Unit III. The Cell and Tissues PDF

UNIT III. THE CELL AND TISSUES PART I. Overview of the Cellular Basis of Life In the late 1600s, Robert Hooke was looking through a crude microscope at some plant tissue—cork. He saw some cubelike structures that reminded him of the long rows of monk’s rooms (or cells) at the monast...

UNIT III. THE CELL AND TISSUES PART I. Overview of the Cellular Basis of Life In the late 1600s, Robert Hooke was looking through a crude microscope at some plant tissue—cork. He saw some cubelike structures that reminded him of the long rows of monk’s rooms (or cells) at the monastery, so he named these structures “cells.” The living cells that had formed the cork were long since dead; only the plant cell walls remained. However, the name stuck and is still used to describe the smallest unit of all living things. Since the late 1800s, cell research has been exceptionally fruitful and has provided us with the following four concepts collectively known as the cell theory: ▪ A cell is the basic structural and functional unit of living organisms. So, when you define cell properties, you are in fact defining the properties of life. ▪ The activity of an organism depends on the collective activities of its cells.  Anatomy of the Cell Cells are not all the same All cells share general structures Cells are organized into three main regions Nucleus Cytoplasm Plasma membrane Figure 3.1a Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 3.2 STRUCTURE OF CELL, ITS COMPONENTS, THEIR STRUCTURE & FUNCTIONS CELLS Cells have long been recognized as the simplest units of living matter that can maintain life and reproduce themselves. The human body, which is made up of numerous cells, begins as a single, newly fertilized cells. activity of an organism depends on both the individual and the collective activities of its cells. According to the principle of complementarity of structure and function, the biochemical activities of cells are dictated by their shapes or forms, and by the relative number of their specific subcellular structures Continuity of life from one generation to another has a cellular basis. cell is the microscopic package that contains all the parts necessary to survive in an ever-changing world. It follows then that loss of cellular homeostasis underlies virtually every disease Abilities or Functions of Cell ❖ Reproduction by cell division ❖ Use of enzymes and other proteins coded by DNA genes and made via messenger RNA intermediates and ribosomes, ❖ Metabolism, ❖ Response to external and internal stimuli such as change in temperature, pH or levels of nutrients, and ❖ Cell contents are contained within a cell surface membrane that is made from a phospholipid bilayer with proteins embedded in it. ❑ Organ systems do not work in isolation; instead, they work together to promote the well-being of the entire body  The Nucleus Control center of the cell Contains genetic material (DNA) Three regions Nuclear membrane Nucleolus Chromatin Figure 3.1b Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 3.3  Nuclear Membrane Barrier of nucleus Consists of a double phospholipid membrane Contain nuclear pores that allow for exchange of material with the rest of the cell Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 3.4  Nucleoli Nucleus contains one or more nucleoli Sites of ribosome production Ribosomes then migrate to the cytoplasm through nuclear pores Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 3.5  Chromatin Composed of DNA and protein Scattered throughout the nucleus Chromatin condenses to form chromosomes when the cell divides Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 3.6  Plasma Membrane Barrier for cell contents Double phospholipid layer Hydrophilic heads Hydrophobic tails Other materials in plasma membrane Protein Cholesterol Glycoproteins Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 3.7a  Plasma Membrane Figure 3.2 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 3.7b  Plasma Membrane Specializations Microvilli Finger-like projections that increase surface area for absorption Figure 3.3 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 3.8a  Plasma Membrane Specializations Membrane junctions Tight junctions Desmosomes Gap junctions Figure 3.3 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 3.8b  Cytoplasm Material outside the nucleus and inside the plasma membrane Cytosol Fluid that suspends other elements Organelles Metabolic machinery of the cell Inclusions Non-functioning units Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 3.9 Cytoplasmic Organelles Figure 3.4  Cytoplasmic Organelles Ribosomes Made of protein and RNA Sites of protein synthesis Found at two locations Free in the cytoplasm Attached to rough endoplasmic reticulum Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 3.11  Cytoplasmic Organelles Endoplasmic reticulum (ER) Fluid-filled tubules for carrying substances Two types of ER Rough Endoplasmic Reticulum Studded with ribosomes Site where building materials of cellular membrane are formed Smooth Endoplasmic Reticulum Functions in cholesterol synthesis and breakdown, fat metabolism, and detoxification of drugs Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 3.12  Cytoplasmic Organelles Golgi apparatus Modifies and packages proteins Produces different types of packages Secretory vesicles Cell membrane components Lysosomes Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 3.13a Cytoplasmic Organelles  Cytoplasmic Organelles Lysosomes Contain enzymes that digest nonusable materials within the cell Peroxisomes Membranous sacs of oxidase enzymes Detoxify harmful substances Break down free radicals (highly reactive chemicals) Replicate by pinching in half Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 3.14  Cytoplasmic Organelles Mitochondria “Powerhouses” of the cell Change shape continuously Carry out reactions where oxygen is used to break down food Provides ATP for cellular energy Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 3.15  Cytoplasmic Organelles Cytoskeleton Network of protein structures that extend throughout the cytoplasm Provides the cell with an internal framework Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 3.16a  Cytoplasmic Organelles Cytoskeleton Three different types Microfilaments Intermediate filaments Microtubules Figure 3.6 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 3.16b  Cytoplasmic Organelles Centrioles Rod-shaped bodies made of microtubules Direct formation of mitotic spindle during cell division Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 3.17  Cellular Projections Not found in all cells Used for movement Cilia moves materials across the cell surface Flagellum propels the cell Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 3.18 Cell Diversity The trillions of cells in the human body include over 200 different cell types that vary greatly in size, shape, and function. They include sphere-shaped fat cells, disc-shaped red blood cells, branching nerve cells, and cube-shaped cells of kidney tubules. A cell’s shape reflects its function. For example, the flat, tile- like epithelial cells that line the inside of your cheek fit closely together, forming a living barrier that protects underlying tissues from bacterial invasion. The shapes of cells and the relative numbers of the various organelles they contain relate to specialized cell functions.  Cell Diversity 1. Cells That Connect The Body Part ▪ Fibroblast - This cell has an elongated shape, like the cable-like fibers that it secretes. It has an abundant rough ER and a large Golgi apparatus to make and secrete the protein building blocks of these fibers. ▪ Erythrocyte (RBC)-This cell carries oxygen in the blood. Its biconcave disc shape provides extra surface area for the uptake of oxygen and streamlines the cell so it flows easily through the bloodstream. 2. Cells that cover and line body organs ▪ Epithelial cell - The hexagonal shape of this cell is exactly like a “cell” in a honeycomb of a beehive. This shape allows epithelial cells to pack together in sheets. An epithelial cell has abundant intermediate filaments and desmosomes that resist tearing when the epithelium is rubbed or pulled.  Cell Diversity 3. Cells That Move The Organs And Body Parts ▪ Skeletal, cardiac, and smooth muscle cells - These cells are elongated and filled with abundant contractile filaments, so they can shorten forcefully and move the bones, pump blood, or change the size of internal organs to move substances around the body.  Cell Diversity 4. Cell That Stores Nutrients ▪ Fat cell - The huge spherical shape of a fat cell is produced by a large lipid droplet in its cytoplasm. 5. Cell That Fights Disease ▪ White Blood Cells such as the macrophage (a phagocytic cell). This cell extends long pseudopods (“false feet”) to crawl through tissue to reach infection sites. The many lysosomes within the cell digest the infectious microorganisms (such as bacteria) that it “eats.”  Cell Diversity 6. Cell that gathers information and controls body functions ▪ Nerve cell (neuron). This cell has long processes (extensions) for receiving messages and transmitting them to other structures in the body. The processes are covered with an extensive plasma membrane, and a plentiful rough ER synthesizes membrane components and signaling molecules called neurotransmitters. 7. Cells of reproduction ▪ Oocyte (female) - The largest cell in the body, this egg cell contains several copies of all organelles, for distribution to the daughter cells that arise when the fertilized egg divides to become an embryo. ▪ Sperm (male) - This cell is long and streamlined, built for swimming to the egg for fertilization. Its flagellum acts as a motile whip to propel the sperm.  Cellular Physiology: Membrane Transport ❖ Each of the cell’s internal parts is designed to perform a specific function for the cell. Most cells have the ability to metabolize (use nutrients to build new cell material, break down substances, and make ATP), digest foods, dispose of wastes, reproduce, grow, move, and respond to a stimulus (irritability). Membrane Transport – movement of substance into and out of the cell Transport is by two basic methods Passive transport No energy is required Active transport The cell must provide metabolic energy  Solutions and Transport Solution – homogeneous mixture of two or more components. The fluid environment on both sides of the plasma membrane is an example of a solution. Examples include the air we breathe (a mixture of gases), seawater (a mixture of water and salts), and rubbing alcohol (a mixture of water and alcohol). Solvent – dissolving medium. Water is the body’s chief solvent. Solutes – components in smaller quantities within a solution. So tiny that the molecules cannot be seen with the naked eye and do not settle out. Intracellular fluid – nucleoplasm and cytosol. Is a solution containing small amounts of gases (oxygen and carbon dioxide), nutrients, and salts, dissolved in water. Interstitial fluid – fluid on the exterior of the cell, ( “soup”). It contains thousands of ingredients, including nutrients (amino acids, sugars, fatty acids, vitamins), regulatory substances such as hormones and neurotransmitters, salts, and waste products. - To remain healthy, each cell must extract from this soup the exact amounts of the substances it needs at specific times and reject the rest.  Selective Permeability The plasma membrane allows some materials to pass while excluding others This permeability includes movement into and out of the cell Means that a barrier allows some substances to pass through it while excluding others. Thus, it allows nutrients to enter the cell but keeps many undesirable or unnecessary substances out. At the same time, valuable cell proteins and other substances are kept within the cell, and wastes are allowed to pass out of it. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 3.22 Homeostatic Imbalance ▪ The property of selective permeability is typical only of healthy, unharmed cells. When a cell dies or is badly damaged, its plasma membrane can no longer be selective and becomes permeable to nearly everything. ▪ We see this problem when someone has been severely burned. Precious fluids, proteins, and ions “weep” (leak out) from the dead and damaged cells at the burn site. ▪ Substances move through the plasma membrane in basically two ways—passively or actively. In passive processes, substances are transported across the membrane without any energy input from the cell. In active processes, the cell provides the metabolic energy (ATP) that drives the transport process.  Passive Transport Processes Diffusion Particles tend to distribute themselves evenly within a solution. Movement is from high concentration to low concentration, or down a concentration gradient. Types of diffusion a. Simple diffusion Unassisted process Solutes are lipid-soluble materials or small enough to pass through membrane pores ▪ Osmosis – simple diffusion of water Highly polar water easily crosses the plasma membrane b. Facilitated diffusion Substances require a protein carrier for passive transport Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 3.24b  Passive Transport Processes Filtration Water and solutes are forced through a membrane by fluid, or hydrostatic pressure A pressure gradient must exist Solute-containing fluid is pushed from a high pressure area to a lower pressure area  Active Transport Processes Whenever a cell uses ATP to move substances across the membrane, the process is active. Transport substances that are unable to pass by diffusion They may be too large They may not be able to dissolve in the fat core of the membrane They may have to move against a concentration gradient They may have to move “uphill” against their concentration gradients Two common forms of active transport Solute pumping Bulk transport This is opposite to the direction in which substances would naturally flow by diffusion, which explains the need for energy in the form of ATP.  Active Transport Processes Solute pumping Amino acids, some sugars and ions are transported by solute pumps and in most cases these substances move against concentration (or electrical) gradients. This is opposite to the direction in which substances would naturally flow by diffusion, which explains the need for energy in the form of ATP. ATP energizes protein carriers, and in most cases, moves substances against concentration gradients.  Active Transport Processes Operation of the sodium-potassium pump, a solute pump ▪ This process is absolutely necessary for normal transmission of nerve impulses. ▪ There are more sodium ions outside the cells than inside, so those inside tend to remain in the cell unless the cell uses ATP to force, or “pump,” them out. ATP is split into ADP and Pi (inorganic phosphate), and the phosphate is then attached to the sodium-potassium pump in a process called phosphorylation. ▪ Likewise, there are more potassium ions inside cells than in the extracellular fluid, and potassium ions that leak out of cells must be actively pumped back inside. Because each of the pumps in the plasma membrane transports only specific substances, active transport provides a way for the cell to be very selective in cases where substances cannot pass by ❖ ATP provides the energy for a “pump” protein diffusion. (No pump—no transport.) to move three sodium ions out of the cell and two potassium ions into the cell.  Active Transport Processes Bulk transport/Vesicular Transport Some substances cannot get through the plasma membrane by active or passive transport. Vesicular transport, which involves help from ATP to fuse or separate membrane vesicles and the cell membrane, moves substances into or out of cells “in bulk” without their actually crossing the plasma membrane directly. Two Types: 1. Exocytosis - is the mechanism that cells use to actively secrete hormones, mucus, and other cell products or to eject certain cellular wastes. Moves materials out of the cell Material is carried in a membranous vesicle Vesicle migrates to plasma membrane Vesicle combines with plasma membrane Material is emptied to the outside Active Transport Processes  Active Transport Processes 2. Endocytosis Extracellular substances are engulfed by being enclosed in a membranous vesicle. Once the vesicle is formed, it detaches from the plasma membrane and moves into the cytoplasm, where it typically fuses with a lysosome and its contents are digested (by lysosomal enzymes). Types of endocytosis Phagocytosis – cell eating. If the engulfed substances are relatively large particles, such as bacteria or dead body cells, and the cell separates them from the external environment by pseudopods. Pinocytosis – cell drinking. The cell “gulps” droplets of extracellular fluid. The plasma membrane indents to form a tiny pit, or “cup,” and then its edges fuse around the droplet of extracellular fluid containing dissolved proteins or fats.  Active Transport Processes ▪ Sequence of events in endocytosis. A vesicle forms by forming a pit in the membrane 1) Once the vesicle detaches from the plasma membrane, its contents may be digested within a lysosome 2) - Then released to the cytosol. Alternatively, the vesicle may be transported across the cell intact and then released to the cell exterior by exocytosis - If present, its membrane components and receptors are recycled to the plasma membrane.  Cell Life Cycle Is the series of changes a cell goes through from the time it is formed until it divides. Cells have two major periods 1) Interphase - Cell grows - Cell carries on it’s usual metabolic processes - the longer phase of the cell cycle, the cell is very active and is preparing for cell division. A more accurate name for interphase would be metabolic phase. 2) Cell division - Cell replicates itself - Function is to produce more cells for growth and repair processes  DNA Replication Genetic material duplicated and readies a cell for division into two cells Occurs toward the end of interphase DNA uncoils and each side serves as a template Figure 3.13 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 3.32  Events of Cell Division Mitosis Division of the nucleus Results in the formation of two daughter nuclei Cytokinesis Division of the cytoplasm Begins when mitosis is near completion Results in the formation of two daughter cells Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 3.33  Stages of Mitosis 1. Interphase ▪ No cell division occurs ▪ The cell carries out normal metabolic activity and growth 2. Prophase ▪ First part of cell division ▪ Centromeres migrate to the poles 3. Metaphase ▪ Spindle from centromeres are attached to chromosomes that are aligned in the center of the cell  Stages of Mitosis 4. Anaphase ▪ Daughter chromosomes are pulled toward the poles ▪ The cell begins to elongate 5. Telophase ▪ Daughter nuclei begin forming ▪ A cleavage furrow (for cell division) begins to form Stages of Mitosis  Protein Synthesis Proteins are key substances for all aspects of cell life. Proteins have many functions ▪ Building materials for cells - Fibrous (structural) proteins ▪ Act as enzymes (biological catalysts) - speed up every chemical reaction that occurs in cells, are functional proteins. It follows, then, that every cell needs to produce proteins, a process called protein synthesis. ▪ This is accomplished with the DNA blueprints known as genes and with the help of the nucleic acid RNA. ▪ RNA is essential for protein synthesis.  Genes: The Blueprint for Protein Structure In addition to replicating itself for cell division, DNA serves as the master blueprint for protein synthesis. Gene – A DNA segment that carries a blueprint for building one protein. DNA’s information is encoded in the sequence of bases. Each sequence of three bases (a triplet) calls for a particular amino acid. (Amino acids are the building blocks of proteins and are joined during protein synthesis.  Role of RNA By itself, DNA is rather like a coded message; its information is not useful until it is decoded. Most ribosomes—the manufacturing sites for proteins—are in the cytoplasm, but DNA never leaves the nucleus during interphase. Thus, DNA requires not only a decoder but also a trusted messenger to carry the instructions for building proteins to the ribosomes. These messenger and decoder functions are carried out by a second type of nucleic acid, called ribonucleic acid, or RNA. RNA differs from DNA in being single-stranded. Three varieties of RNA play a special role in protein synthesis: a. Ribosomal RNA (rRNA) helps form the ribosomes, where proteins are built. b. Messenger RNA (mRNA) molecules are long, single nucleotide strands that resemble half of a DNA molecule. They carry the “message” containing instructions for protein synthesis from the DNA (gene) in the nucleus to the ribosomes in the cytoplasm. c. Transfer RNA (tRNA) molecules are small, cloverleaf-shaped molecules that  The Process of Protein Synthesis Protein synthesis involves two major phases: 1. Transcription ▪ Transfer of information from DNA’s base sequence to the complimentary base sequence of mRNA 2. Translation ▪ Base sequence of nucleic acid is translated to an amino acid sequence ▪ Amino acids are the building blocks of proteins Protein Synthesis  PART II. Body Tissues Cells are specialized for particular functions Tissues Groups of cells with similar structure and function Four primary types 1. Epithelium 2. Connective tissue 3. Nervous tissue 4. Muscle  Epithelial Tissues Found in different areas Functions ▪ Body coverings ▪ Protection ▪ Body linings ▪ Absorption ▪ Filtration ▪ Glandular tissue ▪ Secretion Epithelium Characteristics Cells fit closely together Tissue layer always has one free surface The lower surface is bound by a basement membrane Avascular (have no blood supply) Regenerate easily if well nourished  Classification of Epithelium ❑ Number of cell layers Simple – one layer Stratified – more than one layer ❑ Shape of cells Squamous – flattened Cuboidal – cube-shaped Columnar – column-like  Simple Epithelium ▪ Simple squamous Single layer of flat cells Usually forms membranes o Lines body cavities o Lines lungs and capillaries ▪ Simple cuboidal Single layer of cube-like cells Common in glands and their ducts Forms walls of kidney tubules Simple Epithelium Simple columnar Single layer of tall cells Often includes goblet cells, which produce mucus Lines digestive tract  Simple Epithelium ▪ Pseudostratified Single layer, but some cells are shorter than others Often looks like a double cell layer Sometimes ciliated, such as in the respiratory tract May function in absorption or secretion ▪ Stratified squamous ▪ Cells at the free edge are flattened ▪ Found as a protective covering where friction is common ▪ Locations ▪ Skin ▪ Mouth ▪ Esophagus  Stratified Epithelium Stratified cuboidal Two layers of cuboidal cells Stratified columnar Surface cells are columnar, cells underneath vary in size and shape Stratified cuboidal and columnar Rare in human body Found mainly in ducts of large glands Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 3.50  Stratified Epithelium ▪ Transitional epithelium Shape of cells depends upon the amount of stretching Lines organs of the urinary system Glandular Epithelium ▪ Gland – one or more cells that secretes a particular product ▪Two major gland types ▪ Secretion - typically contains protein Endocrine gland molecules in an aqueous (water- o Ductless based) fluid. The term secretion also indicates an active process in which o Secretions are hormones the glandular cells obtain needed Exocrine gland materials from the blood and use them to make their products, which they then o Empty through ducts to the discharge by exocytosis. epithelial surface o Include sweat and oil glands  Connective Tissue ▪ Found everywhere in the body ▪ Includes the most abundant and widely distributed tissues ▪ Functions Binds body tissues together Supports the body Provides protection Connective Tissue Characteristics ▪ Variations in blood supply Some tissue types are well vascularized Some have poor blood supply or are avascular ▪ Extracellular matrix Non-living material that surrounds living cells  Extracellular Matrix ▪ Two main elements Connective Tissue Types Ground substance – ▪ Bone (osseous tissue) mostly water along with Composed of: adhesion proteins and polysaccharide o Bone cells in molecules lacunae (cavities) Fibers o Hard matrix of calcium salts Produced by the cells Three types o Large numbers of collagen fibers Collagen fibers Used to protect and Elastic fibers support the body Reticular fibers  Connective Tissue Types a. Hyaline cartilage Most common cartilage Composed of: o Abundant collagen fibers o Rubbery matrix Entire fetal skeleton is hyaline cartilage b. Elastic cartilage Provides elasticity Example: supports the external ear  Connective Tissue Types ▪ Fibrocartilage Highly compressible Example: forms cushion-like discs between vertebrae  Connective Tissue Types ▪ Dense connective tissue Main matrix element is collagen fibers Cells are fibroblasts Examples o Tendon – attach muscle to bone o Ligaments – attach bone to bone  Connective Tissue Types ▪ Areolar connective tissue Most widely distributed connective tissue Soft, pliable tissue Contains all fiber types Can soak up excess fluid ▪ Adipose tissue Matrix is an areolar tissue in which fat globules predominate Many cells contain large lipid deposits Functions o Insulates the body o Protects some organs Slide 3.60 o Serves as a site of fuel storage  Connective Tissue Types ▪ Reticular connective tissue Delicate network of interwoven fibers Forms stroma (internal supporting network) of lymphoid organs o Lymph nodes o Spleen o Bone marrow ▪ Blood Blood cells surrounded by fluid matrix Fibers are visible during clotting Functions as the transport vehicle for materials Slide 3.62  Muscle Tissue ▪ Function is to produce movement ▪ Three types 1) Skeletal muscle 2) Cardiac muscle 3) Smooth muscle 1) Skeletal muscle o Can be controlled voluntarily o Cells attach to connective tissue o Cells are striated o Cells have more than one nucleus Slide 3.64  Muscle Tissue Types 2. Cardiac muscle Found only in the heart Function is to pump blood (involuntary) Cells attached to other cardiac muscle cells at intercalated disks Cells are striated One nucleus per cell 3. Smooth muscle Involuntary muscle Surrounds hollow organs Attached to other smooth muscle cells Figure 3.19c No visible striations Slide 3.66 One nucleus per cell  Nervous Tissue ▪ Neurons and nerve support cells ▪ Function is to send impulses to other areas of the body Irritability Conductivity Tissue Repair (Wound Healing) ❖ The body has many techniques for protecting ❖ itself from uninvited guests or injury. ❖ Intact physical barriers such as the skin and mucous membranes, cilia, and the strong acid produced by stomach glands are just three examples of body defenses exerted at the tissue level. ❖ When tissue injury does occur, it stimulates the body’s inflammatory and immune responses, and the healing process begins almost immediately. ❖ Inflammation is a general (nonspecific) body response that attempts to prevent further injury. ❖ The immune response, in contrast, is extremely specific and mounts a vigorous attack against recognized invaders, including bacteria, viruses, and toxins.  Tissue Repair Tissue repair, or wound healing, occurs in two major ways: by regeneration and by fibrosis. Regeneration Replacement of destroyed tissue by the same kind of cells Fibrosis Repair by dense fibrous connective tissue, that is, by the formation of scar tissue. Determination of method Type of tissue damaged Severity of the injury  Clean cuts (incisions) heal much more successfully than ragged tears of the tissue. Tissue Injury Sets The Following Series Of Events Into Motion: Inflammation sets the stage ❑ Injured tissue cells and others release inflammatory chemicals that make the capillaries very permeable. ❑ This allows fluid rich in clotting proteins and other substances to seep into the injured area from the bloodstream ❑ The leaked clotting proteins construct a clot, which “plugs the hole” to stop blood loss and hold the edges of the wound together. The injured area becomes walled off, preventing bacteria or other harmful substances from spreading to surrounding tissues. Where the clot is exposed to air, it quickly dries and hardens, forming a scab.  Granulation tissue forms ❑ Granulation tissue is delicate pink tissue composed largely of new capillaries that grow into the damaged area from undamaged blood vessels nearby. ❑ These capillaries are fragile and bleed freely, as when a scab is picked away from a skin wound. ❑ Granulation tissue also contains phagocytes, which eventually dispose of the blood clot, and connective tissue cells (fibroblasts), which produce the building blocks of collagen fibers (scar tissue) to permanently bridge the gap. Regeneration and fibrosis effect permanent repair ❑ As the surface epithelium begins to regenerate, it makes its way between the granulation tissue and the scab.  ❑ The ability of the different tissue types to regenerate varies widely. ❑ Epithelial tissues such as the skin epidermis and mucous membranes regenerate beautifully. So, too, do most of the fibrous connective tissues and bone. ❑ Skeletal muscle regenerates poorly, and cardiac muscle and nervous tissue within the brain and spinal cord are replaced largely by scar tissue. Homeostatic Imbalance Scar tissue is strong, but it lacks the flexibility of most normal tissues. Perhaps even more important is its inability to perform the normal functions of the tissue it replaces. Thus, if scar tissue forms in the wall of the bladder, heart, or another muscular organ, it may severely hamper the functioning of that organ.  A contracture is a permanent tightening of the skin affecting the underlying tendons or muscles. Contractures develop during the healing process as inelastic fibrous tissue replaces the normal elastic connective tissues. Because fibrous tissue resists stretching, movement of the affected area may be limited.  Developmental Aspects of Tissue Very early in embryonic development, cells begin to specialize to form the primary tissues, and by birth, most organs are well formed and functioning. The body continues to grow and enlarge by forming new tissue throughout childhood and adolescence. Cell division is extremely important during the body’s growth period. Most cells (except neurons and mature red blood cells) undergo mitosis until the end of puberty, when adult body size is reached and overall body growth ends. After this time, only certain cells routinely divide (are mitotic)— for example, cells exposed to abrasion that continually wear away, such as skin and intestinal cells.  Developmental Aspects of Tissue Liver cells stop dividing, but they retain this ability should some of them die or become damaged and need to be replaced. Still other cell groups (for example, heart muscle and nervous tissue) almost completely lose their ability to divide when they are fully mature; that is, they become amitotic. Amitotic tissues are severely handicapped by injury because the lost cells cannot be replaced by the same type of cells. This is why the heart of an individual who has had several severe heart attacks becomes weaker and weaker. Damaged cardiac muscle does not regenerate and is replaced by scar tissue that cannot contract, so the heart becomes less and less capable of acting as an efficient blood pump.  Developmental Aspects of Tissue The aging process begins once maturity has been reached. (Some think it begins at birth.) No one has been able to explain just what causes aging, but there have been many suggestions. Some think it is a result of little “chemical insults” that occur continually through life—for example, the presence of toxic chemicals (such as alcohol, certain drugs, or carbon monoxide) in the blood, or the temporary absence of needed substances such as glucose or oxygen. There is no question that certain events are part of the aging process. For example, with age, epithelial membranes thin and are more easily damaged, and the skin loses its elasticity and begins to sag. The exocrine glands of the body (epithelial tissue) become less active, and we begin to “dry out” as less oil, mucus, and sweat are produced.  Developmental Aspects of Tissue Some endocrine glands produce decreasing amounts of hormones, and the body processes that they control (such as metabolism and reproduction) slow down or stop altogether. Connective tissue structures also show changes with age. Bones become porous and weaken, and tissue repair slows. Muscles begin to waste away. Although a poor diet may contribute to some of these changes, there is little doubt that decreased efficiency of the circulatory system, which reduces nutrient and oxygen delivery to body tissues, is a major factor. Other modifications of cells and tissues may occur at any time. For example, when cells fail to honor normal controls on cell division and multiply wildly, an abnormal mass of proliferating cells, known as a neoplasm (ne′o-plazm″; “new growth”), results. Neoplasms may be benign or malignant.  Developmental Aspects of Tissue Not all increases in cell number involve neoplasms. Certain body tissues (or organs) may enlarge because there is some local irritant or condition that stimulates the cells. This response is called hyperplasia (hi″per-pla′ze-ah). For example, a woman’s breasts enlarge during pregnancy in response to increased hormones; this is a normal but temporary situation that doesn’t have to be treated. In contrast, atrophy (at′ro-fe), or decrease in size, can occur in an organ or body area that loses its normal stimulation. For example, the muscles of a broken leg atrophy while in a cast during the healing period.