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CHAPTER 4 TISSUES Tissue: The Living Fabric • Individual body cells specialized – Each type performs specific functions that maintain homeostasis • Tissues – Groups of cells similar in structure that perform common or related function • Histology – Study of tissues Types of Primary Tissues • Epithelial tissue – Covers • Connective tissue – Supports • Muscle tissue – Produces movement • Nerve tissue – Controls Figure 4.1 Overview of four basic tissue types: epithelial, connective, muscle, and nervous tissues. Nervous tissue: Internal communication • Brain • Spinal cord • Nerves Muscle tissue: Contracts to cause movement • Muscles attached to bones (skeletal) • Muscles of heart (cardiac) • Muscles of walls of hollow organs (smooth) Epithelial tissue: Forms boundaries between different environments, protects, secretes, absorbs, filters • Lining of digestive tract organs and other hollow organs • Skin surface (epidermis) Connective tissue: Supports, protects, binds other tissues together • Bones • Tendons • Fat and other soft padding tissue Studying Human Tissue: Microscopy • Tissue is fixed – Preserved • Cut – Sliced thin enough to transmit light or electrons • Stained – Enhances contrast Epithelial Tissue (Epithelium) • Form boundaries • Two main types (by location) – Covering and lining epithelia • On external and internal surfaces – Glandular epithelia • Secretory tissue in glands Epithelial Tissue Functions • Protection: internal from external environment • Absorption: digestive system/skin • Filtration: capillaries • Excretion: skin • Secretion: skin/digestion • Sensory reception: external & internal Five Characteristics of Epithelial Tissues • • • • • Polarity Specialized contacts Supported by connective tissues Avascular, but innervated Can regenerate Characteristics of Epithelial Tissue: Polarity • Cells have polarity – Apical surface (upper free) exposed to exterior or cavity – Basal surface (lower, attached) • Both surfaces differ in structure and function Apical Surface of Epithelial Tissues • May be smooth & slick • Most have microvilli (e.g., brush border of intestinal lining) – Increase surface area • Some have cilia (e.g., lining of trachea) Basal Surface of Epithelial Tissues • Noncellular basal lamina – Glycoprotein and collagen fibers lies adjacent to basal surface – Adhesive sheet – Selective filter – Scaffolding for cell migration in wound repair Characteristics of Epithelial Tissue: Connective Tissue Support • Reticular lamina – Deep to basal lamina – Network of collagen fibers • Basement membrane – Basal lamina + reticular lamina – Reinforces epithelial sheet – Resists stretching and tearing – Defines epithelial boundary Characteristics of Epithelial Tissue: Specialized Contacts • Covering and lining epithelial tissues fit closely together – Form continuous sheets • Specialized contacts bind adjacent cells – Lateral contacts • Tight junctions • Desmosomes • Gap junctions Characteristics of Epithelial Tissue: Avascular but Innervated • No blood vessels in epithelial tissue – Must be nourished by diffusion from underlying connective tissues • Is supplied by nerve fibers Characteristics of Epithelial Tissue: Regeneration • High regenerative capacity • Stimulated by loss of apical-basal polarity and lateral contacts – Some exposed to friction – Some exposed to hostile substances • If adequate nutrients are present, can replace lost cells by cell division Classification of Epithelia • All epithelial tissues have two names – One indicates number of cell layers • Simple epithelia = single layer of cells • Stratified epithelia = two or more layers of cells – Shape can change in different layers – One indicates shape of cells • Squamous • Cuboidal • Columnar • In stratified epithelia, epithelia classified by cell shape in apical layer Figure 4.2a Classification of epithelia. Apical surface Basal surface Simple Apical surface Basal surface Stratified Classification based on number of cell layers. Cells of Epithelial Tissues • Squamous cells – Flattened and scalelike – Nucleus flattened (ovoid) • Cuboidal cells Squamous Cuboidal – Boxlike – Nucleus round • Columnar cells – Tall; column shaped – Nucleus elongated (ovoid) Columnar Classification of Epithelia: Simple Epithelia • Very thin • Functions: – Absorption – Secretion – Filtration Simple Squamous Epithelium • Cells flattened laterally • Cytoplasm sparse • Function where rapid diffusion is priority – i.e., kidney, lungs • Note description, function, location on next slide – Endothelium: lining of lymphatic vessels, blood vessels, and heart – Mesothelium: epithelium of serous membranes in the ventral body cavity Figure 4.3a Epithelial tissues. Simple squamous epithelium Description: Single layer of flattened cells with disc-shaped central nuclei and sparse cytoplasm; the simplest of the epithelia. Air sacs of lung tissue Nuclei of squamous epithelial cells Function: Allows materials to pass by diffusion and filtration in sites where protection is not important; secretes lubricating substances in serosae. Location: Kidney glomeruli; air sacs of lungs; lining of heart, blood vessels, and lymphatic vessels; lining of ventral body cavity (serosae). Photomicrograph: Simple squamous epithelium forming part of the alveolar (air sac) walls (140x). Simple Cuboidal Epithelia • • • • Single layer of cells Secretion Absorption Forms walls of smallest ducts of glands and many kidney tubules • Note description, function, location on next slide Figure 4.3b Epithelial tissues. Simple cuboidal epithelium Description: Single layer of cubelike cells with large, spherical central nuclei. Simple cuboidal epithelial cells Nucleus Function: Secretion and absorption. Basement membrane Location: Kidney tubules; ducts and secretory portions of small glands; ovary surface. Connective tissue Photomicrograph: Simple cuboidal epithelium in kidney tubules (430x). Simple Columnar Epithelium • • • • Single layer of tall, closely packed cells Absorption Secretion Note description, function, location on next slide Figure 4.3c Epithelial tissues. Simple columnar epithelium Description: Single layer of tall cells with round to oval nuclei; some cells bear cilia; layer may contain mucus-secreting unicellular glands (goblet cells). Microvilli Simple columnar epithelial cell Function: Absorption; secretion of mucus, enzymes, and other substances; ciliated type propels mucus (or reproductive cells) by ciliary action. Location: Nonciliated type lines most of the digestive tract (stomach to rectum), gallbladder, and excretory ducts of some glands; ciliated variety lines small bronchi, uterine tubes, and some regions of the uterus. Mucus of goblet cell Basement membrane Photomicrograph: Simple columnar epithelium of the small intestine mucosa (660x). Pseudostratified Columnar Epithelium • Cells vary in height – Cell nuclei at different levels, so appears stratified, but is not – Secretion – Absorption – Note description, function, location on next slide Figure 4.3d Epithelial tissues. Pseudostratified columnar epithelium Description: Single layer of cells of differing heights, some not reaching the free surface; nuclei seen at different levels; may contain mucus-secreting cells and bear cilia. Cilia Pseudostratified epithelial layer Function: Secrete substances, particularly mucus; propulsion of mucus by ciliary action. Location: Nonciliated type in male’s sperm-carrying ducts and ducts of large glands; ciliated variety lines the trachea, most of the upper respiratory tract. Trachea Photomicrograph: Pseudostratified ciliated columnar epithelium lining the human trachea (800x). Basement membrane Stratified Epithelial Tissues • Two or more cell layers • Regenerate from below – Basal cells divide, cells migrate to surface • More durable than simple epithelia • Protection is major role • Always named for the type at apical surface Stratified Squamous Epithelium • Most widespread of stratified epithelia • Free surface squamous; deeper layers cuboidal or columnar • Located for wear and tear • Those farthest from basal layer (and therefore nutrients) less viable • Note description, function, location on next slide Figure 4.3e Epithelial tissues. Stratified squamous epithelium Description: Thick membrane composed of several cell layers; basal cells are cuboidal or columnar and metabolically active; surface cells are flattened (squamous); in the keratinized type, the surface cells are full of keratin and dead; basal cells are active in mitosis and produce the cells of the more superficial layers. Stratified squamous epithelium Function: Protects underlying tissues in areas subjected to abrasion. Location: Nonkeratinized type forms the moist linings of the esophagus, mouth, and vagina; keratinized variety forms the epidermis of the skin, a dry membrane. Nuclei Basement membrane Connective tissue Photomicrograph: Stratified squamous epithelium lining the esophagus (285x). Stratified Cuboidal Epithelium • Quite rare • Found in some sweat and mammary glands • Typically two cell layers thick Stratified Columnar Epithelium • Limited distribution in body • Small amounts in pharynx, male urethra, and lining some glandular ducts • Also occurs at transition areas between two other types of epithelia • Only apical layer is columnar Transitional Epithelium • • • • • Forms lining of hollow urinary organs Basal layer cells are cuboidal or columnar Ability to change shape with stretch Apical cells vary in appearance Note description, function, location on next slide Figure 4.3f Epithelial tissues. Transitional epithelium Description: Resembles both stratified squamous and stratified cuboidal; basal cells cuboidal or columnar; surface cells dome shaped or squamouslike, depending on degree of organ stretch. Transitional epithelium Function: Stretches readily, permits stored urine to distend urinary organ. Location: Lines the ureters, bladder, and part of the urethra. Photomicrograph: Transitional epithelium lining the bladder, relaxed state (360x); note the bulbous, or rounded, appearance of the cells at the surface; these cells flatten and elongate when the bladder fills with urine. Basement membrane Connective tissue Glandular Epithelia • Gland – One or more cells that makes and secretes an aqueous fluid called a secretion • Classified by – Site of product release: endocrine or exocrine – Relative number of cells forming the gland • Unicellular (e.g., goblet cells) or multicellular Endocrine Glands • Ductless glands – Secretions not released into a duct • Secrete (by exocytosis) hormones that travel through lymph or blood to their specific target organs • Target organs respond in some characteristic way Exocrine Glands • Secretions released onto body surfaces (skin) or into body cavities • More numerous than endocrine glands • Secrete products into ducts • Examples include mucous, sweat, oil, and salivary glands Unicellular Exocrine Glands • The only important unicellular glands are mucous cells and goblet cells • Found in epithelial linings of intestinal and respiratory tracts • All produce mucin – Dissolves in water to form mucus • Slimy protective, lubricating coating Figure 4.4 Goblet cell (unicellular exocrine gland). Microvilli Secretory vesicles containing mucin Golgi apparatus Rough ER Nucleus Multicellular Exocrine Glands • Multicellular exocrine glands are composed of a duct and a secretory unit • Usually surrounded by supportive connective tissue – Supplies blood and nerve fibers – Extends into and divides gland into lobes Classification of Multicellular Glands • By structure and type of secretion – Structure • Simple glands (unbranched duct) or compound glands (branched duct) – Type of secretion • Merocrine: most abundant – secrete products by exocytosis as produced • Holocrine: accumulate products within then rupture • Apocrine: accumulates products within but only apex ruptures – controversy if exist in humans Connective Tissue • Most abundant and widely distributed of primary tissues • Four main classes – Connective tissue proper – Cartilage – Bone – Blood Major Functions of Connective Tissue • • • • • Binding and support Protecting Insulating Storing reserve fuel Transporting substances (blood) Characteristics of Connective Tissue • Have varying degrees of vascularity • Have extracellular matrix – Connective tissue not composed mainly of cells – Largely nonliving extracellular matrix separates cells • Can bear weight • Withstand tension • Endure abuse Structural Elements of Connective Tissue • Three elements – Ground substance – Fibers – Cells • Composition and arrangement varies in different connective tissues Ground Substance • Unstructured material that fills space between cells – Substance through which solutes diffuse between blood capillaries and cells • Components – Interstitial fluid – Cell adhesion proteins ("glue" for attachment) – Proteoglycans • Protein core + large polysaccharides • Trap water in varying amounts, affecting viscosity of ground substance Connective Tissue Fibers • Three types of fibers provide support – Collagen • Strongest and most abundant type • Tough; provides high tensile strength – Elastic fibers • Networks of long, thin, elastin fibers that allow for stretch and recoil – Reticular • Short, fine, highly branched collagenous fibers (different chemistry and form than collagen fibers) • Branch, forming networks that offer more "give" Cells • "Blasts" cells – Immature form; mitotically active; secrete ground substance and fibers – Fibroblasts in connective tissue proper – Chondroblasts in cartilage – Osteoblasts in bone – Hematopoietic stem cells in bone marrow • "Cyte" cells – Mature form; maintain matrix – Chondrocytes in cartilage – Osteocytes in bone Other Cell Types in Connective Tissues • Fat cells – Store nutrients • White blood cells – Neutrophils, eosinophils, lymphocytes – Tissue response to injury/ infection • Mast cells – Initiate local inflammatory response against foreign microorganisms they detect • Macrophages – Phagocytic cells that "eat" dead cells, microorganisms; function in immune system Types of Connective Tissues: Connective Tissue Proper • All connective tissues except bone, cartilage and blood • Two subclasses – Loose connective tissues • Areolar • Adipose • Reticular – Dense connective tissues (also called fibrous connective tissues) • Dense regular • Dense irregular • Elastic Areolar Connective Tissue • Support and bind other tissues – Universal packing material between other tissues • • • • • • • • Most widely distributed Provide reservoir of water and salts Defend against infection Store nutrients as fat Fibroblasts Loose arrangement of fibers Ground substance When inflamed, soaks up fluid edema Figure 4.8a Connective tissues. Connective tissue proper: loose connective tissue, areolar Description: Gel-like matrix with all three fiber types; cells: fibroblasts, macrophages, mast cells, and some white blood cells. Function: Wraps and cushions organs; its macrophages phagocytize bacteria; plays important role in inflammation; holds and conveys tissue fluid. Location: Widely distributed under epithelia of body, e.g., forms lamina propria of mucous membranes; packages organs; surrounds capillaries. Elastic fibers Ground substance Fibroblast nuclei Collagen fibers Epithelium Lamina propria Photomicrograph: Areolar connective tissue, a soft packaging tissue of the body (340x). Adipose Tissue • White fat – Similar to areolar but greater nutrient storage – Cell is adipocyte • Stores nutrients – Not much matrix – Richly vascularized – Shock absorption, insulation, energy storage • Brown fat – Use lipid fuels to heat bloodstream, not to produce ATP Figure 4.8b Connective tissues. Connective tissue proper: loose connective tissue, adipose Description: Matrix as in areolar, but very sparse; closely packed adipocytes, or fat cells, have nucleus pushed to the side by large fat droplet. Function: Provides reserve food fuel; insulates against heat loss; supports and protects organs. Nucleus of adipose (fat) cell Location: Under skin in subcutaneous tissue; around kidneys and eyeballs; within abdomen; in breasts. Adipose tissue Fat droplet Photomicrograph: Adipose tissue from the subcutaneous layer under the skin (350x). Mammary glands Reticular Connective Tissue • Resembles areolar but fibers are reticular fibers • Fibroblasts called reticular cells • Supports free blood cells in lymph nodes, the spleen, and bone marrow Figure 4.8c Connective tissues. Connective tissue proper: loose connective tissue, reticular Description: Network of reticular fibers in a typical loose ground substance; reticular cells lie on the network. Function: Fibers form a soft internal skeleton (stroma) that supports other cell types including white blood cells, mast cells, and macrophages. White blood cell (lymphocyte) Location: Lymphoid organs (lymph nodes, bone marrow, and spleen). Spleen Reticular fibers Photomicrograph: Dark-staining network of reticular connective tissue fibers forming the internal skeleton of the spleen (350x). Dense Regular Connective Tissue • Closely packed bundles of collagen fibers running parallel to direction of pull – White structures with great resistance to pulling – Fibers slightly wavy so stretch a little • Fibroblasts manufacture fibers and ground substance • Few cells • Poorly vascularized Figure 4.8d Connective tissues. Connective tissue proper: dense connective tissue, dense regular Description: Primarily parallel collagen fibers; a few elastic fibers; major cell type is the fibroblast. Function: Attaches muscles to bones or to muscles; attaches bones to bones; withstands great tensile stress when pulling force is applied in one direction. Collagen fibers Location: Tendons, most ligaments, aponeuroses. Nuclei of fibroblasts Shoulder joint Ligament Tendon Photomicrograph: Dense regular connective tissue from a tendon (430x). Dense Irregular Connective Tissue • Same elements but bundles of collagen thicker and irregularly arranged • Resists tension from many directions – Dermis – Fibrous joint capsules – Fibrous coverings of some organs Figure 4.8e Connective tissues. Connective tissue proper: dense connective tissue, dense irregular Description: Primarily irregularly arranged collagen fibers; some elastic fibers; fibroblast is the major cell type. Nuclei of fibroblasts Function: Withstands tension exerted in many directions; provides structural strength. Location: Fibrous capsules of organs and of joints; dermis of the skin; submucosa of digestive tract. Collagen fibers Shoulder joint Fibrous joint capsule Photomicrograph: Dense irregular connective tissue from the fibrous capsule of a joint (430x). Elastic Connective Tissue • Some ligaments very elastic – Those connecting adjacent vertebrae • Contained in walls of larger arteries Figure 4.8f Connective tissues. Connective tissue proper: dense connective tissue, elastic Description: Dense regular connective tissue containing a high proportion of elastic fibers. Function: Allows tissue to recoil after stretching; maintains pulsatile flow of blood through arteries; aids passive recoil of lungs following inspiration. Location: Walls of large arteries; within certain ligaments associated with the vertebral column; within the walls of the bronchial tubes. Elastic fibers Aorta Heart Photomicrograph: Elastic connective tissue in the wall of the aorta (250x). Cartilage • • • • Chondroblasts and chondrocytes Tough yet flexible Lacks nerve fibers Up to 80% water - can rebound after compression • Avascular – Receives nutrients from membrane surrounding it • Perichondrium • Three types of cartilage: – Hyaline cartilage – Elastic cartilage – Fibrocartilage Figure 4.8g Connective tissues. Cartilage: hyaline Description: Firm matrix; collagen fibers form an imperceptible network; chondroblasts produce the matrix and when mature (chondrocytes) lie in lacunae. Function: Supports and reinforces; serves as resilient cushion; resists compressive stress. Chondrocyte in lacuna Location: Forms most of the embryonic skeleton; covers the ends of long bones in joint cavities; forms costal cartilages of the ribs; cartilages of the nose, trachea, and larynx. Costal cartilages Matrix Photomicrograph: Hyaline cartilage from a costal cartilage of a rib (470x). Figure 4.8h Connective tissues. Cartilage: elastic Description: Similar to hyaline cartilage, but more elastic fibers in matrix. Function: Maintains the shape of a structure while allowing great flexibility. Chondrocyte in lacuna Matrix Location: Supports the external ear (pinna); epiglottis. Photomicrograph: Elastic cartilage from the human ear pinna; forms the flexible skeleton of the ear (800x). Figure 4.8i Connective tissues. Cartilage: fibrocartilage Description: Matrix similar to but less firm than that in hyaline cartilage; thick collagen fibers predominate. Function: Tensile strength allows it to absorb compressive shock. Location: Intervertebral discs; pubic symphysis; discs of knee joint. Chondrocytes in lacunae Intervertebral discs Collagen fiber Photomicrograph: Fibrocartilage of an intervertebral disc (125x). Special staining produced the blue color seen. Bone • • • • • • • • • Also called osseous tissue Supports and protects body structures Stores fat and synthesizes blood cells in cavities More collagen than cartilage Has inorganic calcium salts Osteoblasts produce matrix Osteocytes maintain the matrix Osteons – structural units Richly vascularized Figure 4.8j Connective tissues. Others: bone (osseous tissue) Description: Hard, calcified matrix containing many collagen fibers; osteocytes lie in lacunae. Very well vascularized. Function: Supports and protects (by enclosing); provides levers for the muscles to act on; stores calcium and other minerals and fat; marrow inside bones is the site for blood cell formation (hematopoiesis). Central canal Lacunae Lamella Location: Bones Photomicrograph: Cross-sectional view of bone (125x). Blood • Most atypical connective tissue – is a fluid • Red blood cells (RBCs) most common cell type • Also contains white blood cells (WBCs) and platelets • Fibers are soluble proteins that precipitate during blood clotting • Functions in transport Figure 4.8k Connective tissues. Connective tissue: blood Description: Red and white blood cells in a fluid matrix (plasma). Red blood cells (erythrocytes) Function: Transport respiratory gases, nutrients, wastes, and other substances. White blood cells: • Lymphocyte • Neutrophil Location: Contained within blood vessels. Plasma Photomicrograph: Smear of human blood (1670x); shows two white blood cells surrounded by red blood cells. Muscle Tissue • Highly vascularized • Responsible for most types of movement • Three types – Skeletal muscle tissue • Found in skeletal muscle • Voluntary – Cardiac muscle tissue • Found in walls of heart • Involuntary – Smooth muscle tissue • Mainly in walls of hollow organs other than heart • Involuntary Figure 4.9a Muscle tissues. Skeletal muscle Description: Long, cylindrical, multinucleate cells; obvious striations. Part of muscle fiber (cell) Function: Voluntary movement; locomotion; manipulation of the environment; facial expression; voluntary control. Nuclei Location: In skeletal muscles attached to bones or occasionally to skin. Striations Photomicrograph: Skeletal muscle (approx. 440x). Notice the obvious banding pattern and the fact that these large cells are multinucleate. Figure 4.9b Muscle tissues. Cardiac muscle Description: Branching, striated, generally uninucleate cells that interdigitate at specialized junctions (intercalated discs). Intercalated discs Function: As it contracts, it propels blood into the circulation; involuntary control. Striations Location: The walls of the heart. Nucleus Photomicrograph: Cardiac muscle (900x); notice the striations, branching of cells, and the intercalated discs. Figure 4.9c Muscle tissues. Smooth muscle Description: Spindle-shaped cells with central nuclei; no striations; cells arranged closely to form sheets. Function: Propels substances or objects (foodstuffs, urine, a baby) along internal passageways; involuntary control. Nuclei Location: Mostly in the walls of hollow organs. Smooth muscle cell Photomicrograph: Sheet of smooth muscle (720x). Nervous Tissue • Main component of nervous system – Brain, spinal cord, nerves – Regulates and controls body functions • Neurons – Specialized nerve cells that generate and conduct nerve impulses • Neuroglia – Supporting cells that support, insulate, and protect neurons Figure 4.10 Nervous tissues. Nervous tissue Description: Neurons are branching cells; cell processes that may be quite long extend from the nucleus-containing cell body; also contributing to nervous tissue are nonexcitable supporting cells. Nuclei of supporting cells Neuron processes Cell body Axon Dendrites Cell body of a neuron Function: Neurons transmit electrical signals from sensory receptors and to effectors (muscles and glands) which control their activity; supporting cells support and protect neurons. Neuron processes Location: Brain, spinal cord, and nerves. Photomicrograph: Neurons (350x). Covering and Lining Membranes • Composed of at least two primary tissue types – An epithelium bound to underlying connective tissue proper – Are simple organs • Three types – Cutaneous membranes – Mucous membranes – Serous membranes Cutaneous Membranes • Skin • Keratinized stratified squamous epithelium (epidermis) attached to a thick layer of connective tissue (dermis) The cutaneous membrane • Dry membrane (the skin) covers the body surface. Mucous Membranes • Mucosa indicates location not cell composition • All called mucosae – Line body cavities open to the exterior (e.g., Digestive, respiratory, urogenital tracts) • Moist membranes bathed by secretions (or urine) • Epithelial sheet lies over layer of connective tissue called lamina propria • May secrete mucus Figure 4.11b Classes of membranes. Mucous membranes Mucous membranes line body cavities that are open to the exterior. Mucosa of nasal cavity Mucosa of mouth Esophagus lining Mucosa of lung bronchi Serous Membranes • Serosae—found in closed ventral body cavity • Simple squamous epithelium (mesothelium) resting on thin areolar connective tissue • Parietal serosae line internal body cavity walls • Visceral serosae cover internal organs • Serous fluid between layers • Moist membranes • Pleurae, pericardium, peritoneum Figure 4.11c Classes of membranes. Serous membranes Serous membranes line body cavities that are closed to the exterior. Parietal pleura Visceral pleura Visceral Parietal pericardium pericardium Parietal peritoneum Visceral peritoneum Tissue Repair • Necessary when barriers are penetrated • Cells must divide and migrate • Occurs in two major ways – Regeneration • Same kind of tissue replaces destroyed tissue • Original function restored – Fibrosis • Connective tissue replaces destroyed tissue • Original function lost Steps in Tissue Repair: Step 1 • Inflammation sets stage – Release of inflammatory chemicals – Dilation of blood vessels – Increase in vessel permeability – Clotting occurs Figure 4.12. Tissue repair of a nonextensive skin wound: regeneration and fibrosis. Slide 1 Scab Epidermis Vein Blood clot in incised wound Inflammatory chemicals Migrating white blood cell Artery 1 Inflammation sets the stage: • Severed blood vessels bleed. • Inflammatory chemicals are released. • Local blood vessels become more permeable, allowing white blood cells, fluid, clotting proteins, and other plasma proteins to seep into the injured area. • Clotting occurs; surface dries and forms a scab. Steps in Tissue Repair: Step 2 • Organization restores blood supply – The blood clot is replaced with granulation tissue – Epithelium begins to regenerate – Fibroblasts produce collagen fibers to bridge the gap – Debris is phagocytized Figure 4.12. Tissue repair of a nonextensive skin wound: regeneration and fibrosis. Slide 2 Regenerating epithelium Area of granulation tissue ingrowth Fibroblast Macrophage Budding capillary 2 Organization restores the blood supply: • The clot is replaced by granulation tissue, which restores the vascular supply. • Fibroblasts produce collagen fibers that bridge the gap. • Macrophages phagocytize dead and dying cells and other debris. • Surface epithelial cells multiply and migrate over the granulation tissue. Steps in Tissue Repair: Step 3 • Regeneration and fibrosis – The scab detaches – Fibrous tissue matures; epithelium thickens and begins to resemble adjacent tissue – Results in a fully regenerated epithelium with underlying scar tissue Figure 4.12. Tissue repair of a nonextensive skin wound: regeneration and fibrosis. Regenerated epithelium Fibrosed area 3 Regeneration and fibrosis effect permanent repair: • The fibrosed area matures and contracts; the epithelium thickens. • A fully regenerated epithelium with an underlying area of scar tissue results. Slide 3 Regenerative Capacity in Different Tissues • Regenerate extremely well – Epithelial tissues, bone, areolar connective tissue, dense irregular connective tissue, blood-forming tissue • Moderate regenerating capacity – Smooth muscle and dense regular connective tissue • Virtually no functional regenerative capacity – Cardiac muscle and nervous tissue of brain and spinal cord – New research shows cell division does occur • Efforts underway to coax them to regenerate better