Cytology 2024 PDF

Duration: 100 min 1-year biology Grades: 1 First lecturer Dr. Ahmed Atwa Lecturer of stem cell technologies & Tissue culture Cell Biology  Figure 3-1 Page 68-69...

Duration: 100 min 1-year biology Grades: 1 First lecturer Dr. Ahmed Atwa Lecturer of stem cell technologies & Tissue culture Cell Biology  Figure 3-1 Page 68-69 2 The Anatomy of a Model Cell How are living things organized? ❖ Molecules: ❖ are a group of atoms working together ❖ Organelles: ❖ are a group of molecules working together Cells: are a group of organelles working together Tissues: are a group of similar cells working together Organs: are a group of different tissues working together Organ systems (11): are a group of organs working together Organism: is an individual 3  Each of the fundamental tissues is formed by several types of cells and typically by specific associations of cells and extracellular matrix. These characteristic associations facilitate the recognition of the many subtypes of tissues by students. Most organs are formed by an orderly combination of several tissues, except the central nervous system, which is formed almost solely by nervous tissue. The precise combination of these tissues allows the functioning of each organ and of the organism. The small size of cells and matrix components makes histology dependent on the use of microscopes. Advances in chemistry, physiology, immunology, and pathology—and the interactions among these fields— are essential for a better knowledge of tissue biology. Familiarity with the tools and methods of any branch of science is essential for a proper understanding of the subject. This chapter reviews some of the more common methods used to study cells and tissues and the principles involved in these methods. Levels of Structural Organization Smooth muscle cell Molecules 2 Cellular level Cells are made up of molecules Atoms 1 Chemical level Atoms combine to Smooth form molecules muscle tissue Heart 3 Tissue level Cardiovascular Tissues consist of system Blood similar types of cells vessels Epithelial tissue Smooth Blood muscle vessel tissue (organ) 6 Organismal level Connective The human organism is tissue made up of many organ systems 4 Organ level Organs are made up of 5 Organ system level Organ systems consist of different organs different types of tissues that work together closely 5 Cells Cells are: Function: The smallest living units The building blocks of in the body all plants and animals 0.1 mm in diameter in a Come from the division typical cell of preexisting cells Named by Robert Perform vital Hooke who observed physiological functions by microscopy Each cell maintains homeostasis at the cellular level 6  Cellular Diversity It is amazing that from a single cell, the fertilized egg, hundreds of kinds of cells arise, producing the estimated 60 trillion to 100 trillion cells that make up an adult human. Cells vary greatly in size and shape. The smallest cells are visible only through a high-powered microscope. Even the largest, an egg cell (ovum), is barely visible to the unaided eye.  CELLULAR CHEMISTRY To understand cellular structure and function, one must have a knowledge of basic cellular and general body chemistry. All the processes that occur in the body comply with principles of chemistry. Furthermore, many of the dysfunctions of the body have a chemical basis. Elements, Molecules, and Compounds Elements are the simplest chemical substances. Four elements compose over 95% of the body’s mass. These elements and their percentages of body weight are oxygen (O) 65%, carbon (C) 18%, hydrogen (H) 10%, and nitrogen (N) 3%. Additional common elements found in the body include calcium (Ca), potassium (K), sodium (Na), phosphorus (P), magnesium (Mg), and sulfur (S).  CELLULAR STRUCTURE The cell membrane separates the interior of a cell from the extracellular environment. The passage of substances into and out of the cell is regulated by the cell membrane. Most of the metabolic activities of a cell occur within the cytoplasmic organelles. The nucleus functions in protein synthesis and cell reproduction. 11  cell parts 1. Cell (plasma) membrane. The selectively permeable cell membrane gives form to the cell. It controls the passage of molecules into and out of the cell and separates the cell’s internal structures from the extracellular environment. 2. Cytoplasm and organelles. The cytoplasm (si′to˘-plaz″em) is the cellular material between the nucleus and the cell membrane. Organelles (or″ga˘-nelz′) are the specialized structures suspended within the cytoplasm of the cell that perform specific functions. 3. Nucleus. The nucleus (noo′kle-us) is the large spheroid or oval body usually located near the center of the cell. It contains the DNA, or genetic material, that directs the activities of the cell. Within the nucleus, one or more dense bodies called nucleoli (singular, nucleolus) may be seen. Cell Membrane The extremely thin cell (plasma) membrane is composed primarily of phospholipid and protein molecules. Its thickness ranges from 65 to 100 angstroms (Å); that is, it is less than a millionth of an inch thick. The structure of the cell membrane is not fully understood, but most cytologists believe that it consists of a double layer of phospholipids in which larger globular proteins are embedded.  The proteins are free to move within the membrane. As a result, they are not uniformly distributed, but rather form a constantly changing mosaic. Minute openings, or pores, ranging between 7 and 10 Å in diameter extend through the membrane. The two most important functions of the cell membrane are to enclose the components of the cell and to regulate the passage of substances into and out of the cell. A highly selective exchange of substances occurs across the membrane boundary, involving several types of passive and active processes. The permeability of the cell membrane 1) Structure of the cell membrane. Although cell membranes of all cells are composed of phospholipids, there is evidence that their thickness and structural arrangement both of which could affect permeability vary considerably. 2) Size of the molecules. Macromolecules, such as certain proteins, are not allowed into the cell. Water and amino acids are small molecules and can readily pass through the cell membrane. 3) Ionic charge. The protein portion of the cell membrane carries a positive or negative ionic charge. Ions with an opposite charge are attracted to and readily pass through the membrane, whereas those with a similar charge are repelled. 4) Lipid solubility. Substances that are easily dissolved in lipids pass into the cell with no problem, since a portion of the cell membrane is composed of lipid material. 5) Presence of carrier molecules. Specialized carrier molecules within the cell membrane can attract and transporting substances across the membrane, regardless of size, ionic charge, or lipid solubility. 6) Pressure differences. The pressure difference on the two sides of a cell membrane may greatly aid movement of molecules either into or out of a cell.  Transport Across Cell Membrane The contents of a cell are surrounded by its cell membrane or plasma membrane. Thus, any communication between the cell and the extracellular medium is mediated by the cell membranes. These cell membranes serve two important functions: It must retain the dissolved materials of the cell so that they do not simply leak out into the environment. It should also allow the necessary exchange of materials into and out of the cell. There are two major methods for moving molecules across a membrane, and it is related to whether cell energy is used. Passive mechanisms, such as diffusion, require no energy to function, whereas active transport does. In passive transport, an ion or molecule crosses the membrane and moves down its concentration or electrochemical gradient. The different types of transport mechanisms across cell membranes are as follows: Simple diffusion Facilitated diffusion Osmosis  Diffusion Diffusion is a spontaneous process in which a substance moves from a region of high concentration to a region of low concentration, eventually eliminating the concentration difference between the two regions. Simple Diffusion Transport across the plasma membrane occurs unaided in simple diffusion, i.e., molecules of gases such as carbon dioxide and oxygen, as well as small molecules like ethanol, enter the cell by crossing the cell membrane without the assistance of any permease. A small molecule in an aqueous solution dissolves into the phospholipid bilayer, crosses it, and then dissolves into the aqueous solution on the opposite side during simple diffusion. The relative rate of molecule diffusion across the phospholipid bilayer is proportional to the concentration gradient across the membrane. Facilitated Diffusion This is a type of passive transport in which molecules that cross the cell membrane move quickly due to the presence of specific permeases in the membrane. Facilitated diffusion occurs only in the direction of a concentration gradient and does not require metabolic energy. It is distinguished by the following characteristics: The rate of molecule transport across the membrane is much faster than would be expected from simple diffusion. This is a specific process; each facilitated diffusion protein transports only one type of molecule. There is a maximum rate of transport, which means that when the concentration gradient of molecules across the membrane is low, increasing the concentration gradient results in an increase in the rate of transport. Osmosis Water molecules can transport through the cell membrane. The movement of water molecules through the cell membrane is caused by differences in the concentration of the solute on its two sides. Osmosis is the process by which water molecules pass through a membrane from a region of higher water concentration to a region of lower water concentration. The process by which water molecules enter the cell is known as endosmosis, whereas the process by which water molecules exit the cell is known as exosmosis. Excessive exosmosis causes the cytoplasm and cell membrane in plant cells to shrink away from the cell wall. This is known as plasmolysis. It is due to plasmolysis that a plant loses its support and wilts. When two compartments of different solute concentrations are separated by a semipermeable membrane, the compartment with higher solute concentration is called hypertonic relative to the compartment of lower solute concentration, which is described as hypotonic. If a cell is placed in a hypotonic solution, it rapidly gains water by osmosis and swells. Conversely, a cell placed into a hypertonic solution rapidly loses water by osmosis and shrinks. When the internal solute concentration equals the external solute concentration, it is said to be isotonic. Here, no net movement of water in or out of the cells occurs. The amount of water contained within the cell creates a pressure termed hydrostatic pressure (osmotic pressure). The cell membrane regulates the osmotic pressures of intracellular and intercellular fluids. Endo- and Exocytosis Eukaryotic cells can take up macromolecules from the outside of the cell by a process called endocytosis, and to transport macromolecules to the outside of the cell by a process called exocytosis, as shown in the diagram. In endocytosis: the endocytosed macromolecules are transported inside the cell in a vesicle called an endosome. (These include macromolecules in the plasma membrane itself). Endosomes fuse fuse to form early endosomes. The early endosomes fuse with vesicles from the Golgi to form late endosomes. New vesicles called lysosomes, bud off from the late endosome. The lysosomes are bags of hydrolytic enzymes, which 'digest' macromolecules. They contain around 40 different types of hydrolytic enzymes - proteases, lipases, glycosidases, etc, and they work in an acid environment. in which the proteins are broken down. Some molecules/proteins are retrieved (pink arrows), either from the endosomes and sent back to the plasma membrane for re-use (pink arrows), from late endosomes and sent to the Golgi (pink arrow). In exocytosis, proteins made in the rough ER are transported to the Golgi, sorted, packaged, and sent to the plasma membrane in exocytic vesicles. Three routes for endocytosis: There are three routes for endocytosis - uptake of macromolecules at the plasma membrane, degradation of intracellular organelles (autophagy) and uptake of cells/microorganisms - phagocytosis. All these forms of endocytosis require special receptor molecules at the plasma membrane. 1. Phagocytosis (cellular eating) This is a specialized form of endocytosis, that is usually only carried out by specialized cells, for example, macrophages and other white blood cells. Micro-organisms, or cell debris are taken up into large vesicles, that are bigger than 250nm in diameter. The particles that are going to be phagocytosed have to bind to the membrane first through receptors in the membrane. For example, bacteria are bound to white blood cells when antibodies on the bacteria are recognized by special Fc receptors on white blood cells. The white blood cells then extend fine processes that surround the bacterium. 2- Pinocytosis (cellular drinking) This is sometimes used to mean fluid-phase endocytosis, when the vesicles are smaller than 150nm in diameter. This again requires receptors, and more than 25 different types of receptor are known. For example, most cells take up cholesterol through receptor mediated endocytosis. Cholesterol is transported in blood bound to proteins called low-density lipoproteins (LDL). The cells have receptors for cholesterol called LDL receptors. When LDL binds to the receptors, endocytosis is initiated. Defects in this pathway can result in high blood cholesterol levels, and a predisposition to atherosclerosis, heart disease, and early death from coronary heart disease. As the endocytic vesicles invaginate into the cell, the membrane is surrounded by a network of proteins, including clathrin, which form a cage around the vesicle (called clathrin coated pits). Cells also use this process to continually recycle their plasma membrane. The rate at which they do this varies from cell to cell, but a macrophage can ingest 25% of its own volume of fluid per hour (3% of it's plasma membrane per minute). Fibroblasts endocytose at a lower rate than this. This continual endocytosis, which removes plasma membrane, is balanced by continual exocytosis, so that the cell stays the same size. Synaptic transmission involves exocytosis (to release neurotransmitter) and endocytosis (to recover plasma membrane etc). 3- Receptor-mediated endocytosis Receptor-mediated endocytosis is the best route for the uptake of certain macromolecules. Initially, macromolecules attach to specific receptors, such as clathrin-coated pits at the cell surface. The pits germinate in the membrane to form small clathrin-coated vesicles containing receptor with bonded macromolecules. Eventually, clathrin-coated vesicles merge with the primary endosome and are transferred to the lysosome or recycled to plasma membrane  Bulk Transport - Exocytosis Exocytosis is composed of five main stages: 1. The first stage is called vesicle trafficking. This involves the steps required to move, over a significant distance, the vesicle containing the material that is to be disposed. 2. The next stage that occurs is vesicle tethering, which links the vesicle to the cell membrane by biological material at half the diameter of a vesicle. 3. Next, the vesicle’s membrane and the cell membrane connect and are held together in the vesicle docking step. 4. This stage of exocytosis is then followed by vesicle priming, which includes all the molecular rearrangements and protein and lipid modifications that take place after initial docking. In some cells, there is no priming. 5. The final stage, vesicle fusion, involves the merging of the vesicle membrane with the target membrane. This results in the release of the unwanted materials into the space outside the cell. Some examples of cells releasing molecules via exocytosis include the secretion of proteins of the extracellular matrix and secretion of neurotransmitters into the synaptic cleft by synaptic vesicles. Some examples of cells using exocytosis include: the secretion of proteins like enzymes, peptide hormones and antibodies from different cells, the flipping of the plasma membrane, the placement of integral membrane proteins(IMPs) or proteins that are attached biologically to the cell, and the recycling of plasma membrane bound receptors (molecules on the cell membrane that intercept signals). The proteins of the plasmalemma are synthesized in the rough endoplasmic reticulum and then transported in vesicles to the Golgi complex, where they may be modified and transferred to the cell membrane. This example shows the synthesis and transport of a glycoprotein, which is an integral protein of the membrane  Cytoplasm and Organelles Cytoplasm refers to the material located within the cell membrane but outside the nucleus. The material within the nucleus is frequently called the nucleoplasm. The term protoplasm is sometimes used to refer to the cytoplasm and nucleoplasm collectively. When observed through an electron microscope, distinct cellular components called organelles can be seen in the highly structured cytoplasm. The matrix of the cytoplasm is a jellylike substance that is 80% to 90% water. The organelles and inorganic colloid substances (suspended particles) are dispersed throughout the cytoplasm. Colloid substances have similar ionic charges that space them uniformly.  The Golgi apparatus The Golgi apparatus receives protein and/or lipid-filled vesicles that bud from the ER. The Golgi apparatus contains enzymes that modify proteins and lipids. For example, it can add a chain of sugars to proteins and lipids, thereby making them glycoproteins and glycolipids, which are molecules found in the plasma membrane. The vesicles that leave the Golgi apparatus move to other parts of the cell. Some vesicles proceed to the plasma membrane, where they discharge their contents. Because this is secretion, note that the Golgi apparatus is involved in processing, packaging, and secretion. Other vesicles that leave the Golgi apparatus are lysosomes. The Golgi apparatus is named for Camillo Golgi, who discovered its presence in cells in 1898. The Golgi apparatus consists of a stack of three to twenty slightly curved saccules whose appearance can be compared to a stack of pancakes. In animal cells, one side of the stack (the inner face) is directed toward the ER, and the other side of the stack (the outer face) is directed toward the plasma membrane  Endoplasmic Reticulum Often abbreviated ER, the endoplasmic reticulum (en″doplaz ′mik re˘-tik′yu˘-lum) is widely distributed throughout the cytoplasm as a complex network of interconnected membranes. 1. a rough, or granular, endoplasmic reticulum (rough ER), characterized by numerous small granules called ribosomes that are attached to the outer surface of the membranous wall. 2. A smooth endoplasmic reticulum (smooth ER) that lacks ribosomes. Ribosomes Ribosomes are composed of two subunits, one large and one small. Each subunit has its own mix of proteins and rRNA. Protein synthesis occurs at the ribosomes. Ribosomes are found free within the cytoplasm either singly or in groups called polyribosomes. Proteins synthesized by cytoplasmic ribosomes are used inside the cell for various purposes. Those produced by ribosomes attached to endoplasmic reticulum may eventually be secreted from the cell.  Lysosomes lysosome, subcellular organelle that is found in nearly all types of eukaryotic cells (cells with a clearly defined nucleus) and that is responsible for the digestion of macromolecules, old cell parts, and microorganisms. Each lysosome is surrounded by a membrane that maintains an acidic environment within the interior via a proton pump. Lysosomes contain a wide variety of hydrolytic enzymes (acid hydrolases) that break down macromolecules such as nucleic acids, proteins, and polysaccharides. These enzymes are active only in the lysosome’s acidic interior; their acid-dependent activity protects the cell from self-degradation in case of lysosomal leakage or rupture, since the pH of the cell is neutral to slightly alkaline. Lysosomes were discovered by the Belgian cytologist Christian René de Duve in the 1950s. (De Duve was awarded a share of the 1974 Nobel Prize for Physiology or Medicine for his discovery of lysosomes and other organelles known as peroxisomes.) For this reason, lysosomes are frequently called “suicide packets.” PEROXISOMES Specialized Organelles for Detoxification Structure: Peroxisomes are similar in size and shape to lysosomes, but they contain different enzymes. The interior of a peroxisome is filled with oxidative enzymes, such as catalase. Function: 1. Breakdown of fatty acids: Peroxisomes break down long-chain fatty acids into shorter fatty acids, which can then be transported to mitochondria for further oxidation. 2. Detoxification of harmful substances: Peroxisomes play a vital role in neutralizing harmful substances, such as reactive oxygen species (ROS) and toxins. Catalase enzyme breaks down hydrogen peroxide, a harmful ROS, into water and oxygen. 3. Synthesis of lipids and cholesterol: Peroxisomes are involved in the synthesis of certain lipids, including plasmalogens and bile acids, as well as cholesterol. Intercellular Adhesion & Intercellular Junctions Several membrane-associated structures contribute to cohesion and communication between cells. They are present in most tissues but are prominent in epithelia, which is why they are described in this chapter. Epithelial cells are extremely cohesive, and relatively strong mechanical forces are necessary to separate them. Intercellular adhesion is especially marked in epithelial tissues that are subjected to traction and pressure (eg, the skin). Adhesion is due in part to the binding action of a family of transmembrane glycoproteins called cadherins. Cadherins lose their adhesive properties in the absence of Ca2+. Interdigitations between folds of membranes of neighbor cells also help to increase intercellular adhesion. The lateral membranes of many epithelial cells often exhibit several types of membrane modifications, the intercellular junctions. One type of junction provides a mechanism for communication between adjacent cells. Other junctions serve as sites of adhesion and as seals to prevent the flow of material through the space between epithelial cells. In several epithelia the various junctions are present in a definite order from the apex toward the base of the cell. Tight junctions, or zonulae occludens (singular, zonula occludens), are the most apical of the junctions. The Latin terminology gives important information about the geometry of the junction. "Zonula" refers to the fact that the junction forms a band completely encircling the cell, and "occludens" refers to the membrane fusions that close off the intercellular space. In properly stained thin sections viewed in the transmission electron microscope, the outer leaflets of membranes of neighbor cells are seen to fuse, giving rise to a penta-laminar sheet. Gap or communicating junctions can occur almost anywhere along the lateral membranes of epithelial cells. Gap junctions are found in nearly all mammalian tissues, skeletal muscle being a major exception. They are seen, in conventional transmission electron micrographs, as a close (2-nm) apposition of adjacent cell membranes. In specimens submitted to cryofracture, these junctions are seen as aggregates of intramembrane particles arranged in circular patches in the plasma membrane Gap junction. A: Model of a gap junction in an oblique view. Channels (arrow) are formed by pairs of adjacent connexons, which are in turn composed of six protein subunits that span the lipid bilayer of each cell membrane. The channel measures about 1.5 nm in diameter, limiting the size of the molecules that can pass through it. Nutrients and signal molecules may be transported between cells without loss of material into the intercellular space. B: Gap junction as seen on a cryofracture preparation. The junction appears as a plaquelike accumulation of intramembrane protein particles.  The individual unit of the gap junction is called a connexon. Each connexon is formed by six gap junction proteins called connexins, which join leaving a hydrophilic pore about 1.5 nm in diameter in the center. Connexons of adjacent cells are aligned to form a hydrophilic channel between the two cells Each gap junction is formed by tens or hundreds of aligned pairs of connexons. Connexins belong to a family of related proteins that is distributed differently and forms channels with differing physiological properties. Gap junctions permit the exchange between cells of molecules with molecular mass