesterone.  LH also plays a role in the development of ova, induction of ovulation, and stimulation of estradiol (a form of estrogen) and progesterone production by the ovaries.  Estrogens (such as estradiol) and progesterone are released from the developing follicles. Estrogen is the reproductive hormone in females that assists in endometrial regrowth, ovulation, and calcium absorption; it is also responsible for the secondary sexual characteristics of females such as breast development, flaring of the hips, and a shorter period necessary for bone maturation. Progesterone assists in endometrial re-growth and inhibition of FSH and LH release. These hormones together regulate the ovarian and menstrual cycles. The ovarian cycle governs the preparation of endocrine tissues and release of eggs, while the menstrual cycle governs the preparation and maintenance of the uterine lining. These cycles occur concurrently and are coordinated over a 22–32 day cycle, with an average length of 28 days:  the first half of the ovarian cycle is the follicular phase. Slowly rising levels of FSH and LH cause the growth of follicles on the surface of the ovary. This process prepares the egg for ovulation.  As the follicles grow, they begin releasing estrogens and a low level of progesterone.  Estrogen levels increase over the course of the follicular phase as the follicles continue to develop. In the menstrual cycle, menstrual flow occurs at the beginning of the follicular phase when estrogen levels are low (when the follicles are only just beginning to develop); rising levels of estrogen then cause the endometrium to proliferate (grow), replacing the blood vessels and glands that deteriorated during the end of the last cycle. O. Alturshani\K. Mohamed\W. Owgie page 38  Benghazi University, Zoology Department- Biology – for Premedical Students  Ovulation occurs just prior to the middle of the cycle (approximately day 14), when the high level of estrogen produced by the developing follicles causes FSH and especially LH to rise rapidly, then fall. The spike in LH causes ovulation: the most mature follicle ruptures and releases its egg. The follicles that did not rupture degenerate and their eggs are lost. The level of estrogen decreases when the extra follicles degenerate.  Following ovulation, the ovarian cycle enters its luteal phase, and the menstrual cycle enters its secretory phase, both of which run from about day 15 to 28. The luteal and secretory phases refer to changes in the ruptured follicle. The cells in the follicle undergo physical changes and produce a structure called a corpus luteum.  The corpus luteum produces estrogen and progesterone. The progesterone facilitates the regrowth of the uterine lining and inhibits the release of further FSH and LH.  The uterus is being prepared to accept a fertilized egg, should it occur during this cycle.  The inhibition of FSH and LH prevents any further eggs and follicles from developing, while the progesterone is elevated. The level of estrogen produced by the corpus luteum increases to a steady level for the next few days. Estrogen enhances the effects of progesterone.  It takes about seven days for an egg to travel through the fallopian tube from the ovary to the uterus, and it must be fertilized while in the fallopian tube. If no fertilized egg is implanted into the uterus, the corpus luteum degenerates and the levels of estrogen and progesterone decrease. The endometrium begins to degenerate as the progesterone levels drop, initiating the next menstrual cycle. The decrease in progesterone also allows the hypothalamus to send GnRH to the anterior pituitary, releasing FSH and LH and starting the cycles again.The figure below visually compares the ovarian and uterine cycles as well as the hormone levels controlling these cycles. The figure below visually compares the ovarian and uterine cycles as well as the hormone levels controlling these cycles. O. Alturshani\K. Mohamed\W. Owgie page 39  Benghazi University, Zoology Department- Biology – for Premedical Students The purpose and stages of development Your body consist of over 30 trillion cells, but you began as a single cell: a fertilized egg, or zygote. How did you become the large, organized multicellular individual that you are today? Development! As an animal embryo develops, its cells divide, grow, and migrate in specific patterns to make a more and more elaborate body (plant cells perform differential expansion instead of migration). To function correctly, that body needs well-defined axes (such as head vs. tail). It also needs a specific collection of many-celled organs and other structures, positioned in the right spots along the axes and connected up with one another in the right ways. How are all of these complex processed accomplished and coordinated? They occur via four essential stages in early animal development:  Fertilization: the process of a single sperm cell combining with single egg cell to form a zygote.  Cleavage: rapid, multiple rounds of mitotic cell division where the overall size of the embryo does not increase. The developing embryos is called a blastula following completion of cleavage.  Gastrulation: the dramatic rearrangement (movement) of cells in the blastula to create the embryonic tissue layers. These tissue layers will go on to produce the tissues and organs of the adult animal.  Organogenesis: the process of organ and issue formation via cell division and differentiation. Gastrulation and organogenesis together contribute to morphogenesis: the biological processes that result in an organism‘s shape and body organization. For this class session, we will discuss the first two steps above, fertilization and cleavage. We will carry on with gastrulation and organogenesis in the next class session. Development Step 1: Fertilization Fertilization is the process in which a single haploid sperm fuses with a single haploid egg fuse to form a zygote. The sperm and egg cells each possess specific features that make this process possible: The egg is the largest cell produced in most animals species. Human eggs are approximately 16 times larger than a human sperm cell. The eggs of different species contain varying amounts of yolk, nutrients to support growth of the developing embryo. The egg is surrounded by a jelly layer, composed of glycoproteins (proteins that have sugars stuck to them), that releases species-specific chemoattractants (chemical-attractors) that activate and guide sperm to the egg. O. Alturshani\K. Mohamed\W. Owgie page 40  Benghazi University, Zoology Department- Biology – for Premedical Students This diagram represents the components of a mammalian/human egg cell prior to the process of fertilization. It shows the outer jelly coating, the plasma (cell) membrane with sperm cell receptors and the cortical granules just inside the plasma (cell) membrane. Image credit:By Chippolito – Own work,CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=18851511 The sperm is one of the smallest cells produced in most animal species. The sperm consists of head containing tightly packed DNA, a flagellar tail for swimming, and many mitochondria to provide power for sperm movement. The plasma membrane of the sperm contains proteins called bindin, which are species-specific proteins that recognize and bind to receptors on the egg plasma membrane. In addition to the nucleus, the sperm head also contains an organelle called the acrosome, which contains digestive enzymes that will degrade the jelly layer/zona pellucida to allow the sperm to reach the egg plasma membrane. To ensure that the offspring has only one complete diploid set of chromosomes, only one sperm can fuse with one egg. Fusion of more than one sperm with an egg, or polyspermy, is genetically incompatible with life and results in zygote death. There are two mechanisms that prevent polyspermy: the ―fast block‖ to polyspermy and the ―slow block‖ polyspermy. O. Alturshani\K. Mohamed\W. Owgie page 41  Benghazi University, Zoology Department- Biology – for Premedical Students Fertilization is the process in which sperm and egg fuse to form a zygote. The acrosomal reaction help the sperm degrade the glycoprotein matrix protecting the egg and allow the sperm to transfer its nucl eus.Image credit: LadyofHats Public Domain, Development Step 2: Cleavage and Blastula Stage After fertilization successfully activates the egg, the egg begins a series of rapid cell divisions called cleavage, illustrated below. Normal cell division occurs every 18-24 hours, but cleavage cell divisions can occur as frequently as every 10 minutes. During cleavage, the cells divide without an increase in size; that is, one large single-celled zygote divides into multiple smaller cells called blastomeres. After the cleavage has produced over 100 blastomeres, the embryo is called a blastula. The blastula is usually a spherical layer of blastomeres that are considered to be the first embryonic tissue, the blastoderm. The blastoderm surrounds a fluid- filled or yolk-filled cavity, called the blastocoel (coelum = body cavity). The very early stages of mammalian development are extremely similar to other animals. Later stages of cleavage are a little different though, resulting in a structure called the blastocyst, which has an inner cell mass and an outer cell layer called the trophoblast. The inner cell mass will go on to form the embryo, and the trophoblast will contribute to the embryonic portion of placenta and nourish the embryo. O. Alturshani\K. Mohamed\W. Owgie page 42  Benghazi University, Zoology Department- Biology – for Premedical Students Development Step 3: Gastrulation At the end of cleavage, the typical blastula is a ball of cells with a hollow cavity in the middle (the blastocoel). The next stage in embryonic development is gastrulation, in which the cells in the blastula rearrange themselves to form three layers of cells and form the body plan. The embryo during this stage is called a gastrula. 1. The formation of the embryonic tissues, called germ layers. The germ layers include the endoderm, ectoderm, and mesoderm. Each germ layer will later differentiate into different tissues and organ systems. 2. The three germs layers, shown below, are the endoderm, the ectoderm, and the mesoderm. The ectoderm gives rise to the nervous system and the epidermis. The mesoderm gives rise to the muscle cells and connective tissue in the body. The endoderm gives rise to columnar cells found in the digestive system and many internal organs. The three germ layers give rise to different cell types in the animal body. (credit: modification of work by NIH, NCBI) O. Alturshani\K. Mohamed\W. Owgie page 43  Benghazi University, Zoology Department- Biology – for Premedical Students Adult Animal Tissues The cells in complex multicellular organisms are organized into tissues, groups of similar cells that work together on a specific task. Organs are structures made up of two or more tissues organized to carry out a particular function, and groups of organs with related functions make up the different organ systems. The result of gastrulation is the formation of the three embryonic tissue layers, or germ layers. Over the course of development, these cells will proliferate, migrate, and differentiate into the four primary adult tissues: epithelial tissue, connective tissue, muscle tissue, and nervous tissue. Every organ is made up of two or more of these tissues Embryonic Tissues in Amniotes The terrestrially-adapted amniotic egg is the defining characteristic of amniotes (reptiles, birds, and mammals). The evolution of amniotic membranes meant that the embryos of amniotes were provided with their own aquatic environment, which led to less dependence on water for development and thus allowed the amniotes to branch out into drier environments. This was a significant development that distinguished them from amphibians, which were restricted to moist environments due their shell-less eggs.  The amnion, or inner amniotic membrane, surrounds the embryo itself, enclosing the aqueous environment that the embryo develops in to protect the embryo from mechanical shock and support hydration  The chorion. which surrounds the embryo and yolk sac, facilitates exchange of oxygen and carbon dioxide between the embryo and the egg‘s external environment.  The allantois stores nitrogenous wastes produced by the embryo and also facilitates respiration in combination with the chorion. The yolk sac encloses the nutrient-rich yolk and transports nutrients from the yolk to the embryo Most mammals do not lay eggs (though some do!), but they still have amniotic tissues that function as part of the placenta and umbilical cord, as shown below. In essence, pregnancy in placental mammals is the result of internalization and incorporation of the amniotic egg into the uterus, resulting in direct nourishment embryo inside of the amniotic egg rather than laying it outside of the body with a predefined amount of yolk. O. Alturshani\K. Mohamed\W. Owgie page 44  Benghazi University, Zoology Department- Biology – for Premedical Students As you can see above, the chorion separates the fetal and maternal sides of the placenta, and the aminon surrounds the developing fetus. Just as in the amniotic egg:  the chorion regulates gas exchange  the amnion encloses the fluid-filled cavity to provide an aqueous environment for the developing fetus  together, the yolk sac, consisting of blood vessels that transport nutrients to the embryo, and  the allantois, which functions in waste disposal, both function as part of the mammalian umbilical cord (not labeled above) O. Alturshani\K. Mohamed\W. Owgie page 45  Benghazi University, Zoology Department- Biology – for Premedical Students Development Step 4: Organogenesis Gastrulation leads to the formation of the three germ layers that give rise, during further development, to the different organs in the animal body. This process is called organogenesis. In vertebrates, one of the primary steps during organogenesis is the formation of the nervous system. Interestingly, the nervous system originates from ectodermal, not mesodermal tissue. During the formation of the neural system, induction causes some cells at the edge of the ectoderm to become epidermis cells. The remaining cells in the center form the neural plate, which will go on to form the nervous system. Immediately beneath the neural plate is a rod-shaped mesodermal structure called the notochord. The notochord signals the neural plate cells to fold over to form a tube called the neural tube, as illustrated below. During later development, the notochord will disappear (it goes on to help form the spongy discs between the vertebrae), and the neural tube will give rise to the brain The mesoderm that lies on either side of the vertebrate neural tube then forms a set of temporary structures called somites (also called ―primitive segments‖), shown below. Later in development the cells within the somites will migrate to different parts of the body to develop into bone, skeletal muscle, and connective tissue of the skin. The specific pattern of induction from nearby tissues, including the ectoderm, the neural tube, the notochord, and surrounding mesoderm, will determine what type of tissue a particular region of a somite will become. Dorsal view of human embryo. Somites (primitive segments) are visible on either side of the neural tube. Image credit: Henry Gray (1918) Anatomy of the Human Body, Bartleby.com: Gray’s Anatomy, Plate 20, Public Domain) O. Alturshani\K. Mohamed\W. Owgie page 46  Benghazi University, Zoology Department- Biology – for Premedical Students O. Alturshani\K. Mohamed\W. Owgie page 47  Benghazi University, Zoology Department- Biology – for Premedical Students Tissues The approximately 200 distinctly different types of cells composing the human body are arranged and cooperatively organized into four basic tissues. A tissue is a functional collection of cells and associated intercellular material that is specialized to carry out a specific role. Groups of these tissues are assembled in various organizational and functional arrangements into organs, which carry out functions of the body. The four basic tissues types are epithelium tissue, connective tissue, muscle tissue, and nervous tissue. Epithelium Tissue The outer surface of the body and the luminal surface of cavities within the body are lined by one or more layers of cells that completely cover them. Such layers of cells are called epithelia (singular=epithelium). Epithelia also line the ducts and Secretory elements of glands (which develop as outgrowths from epithelium lined surface). General Features of Epithelium Tissue Some features of epithelia are summarized in the following outline : 1. All surfaces in the body are covered or lined by an epithelium; excluding the joint cavities ; therefore, serve as barrier membranes to separate the organism from various external and internal environments. 2. Epithelia rests on a basement membrane. 3. Epithelia are generally avascular; so nourishment of an epithelium occurs by diffusion from the underlying connective tissue vasculature. 4. Epithelial tissue possess a remarkable capability for renewal and regeneration. 5. The cells are compactly arranged on a thin, structureless basement membrane. 6. Due to the compact arrangement, intercellular spaces are usually absent. 7. Epithelia are diverse in origin; they are derived from all three primary germ layers (ectoderm, mesoderm, and endoderm). 8. Finally, the diversity of epithelial function includes ; protection, lubrication, secretion, sensory, digestion, absorption, transduction, and reproduction. Types of Epithelia Epithelia are divided into two main groups according to their structure and function into : covering epithelia and glandular epithelia. O. Alturshani\K. Mohamed\W. Owgie page 48  Benghazi University, Zoology Department- Biology – for Premedical Students Classification of Epithelium Tissues An epithelium may consist of only one layer of cells resting on basement membrane when it is called a uni-layered or simple epithelium. Alternatively, it is composed of more than one cell layer multi-layered or stratified. Simple epithelia may be further classified according to the shape of the cells constituting them. (1)In some epithelia the cells are flattened, their height being very little as compared to their width. Such an epithelium is called a simple squamous epithelium. Locations : glandular ducts of small caliber, lining the pleural, pericardial, and peritoneal cavities (mesothelium); lining the cardiovascular and lymph channels (endothelium); respiratory bronchioles and alveoli of the lungs, Bowman's capsule in the kidney. (2)When the height and width of the cells of the epithelium are or less equal (i.e., they look like squares in section) it is described as simple cuboidal epithelium. Locations : ducts of many glands, lining certain kidneys tubules, rete testis, and covering the free surface of the ovary. (3)When the height of the cells of the epithelium is distinctly greater than their width, it is described as a simple columnar epithelium. Locations : much of digestive tract (stomach, intestine, gall-bladder), portions of female reproductive tract (oviduct and uterus). (4)The fourth variety is termed pseudostratified columaner epithelium ; despite an apparently stratified appearance, all cells rest on the basement membrane. Locations : large portion of the respiratory passages, Eustachian tube, and portions of the male and female urethra. stratified epithelia there are two or more layers of cells, only the basal (lowermost) layer of cells rests on the basement membrane. According to the shape of the surface (outermost) layer of cells, a stratified squamous, stratified cuboidal, or stratified columnar epithelium can be distinguished. A fourth variety of stratified epithelia is termed transitional epithelium. Originally, this epithelium was termed transitional because it was considered to be an intermediate between stratified squamous and stratified columnar. The appearance of this epithelium varies tremendously depending on whether it is in its contracted or expanded state. Locations: limited to lining some portions of the urinary tract, namely , the renal pelvis , ureters and urinary bladder. O. Alturshani\K. Mohamed\W. Owgie page 49  Benghazi University, Zoology Department- Biology – for Premedical Students Table : Classification of Epithelia Shape of Type Locations Functions Structure cells Simple Squamous Flattened Lining: pulmonary Limiting membrane, alveoli, loop of fluid transport, and Henle and blood gaseous exchange. and lymphatic vessels. Cuboidal Cuboidal Ducts of many Secretion, absorption glands, covering and protection. of ovary and form kidney tubules. Columnar Columnar Lining: much of Transportation and digestive, secretion. gallbladder and uterus. Pseudostratified All cells rests on Lining: most of Secretion and basal lamina but trachea and male protection. not all reach urethra. epithelial surface. Stratified Squamous – Flattened Lining: mouth Protection and nonkeratinized epiglottis and secretion. vagina. Squamous keratinized Flattened Epidermis of skin Protection. Cuboidal Cuboidal Lining: ducts of Absorption and sweat glands. secretion. Columnar Columnar Conjunctiva of Secretion and eye and portions absorption. of male urethra. Transitional Dome-shaped Lining: urinary Protection and (relaxed), tract. distensile. flattened (distended). O. Alturshani\K. Mohamed\W. Owgie page 50  Benghazi University, Zoology Department- Biology – for Premedical Students Classification of epithelium tissue O. Alturshani\K. Mohamed\W. Owgie page 51  Benghazi University, Zoology Department- Biology – for Premedical Students Classification of epithelium tissues O. Alturshani\K. Mohamed\W. Owgie page 52  Benghazi University, Zoology Department- Biology – for Premedical Students O. Alturshani\K. Mohamed\W. Owgie page 53  Benghazi University, Zoology Department- Biology – for Premedical Students O. Alturshani\K. Mohamed\W. Owgie page 54  Benghazi University, Zoology Department- Biology – for Premedical Students Connective tissues The term connective tissue is applied to a tissue that fills the interstices between more specialized elements; serves to hold them together and support them. General Features of connective Tissue 1. Most connective tissues originate from mesoderm, middle germ layer of the embryonic tissue. 2. Connective tissues are responsible for providing and maintaining form in the body and also provide a matrix that connect and binds the cells and organs and ultimately gives support to the body. 3. Structurally, connective tissue is formed by three classes of components: cells, fibers, and ground substance. 4. Unlike the other tissues (epithelium, muscle, and nerve), which are formed mainly by cells, the major constituent of connective tissue is the extracellular matrix. Functions of Connective Tissue Although many functions are attributed to connective tissue, its primary functions include: 1. Providing structural support. 2. Serving as a medium for exchange. 3. Aiding in the defense and protection of the body. 4. Forming a site for storage of fat. Basic Component of Connective Tissue Connective tissues are made up of a matrix consisting of living cells and a non-living substance, called the ground substance. The ground substance is made of an organic substance (usually a protein) and an inorganic substance (usually a mineral or water). The principal cell of connective tissues is the fibroblast. This cell makes the fibers found in nearly all of the connective tissues. Fibroblasts are motile, able to carry out mitosis, and can synthesize whichever connective tissue is needed. Macrophages, lymphocytes, and, occasionally, leukocytes can be found in some of the tissues. Some tissues have specialized cells that are not found in the others. The matrix in connective tissues gives the tissue its density. When a connective tissue has a high concentration of cells or fibers, it has proportionally a less dense matrix. The organic portion or protein fibers found in connective tissues are either collagen, elastic, or reticular fibers. Collagen fibers provide strength to the tissue, preventing it from being torn or separated from the surrounding tissues. Elastic fibers are made of the protein elastin; this fiber can stretch to one and one half of its length and return to its original size and shape. Elastic fibers provide flexibility to the tissues. Reticular fibers are the third type of protein fiber found in connective tissues. This fiber consists of thin strands of collagen that form a network of fibers to support the tissue and other organs to which it is connected. O. Alturshani\K. Mohamed\W. Owgie page 55  Benghazi University, Zoology Department- Biology – for Premedical Students The various types of connective tissues, the types of cells and fibers they are made of, and sample locations of the tissues is summarized in Table 33.3. Loose/Areolar Connective Tissue Loose connective tissue, also called Areolar connective tissue, has a sampling of all of the components of a connective tissue. As illustrated in Figure 33.12, loose connective tissue has some fibroblasts; macrophages are present as well. Collagen fibers are relatively wide and stain a light pink, while elastic fibers are thin and stain dark blue to black. The space between the formed elements of the tissue is filled with the matrix. The material in the connective tissue gives it a loose consistency similar to a cotton ball that has been pulled apart. Loose connective tissue is found around every blood vessel and helps to keep the vessel in place. The tissue is also found around and between most body organs. In summary, Areolar tissue is tough, yet flexible, and comprises membranes. O. Alturshani\K. Mohamed\W. Owgie page 56  Benghazi University, Zoology Department- Biology – for Premedical Students Fibrous Connective Tissue Fibrous connective tissues contain large amounts of collagen fibers and few cells or matrix material. The fibers can be arranged irregularly or regularly with the strands lined up in parallel. Irregularly arranged fibrous connective tissues are found in areas of the body where stress occurs from all directions, such as the dermis of the skin. Regular fibrous connective tissue, shown in Figure 33.13, is found in tendons (which connect muscles to bones) and ligaments (which connect bones to bones). O. Alturshani\K. Mohamed\W. Owgie page 57  Benghazi University, Zoology Department- Biology – for Premedical Students Adipose Tissue Adipose tissue, or fat tissue, is considered a connective tissue even though it does not have fibroblasts or a real matrix and only has a few fibers. Adipose tissue is made up of cells called adipocytes that collect and store fat in the form of triglycerides, for energy metabolism. Adipose tissues additionally serve as insulation to help maintain body temperatures, allowing animals to be endothermic, and they function as cushioning against damage to body organs. Under a microscope, adipose tissue cells appear empty due to the extraction of fat during the processing of the material for viewing, as seen in Figure 33.16. The thin lines in the image are the cell membranes, and the nuclei are the small, black dots at the edges of the cells. Dense irregular connective tissue O. Alturshani\K. Mohamed\W. Owgie page 58  Benghazi University, Zoology Department- Biology – for Premedical Students Specialized connective tissue : cartilage, bone, and blood are specialized connective tissues. (a). Cartilage possesses cells called chondrocytes, which occupy small cavities called lacunae within the extracellular matrix they secreted. The substance of cartilage is neither vascularized nor supplied with nerves or lymphatic vessels; however, the cells receive their nourishment from blood vessels of surrounding connective tissue by diffusion through the matrix. The extracellular matrix is composed of glycosaminoglycans and proteoglycans, which are intimately associated with the collagen and elastic fibers embedded within the matrix. Types of Cartilage There are three types of cartilage according to the fibers present in the matrix. 1. Hyaline cartilage : a bluish-gray, semitranslucent, substance, is the most common cartilage of the body and possess perichondrium. It is located in the nose and larynx, in the tracheal rings. Also , it is this cartilage that forms the cartilage template of many of bones during embryonic development 2. Elastic cartilage :in most respects, elastic cartilage is identical to hyaline cartilage and is often associated with it. The outer fibrous layer of the perichondrium is rich in elastic fibers. The matrix of elastic cartilage possesses abundant, fine to coarse branching elastic fibers. It is located in the pinna of the ear, and the external and internal auditory tubes. 3. Fibrocartilage : unlike hyaline and elastic cartilage, does not possess a perichondrium. It is present in Intervertebral disc. Hyaline cartilage O. Alturshani\K. Mohamed\W. Owgie page 59  Benghazi University, Zoology Department- Biology – for Premedical Students Elastic cartilage O. Alturshani\K. Mohamed\W. Owgie page 60  Benghazi University, Zoology Department- Biology – for Premedical Students (b). Bone : is the primary structural framework for support and protection of the organs of the body, including the brain and spinal cord and lungs. Bone is covered on its external surface, except at synovial articulations, with periosteum. bone is composed of cells lying in an extracellular matrix that has become calcified. The cells of bone include osteoprogenitor cells, which differentiate into osteoblasts. Osteoblasts are responsible for secreting the matrix. When these cells are surrounded by matrix , they become osteocytes, and occupy by lacunae. Osteoclast which are multinucleated giant cells involved in the resorption and remodeling of bone tissue. Types of Bone Gross observation of bone in cross section shows dense areas with-out cavities corresponding to compact bone, and areas with numerous interconnecting cavities corresponding cancellous (spongy) bone. Microscopic examination of bone shows collagen fibers arranged in lamellae that are parallel to each other or concentrically organized around a vascular canal, which containing blood vessels, nerves, and loose connective tissue is called a Haversian system or osteon. The haversian canals communicate with each other through transverse or oblique Volkmann's canals. O. Alturshani\K. Mohamed\W. Owgie page 61  Benghazi University, Zoology Department- Biology – for Premedical Students Compact bone O. Alturshani\K. Mohamed\W. Owgie page 62  Benghazi University, Zoology Department- Biology – for Premedical Students (c). Blood Blood is a specialized connective tissue composed of formed elements– Red blood cells ( RBCs; erythrocytes), White blood cells (WBCs; leukocytes), and Platelets (Thrombocytes). suspended in a fluid component (the extracellular matrix), known as plasma. Blood components Plasma: The translucent, yellowish, somewhat viscous supernatant obtained when whole blood is centrifuged is the plasma The major component of plasma is water, constituting about 90% of its volume. Proteins constitute 9% , and inorganic salts, ions, nitrogenous compounds, nutrients, and gases constitute the remaining 1%. The main plasma proteins are albumins, globulins, lipoproteins, and proteins that participate in blood coagulation, such as prothrombin and fibrinogen. Formed elements: 1. Erythrocytes :(red blood cells) , the smallest and most numerous cells of blood, have no nuclei and are responsible for the transport of oxygen and carbon dioxide to and from the tissues of the body. Each erythrocytes resembles a biconcave-shaped disk. Human erythrocytes have an average life span of 120 days, after that they destroyed by macrophage of the spleen, bone marrow, and liver. 2. Leukocytes: (white blood cells) migrate to the tissues, where they perform multiple functions. Human leukocytes have an average life span of days to years. According to the of granules in their cytoplasm and the shape of their nuclei, leukocytes are divided into groups: granulocytes (neutrophils, eosinophils, and basophils) and agranulocytes (lymphocytes and monocytes). The functions of white blood cells include: phagocytes, destroy bacteria, kill parasitic invaders and initiators of the inflammatory process. 3. Platelets: blood platelets (Thrombocytes) are non nucleated disk like cell fragments, 2-4 mm in diameter and life span is 7-11 days. Platelets promote blood clotting and help repair gaps in the walls of blood vessels, preventing loss of blood. O. Alturshani\K. Mohamed\W. Owgie page 63  Benghazi University, Zoology Department- Biology – for Premedical Students Formed elements of blood Lymph : is a transudate from blood and contains the same proteins as in plasma, but in smaller amounts, and in somewhat different proportions. Suspended in lymph there are cells that are chiefly lymphocytes. The lymph returned to the circulation through a separates system of lymphatic vessels (usually called lymphatics). The fluid passing through the lymphatic vessels is called lymph. O. Alturshani\K. Mohamed\W. Owgie page 64  Benghazi University, Zoology Department- Biology – for Premedical Students Muscular tissue Although many cells of multicellular organisms have limited contractile abilities, it is the capability of muscle cells, which are specialized for contraction, that permits animals to move. General features of muscular tissue 1. Muscle tissue is composed of differentiated cells containing contractile proteins. 2. Most muscle cells are of mesodermal origin. 3. Muscle tissue is made up basically of cells that are called muscle fibers. 4. Unique terms are often used to describe the components of muscle cells. Thus, muscle cell membrane is referred to as sarcolemma; the cytoplasm, as sarcoplasm; the smooth endoplasmic reticulum, as sarcoplasmic reticulum. 5. Cells of muscle are elongated and are called either striated muscle cells or smooth muscle cells, depending on the respective presence or absence of a regularly repeated arrangement of myofibrillar contractile proteins, myofilaments. 6. Striated muscle cells display characteristic alternations of light and dark cross-bands, which are absent in smooth muscle. 7. There are two types of striated muscle : skeletal, accounting for most of the voluntary muscle mass of the body, and involuntary cardiac, limited almost exclusively to the heart. The third type is unstriated, smooth muscle cells are located in the walls of blood vessels and the viscera as well as in the dermis of skin. Types of muscle There are three types of muscles : (a). Skeletal muscle : is composed of long, cylindrical, not branched muscle fiber, and multinucleated cells that undergo voluntary contraction to facilitate movement of the body. (b). Cardiac muscle: is composed of long, branched muscle fiber with a single, large, oval, centrally placed nucleus, nonvoluntary striated muscle limited to the heart. (c). Smooth muscle: is composed of short, spindle-shaped muscle fiber, with a centrally placed nucleus, it is not under voluntary control, and found in the walls of a hollow viscera. O. Alturshani\K. Mohamed\W. Owgie page 65  Benghazi University, Zoology Department- Biology – for Premedical Students Types Skeletal muscle Cardiac muscle Smooth muscle Site Skeleton Heart Viscera Control Voluntary Involuntary Involuntary Striations Striated Less striated Non-striated Contraction Quick Rhythmic Slow O. Alturshani\K. Mohamed\W. Owgie page 66  Benghazi University, Zoology Department- Biology – for Premedical Students Nervous tissue Nervous tissue, composed of as many as a trillion neurons with multitudes of interconnections, forms the complex system of neuronal communication within the body. General features of nervous tissue 1. The nervous system develops from the ectoderm. 2. The specialized cells that constitute the functional units of the nervous system are called neurons. 3. The nervous system is organized anatomically into the central nervous system (CNS), which comprise the brain and spinal cord, and the peripheral nervous system (PNS), which include cranial nerves, emanating from the brain; spinal nerves, emanating from the spinal cord. 4. Within the CNS, neurons are supported by a special kind of connective tissue that is called neuroglia; while within PNS, the supporting cells is called schwann cells. Cells of the nervous system The cells of the nervous system are divided into two categories : neurons, which are responsible for the receptive, integrative, and motor functions of the nervous system; and neruroglia cells, which support and protect neurons. Neurons structure A neuron consists of a cell body that gives off a variable number of processes. The cell body is also called the soma. Like a typical cell it consists of a mass of cytoplasm surrounded by a cell membrane. The cytoplasm contains a large central nucleus (usually with a prominent nucleolus), numerous mitochondria, lysosome, and