Chapter4

Synovial Membranes

(si-NŌ-vē-al; syn- = together, referring here to a place where bones come together; -ova = egg, because of their resemblance to the slimy egg white of an uncooked egg) line the cavities of freely movable joints (joint cavities). Like serous membranes, synovial membranes line structures that do not open to the exterior. Unlike mucous, serous, and cutaneous membranes, they lack an epithelium and are therefore not epithelial membranes. Synovial membranes are composed of a discontinuous layer of cells called synoviocytes (si-NŌ-vē-ō-sīts), which are closer to the synovial cavity (space between the bones), and a layer of connective tissue (areolar and adipose) deep to the synoviocytes (Figure 4.9d). Synoviocytes secrete some of the components of synovial fluid. Synovial fluid lubricates and nourishes the cartilage covering the bones at movable joints and contains macrophages that remove microbes and debris from the joint cavity.

osteocytes.

A mature bone cell that maintains the daily activities of bone tissue.

BONE TISSUE

Bones store calcium and phosphorus; house red bone marrow, which produces blood cells; and contain yellow bone marrow, a storage site for triglycerides. Bones are organs composed of several different connective tissues, including bone or osseous tissue (OS-ē-us), the periosteum, red and yellow bone marrow, and the endosteum (a membrane that lines a space within bone that stores yellow bone marrow). Bone tissue is classified as either compact or spongy, depending on how its extracellular matrix and cells are organized.

General Features of Connective Tissue

Connective tissue consists of two basic elements: extracellular matrix and cells. A connective tissue's extracellular matrix (MĀ-triks) is the material located between its widely spaced cells. The extracellular matrix consists of protein fibers and ground substance, the material between the cells and the fibers. The extracellular fibers are secreted by the connective tissue cells and account for many of the functional properties of the tissue in addition to controlling the surrounding watery environment via specific proteoglycan molecules (described shortly). The structure of the extracellular matrix determines much of the tissue's qualities. For instance, in cartilage, the extracellular matrix is firm but pliable. The extracellular matrix of bone, by contrast, is hard and inflexible. Recall that, in contrast to epithelial tissue, connective tissue does not usually occur on body surfaces. Also, unlike epithelial tissue, connective tissue usually is highly vascular; that is, it has a rich blood supply. Exceptions include cartilage, which is avascular, and tendons, with a scanty blood supply. Except for cartilage, connective tissue, like epithelial tissue, is supplied with nerves.

C. ELASTIC CONNECTIVE TISSUE

Description Elastic connective tissue contains predominantly elastic fibers with fibroblasts between them; unstained tissue is yellowish. Location Lung tissue, walls of elastic arteries, trachea, bronchial tubes, true vocal cords, suspensory ligaments of penis, some ligaments between vertebrae. Function Allows stretching of various organs; is strong and can recoil to original shape after being stretched. Elasticity is important to normal functioning of lung tissue (recoils in exhaling) and elastic arteries (recoil between heartbeats to help maintain blood flow).

B. FIBROCARTILAGE

Description Fibrocartilage has chondrocytes among clearly visible thick bundles of collagen fibers within extracellular matrix; lacks perichondrium. Location Pubic symphysis (where hip bones join anteriorly), intervertebral discs, menisci (cartilage pads) of knee, portions of tendons that insert into cartilage. Function Support and joining structures together. Strength and rigidity make it the strongest type of cartilage.

B. ADIPOSE TISSUE

Description Adipose tissue has cells derived from fibroblasts (called adipocytes) that are specialized for storage of triglycerides (fats) as a large, centrally located droplet. Cell fills up with a single, large triglyceride droplet, and cytoplasm and nucleus are pushed to periphery of cell. With weight gain, amount of adipose tissue increases and new blood vessels form. Thus, an obese person has many more blood vessels than does a lean person, a situation that can cause high blood pressure, since the heart has to work harder. Most adipose tissue in adults is white adipose tissue (just described). Brown adipose tissue (BAT) is darker due to very rich blood supply and numerous pigmented mitochondria that participate in aerobic cellular respiration. BAT is widespread in the fetus and infant; adults have only small amounts. Location Wherever areolar connective tissue is located: subcutaneous layer deep to skin, around heart and kidneys, yellow bone marrow, padding around joints and behind eyeball in eye socket. Function Reduces heat loss through skin; serves as an energy reserve; supports and protects organs. In newborns, BAT generates heat to maintain proper body temperature. Adipose tissue is also an excellent source of stem cells, which are used in rejuvenation medicine to repair or replace damaged tissue.

A. AREOLAR CONNECTIVE TISSUE

Description Areolar connective tissue is one of the most widely distributed connective tissues; consists of fibers (collagen, elastic, reticular) arranged randomly and several kinds of cells (fibroblasts, macrophages, plasma cells, adipocytes, mast cells, and a few white blood cells) embedded in semifluid ground substance (hyaluronic acid, chondroitin sulfate, dermatan sulfate, and keratan sulfate). Location In and around nearly every body structure (thus, called "packing material" of the body): in subcutaneous layer deep to skin; papillary (superficial) region of dermis of skin; lamina propria of mucous membranes; around blood vessels, nerves, and body organs. Function Strength, elasticity, support.

I. STRATIFIED COLUMNAR EPITHELIUM

Description Basal layers in stratified columnar epithelium usually consist of shortened, irregularly shaped cells; only apical layer has columnar cells; uncommon. Location Lines part of urethra; large excretory ducts of some glands, such as esophageal glands; small areas in anal mucous membrane; part of conjunctiva of eye. Function Protection and secretion. w0231-233

B. CARDIAC MUSCLE TISSUE

Description Cardiac muscle tissue consists of branched, striated fibers with usually only one centrally located nucleus (occasionally two). Attach end to end by transverse thickenings of plasma membrane called intercalated discs (in-TER-ka-lāt-ed; intercalate = to insert between), which contain desmosomes and gap junctions. Desmosomes strengthen tissue and hold fibers together during vigorous contractions. Gap junctions provide route for quick conduction of electrical signals (muscle action potentials) throughout heart. Involuntary (not conscious) control. Location Heart wall. Function Pumps blood to all parts of body. w0296-298

F. CILIATED PSEUDOSTRATIFIED COLUMNAR EPITHELIUM

Description Ciliated pseudostratified columnar epithelium appears to have several layers because cell nuclei are at various levels. All cells are attached to basement membrane in a single layer, but some cells do not extend to apical surface. When viewed from side, these features give false impression of a multilayered tissue (thus the name pseudostratified; pseudo = false). Contains cells that extend to surface and secrete mucus (globlet cells) or bear cilia. Location Lines airways of most of upper respiratory tract. Function Secretes mucus that traps foreign particles, and cilia sweep away mucus for elimination from body.

D. CILIATED SIMPLE COLUMNAR EPITHELIUM

Description Ciliated simple columnar epithelium is a single layer of ciliated columnlike cells with oval nuclei near base of cells. Goblet cells are usually interspersed. Location Lines some bronchioles (small tubes) of respiratory tract, uterine (fallopian) tubes, uterus, some paranasal sinuses, central canal of spinal cord, and ventricles of brain. Function Cilia beat in unison, moving mucus and foreign particles toward throat, where they can be coughed up and swallowed or spit out. Coughing and sneezing speed up movement of cilia and mucus. Cilia also help move oocytes expelled from ovaries through uterine (fallopian) tubes into uterus. w0216a

Mature Connective Tissue: Supporting Connective Tissue—Bone Tissue

Description Compact bone tissue consists of osteons (haversian systems) that contain lamellae, lacunae, osteocytes, canaliculi, and central (haversian) canals. By contrast, spongy bone tissue (see Figure 6.3) consists of thin columns called trabeculae; spaces between trabeculae are filled with red bone marrow. Location Both compact and spongy bone tissue make up the various parts of bones of the body. Function Support, protection, storage; houses blood-forming tissue; serves as levers that act with muscle tissue to enable movement. w0283-285

B. DENSE IRREGULAR CONNECTIVE TISSUE

Description Dense irregular connective tissue is made up of collagen fibers; usually irregularly arranged with a few fibroblasts. Location Often occurs in sheets, such as fasciae (tissue beneath skin and around muscles and other organs), reticular (deeper) region of dermis of skin, fibrous pericardium of heart, periosteum of bone, perichondrium of cartilage, joint capsules, membrane capsules around various organs (kidneys, liver, testes, lymph nodes); also in heart valves. Function Provides tensile (pulling) strength in many directions.

Mature Connective Tissue: Connective Tissue Proper—Dense Connective Tissue

Description Dense regular connective tissue forms shiny white extracellular matrix; mainly collagen fibers regularly arranged in bundles with fibroblasts in rows between them. Collagen fibers (protein structures secreted by fibroblasts) are not living, so damaged tendons and ligaments heal slowly. Location Forms tendons (attach muscle to bone), most ligaments (attach bone to bone), and aponeuroses (sheetlike tendons that attach muscle to muscle or muscle to bone). Function Provides strong attachment between various structures. Tissue structure withstands pulling (tension) along long axis of fibers. w0265-267_c

C. ELASTIC CARTILAGE

Description Elastic cartilage has chondrocytes in threadlike network of elastic fibers within extracellular matrix; perichondrium present. Location Lid on top of larynx (epiglottis), part of external ear (auricle), auditory (eustachian) tubes. Function Provides strength and elasticity; maintains shape of certain structures.

Mature Connective Tissue: Supporting Connective Tissue—Cartilage A. HYALINE CARTILAGE

Description Hyaline cartilage (hyalinos = glassy) contains a resilient gel as ground substance and appears in the body as a bluish-white, shiny substance (can stain pink or purple when prepared for microscopic examination; fine collagen fibers are not visible with ordinary staining techniques); prominent chondrocytes are found in lacunae surrounded by perichondrium (exceptions: articular cartilage in joints and cartilage of epiphyseal plates, where bones lengthen during growth). Location Most abundant cartilage in body; at ends of long bones, anterior ends of ribs, nose, parts of larynx, trachea, bronchi, bronchial tubes, embryonic and fetal skeleton. Function Provides smooth surfaces for movement at joints, flexibility, and support; weakest type of cartilage and can be fractured.

Connective Tissues 1MUCOUS (MUCOID) CONNECTIVE TISSUE

Description Mucous (mucoid) connective tissue has widely scattered fibroblasts embedded in viscous, jellylike ground substance that contains fine collagen fibers. Location Umbilical cord of fetus. Function Support. w0253-255

Nervous Tissue

Description Nervous tissue consists of (1) neurons (nerve cells), which consist of cell body and processes extending from cell body (one to multiple dendrites and a single axon); and (2) neuroglia, which do not generate or conduct nerve impulses but have other important supporting functions. Location Nervous system. Function Exhibits sensitivity to various types of stimuli; converts stimuli into nerve impulses (action potentials); conducts nerve impulses to other neurons, muscle fibers, or glands.

E. NONCILIATED PSEUDOSTRATIFIED COLUMNAR EPITHELIUM

Description Nonciliated pseudostratified columnar epithelium appears to have several layers because the nuclei of the cells are at various levels. Even though all the cells are attached to the basement membrane in a single layer, some cells do not extend to the apical surface. When viewed from the side, these features give the false impression of a multilayered tissue—thus the name pseudostratified epithelium (pseudo- = false). Contains cells without cilia and also lacks globlet cells. Location Lines epididymis, larger ducts of many glands, and parts of male urethra. Function Absorption and secretion.

C. NONCILIATED SIMPLE COLUMNAR EPITHELIUM

Description Nonciliated simple columnar epithelium is a single layer of nonciliated columnlike cells with oval nuclei near base of cells; contains (1) columnar epithelial cells with microvilli at apical surface and (2) goblet cells. Microvilli, fingerlike cytoplasmic projections, increase surface area of plasma membrane (see Figure 3.1), thus increasing cell's rate of absorption. Goblet cells are modified columnar epithelial cells that secrete mucus, a slightly sticky fluid, at their apical surfaces. Before release, mucus accumulates in upper portion of cell, causing it to bulge and making the whole cell resemble a goblet or wine glass. Location Lines gastrointestinal tract (from stomach to anus), ducts of many glands, and gallbladder. Function Secretion and absorption; larger columnar cells contain more organelles and thus are capable of higher level of secretion and absorption than are cuboidal cells. Secreted mucus lubricates linings of digestive, respiratory, and reproductive tracts, and most of urinary tract; helps prevent destruction of stomach lining by acidic gastric juice secreted by stomach.

C. RETICULAR CONNECTIVE TISSUE

Description Reticular connective tissue is a fine interlacing network of reticular fibers (thin form of collagen fiber) and reticular cells. Location Stroma (supporting framework) of liver, spleen, lymph nodes; red bone marrow; reticular lamina of basement membrane; around blood vessels and muscles. Function Forms stroma of organs; binds smooth muscle tissue cells; filters and removes worn-out blood cells in spleen and microbes in lymph nodes. w0262-264

B. SIMPLE CUBOIDAL EPITHELIUM

Description Simple cuboidal epithelium is a single layer of cube-shaped cells; round, centrally located nucleus. Cuboidal cell shape is obvious when tissue is sectioned and viewed from the side. (Note: Strictly cuboidal cells could not form small tubes; these cuboidal cells are more pie-shaped but still nearly as high as they are wide at the base.) Location Covers surface of ovary; lines anterior surface of capsule of lens of the eye; forms pigmented epithelium at posterior surface of retina of the eye; lines kidney tubules and smaller ducts of many glands; makes up secreting portion of some glands such as thyroid gland and ducts of some glands such as pancreas. Function Secretion and absorption.

Epithelial Tissue: Covering and Lining Epithelium A. SIMPLE SQUAMOUS EPITHELIUM

Description Simple squamous epithelium is a single layer of flat cells that resembles a tiled floor when viewed from apical surface; centrally located nucleus that is flattened and oval or spherical in shape. Location Most commonly (1) lines the cardiovascular and lymphatic system (heart, blood vessels, lymphatic vessels), where it is known as endothelium (en′-dō-THĒ-lē-um; endo- = within; -thelium = covering), and (2) forms the epithelial layer of serous membranes (peritoneum, pleura, pericardium), where it is called mesothelium (mez′-ō-THĒ-lē-um; meso- = middle). Also found in air sacs of lungs, glomerular (Bowman's) capsule of kidneys, inner surface of tympanic membrane (eardrum). Function Present at sites of filtration (such as blood filtration in kidneys) or diffusion (such as diffusion of oxygen into blood vessels of lungs) and at site of secretion in serous membranes. Not found in body areas subject to mechanical stress (wear and tear).

A. SKELETAL MUSCLE TISSUE

Description Skeletal muscle tissue consists of long, cylindrical, striated fibers (striations are alternating light and dark bands within fibers that are visible under a light microscope). Skeletal muscle fibers vary greatly in length, from a few centimeters in short muscles to 30-40 cm (about 12-16 in.) in the longest muscles. A muscle fiber is a roughly cylindrical, multinucleated cell with nuclei at the periphery. Skeletal muscle is considered voluntary because it can be made to contract or relax by conscious control. Location Usually attached to bones by tendons. Function Motion, posture, heat production, protection.

C. SMOOTH MUSCLE TISSUE

Description Smooth muscle tissue consists of nonstriated fibers (lack striations, hence the term smooth). Smooth muscle fiber is a small spindleshaped cell thickest in middle, tapering at each end, and containing a single, centrally located nucleus. Gap junctions connect many individual fibers in some smooth muscle tissue (for example, in wall of intestines). Usually involuntary; can produce powerful contractions as many muscle fibers contract in unison. Where gap junctions are absent, such as iris of eye, smooth muscle fibers contract individually, like skeletal muscle fibers. Location Iris of eyes; walls of hollow internal structures such as blood vessels, airways to lungs, stomach, intestines, gallbladder, urinary bladder, and uterus. Function Motion (constriction of blood vessels and airways, propulsion of foods through gastrointestinal tract, contraction of urinary bladder and gallbladder).

H. STRATIFIED CUBOIDAL EPITHELIUM

Description Stratified cuboidal epithelium has two or more layers of cells; cells in apical layer are cube-shaped; fairly rare type. Location Ducts of adult sweat glands and esophageal glands, part of male urethra. Function Protection; limited secretion and absorption.

G. STRATIFIED SQUAMOUS EPITHELIUM

Description Stratified squamous epithelium has two or more layers of cells; cells in apical layer and several layers deep to it are squamous; cells in deeper layers vary from cuboidal to columnar. As basal cells divide, daughter cells arising from cell divisions push upward toward apical layer. As they move toward surface and away from blood supply in underlying connective tissue, they become dehydrated and less metabolically active. Tough proteins predominate as cytoplasm is reduced, and cells become tough, hard structures that eventually die. At apical layer, after dead cells lose cell junctions they are sloughed off, but they are replaced continuously as new cells emerge from basal cells. Keratinized stratified squamous epithelium develops tough layer of keratin in apical layer of cells and several layers deep to it (see Figure 5.3). (Keratin is a tough, fibrous intracellular protein that helps protect skin and underlying tissues from heat, microbes, and chemicals.) Relative amount of keratin increases in cells as they move away from nutritive blood supply and organelles die. Nonkeratinized stratified squamous epithelium does not contain large amounts of keratin in apical layer and several layers deep and is constantly moistened by mucus from salivary and mucous glands; organelles are not replaced. Location Keratinized variety forms superficial layer of skin; nonkeratinized variety lines wet surfaces (lining of mouth, esophagus, part of epiglottis, part of pharynx, and vagina) and covers tongue. Function Protection against abrasion, water loss, ultraviolet radiation, and foreign invasion. Both types form first line of defense against microbes

J. TRANSITIONAL EPITHELIUM (UROTHELIUM)

Description Transitional epithelium (urothelium) has a variable appearance (transitional). In relaxed or unstretched state, looks like stratified cuboidal epithelium, except apical layer cells tend to be large and rounded. As tissue is stretched, cells become flatter, giving the appearance of stratified squamous epithelium. Multiple layers and elasticity make it ideal for lining hollow structures (urinary bladder) subject to expansion from within. Location Lines urinary bladder and portions of ureters and urethra. Function Allows urinary organs to stretch and maintain protective lining while holding variable amounts of fluid without rupturing.

Connective Tissue Cells

Embryonic cells called mesenchymal cells give rise to the cells of connective tissue. Each major type of connective tissue contains an immature class of cells with a name ending in -blast, which means "to bud or sprout." These immature cells are called fibroblasts in loose and dense connective tissue (described shortly), chondroblasts in cartilage, and osteoblasts in bone. Blast cells retain the capacity for cell division and secrete the extracellular matrix that is characteristic of the tissue. In some connective tissues, once the extracellular matrix is produced, the immature cells differentiate into mature cells with names ending in -cyte, namely, fibrocytes, chondrocytes, and osteocytes. Mature cells have reduced capacities for cell division and extracellular matrix formation and are mostly involved in monitoring and maintaining the extracellular matrix. Connective tissue cells vary according to the type of tissue and include the following (Figure 4.8): 1.Fibroblasts (FĪ-brō-blasts; fibro- = fibers) are large, flat cells with branching processes. They are present in all the general connective tissues, and usually are the most numerous. 2.Macrophages (MAK-rō-fā-jez; macro- = large; -phages = eaters) are phagocytes that develop from monocytes, a type of white blood cell. Fixed macrophages reside in a particular tissue; examples include alveolar macrophages in the lungs or splenic macrophages in the spleen. Wandering macrophages have the ability to move throughout the tissue and gather at sites of infection or inflammation to carry on phagocytosis. 3.Plasma cells (plasmocytes) are found in many places in the body, but most plasma cells reside in connective tissue, especially in the gastrointestinal and respiratory tracts. 4.Mast cells (mastocytes) are involved in the inflammatory response, the body's reaction to injury or infection and can also bind to, ingest, and kill bacteria. 5.Adipocytes (AD-i-pō-sīts) are fat cells or adipose cells, connective tissue cells that store triglycerides (fats). They are found deep to the skin and around organs such as the heart and kidneys. 6.Leukocytes (white blood cells) are not found in significant numbers in normal connective tissue. However, in response to certain conditions they migrate from blood into connective tissue. For example, neutrophils gather at sites of infection, and eosinophils migrate to sites of parasitic invasions and allergic responses.

the four types of tissues that make up the human body

Epithelial tissue,Connective tissue,Muscular tissue ,Nervous tissue

Mature Connective Tissue: Liquid Connective Tissue—Blood

Mature Connective Tissue: Liquid Connective Tissue—Blood

Cell Junctions

are contact points between the plasma membranes of tissue cells. Here we consider the five most important types of cell junctions: tight junctions, adherens junctions, desmosomes, hemidesmosomes, and gap junctions (Figure 4.2).

Membranes

are flat sheets of pliable tissue that cover or line a part of the body. The majority of membranes consist of an epithelial layer and an underlying connective tissue layer and are called epithelial membranes. The principal epithelial membranes of the body are mucous membranes, serous membranes, and the cutaneous membrane, or skin. Another type of membrane, a synovial membrane, lines joints and contains connective tissue but no epithelium.A membrane is a flat sheet of pliable tissues that covers or lines a part of the body.

Loose Connective Tissue

are loosely arranged between cells. The types of loose connective tissue are areolar connective tissue, adipose tissue, and reticular connective tissue

Collagen fibers(KOL-a-jen; colla = glue)

are very strong and resist pulling or stretching, but they are not stiff, which allows tissue flexibility. The properties of different types of collagen fibers vary from tissue to tissue. For example, the collagen fibers found in cartilage and bone form different associations with surrounding molecules. As a result of these associations, the collagen fibers in cartilage are surrounded by more water molecules than those in bone, which gives cartilage a more cushioning effect. Collagen fibers often occur in parallel bundles (see Table 4.5A, dense regular connective tissue). The bundle arrangement adds great tensile strength to the tissue. Chemically, collagen fibers consist of the protein collagen, which is the most abundant protein in your body, representing about 25% of the total. Collagen fibers are found in most types of connective tissues, especially bone, cartilage, tendons (which attach muscle to bone), and ligaments (which attach bone to bone).

Supporting Connective Tissue This type of mature connective tissue includes

cartilage and bone.

The cells of mature cartilage, called

chondrocytes (KON-drō-sīts; chondro- = cartilage), occur singly or in groups within spaces called lacunae (la-KOO-nē = little lakes; singular is lacuna, pronounced la-KOO-na) in the extracellular matrix

Tight Junctions

consist of weblike strands of transmembrane proteins that fuse together the outer surfaces of adjacent plasma membranes to seal off passageways between adjacent cells (Figure 4.2a). Cells of epithelial tissue that lines the stomach, intestines, and urinary bladder have many tight junctions. They inhibit the passage of substances between cells and prevent the contents of these organs from leaking into the blood or surrounding tissues.

CARTILAGE

consists of a dense network of collagen fibers and elastic fibers firmly embedded in chondroitin sulfate, a gel-like component of the ground substance. Cartilage can endure considerably more stress than loose and dense connective tissues. The strength of cartilage is due to its collagen fibers, and its resilience (ability to assume its original shape after deformation) is due to chondroitin sulfate.

Adherens Junctions

contain plaque (PLAK), a dense layer of proteins on the inside of the plasma membrane that attaches both to membrane proteins and to microfilaments of the cytoskeleton (Figure 4.2b). Transmembrane glycoproteins called cadherins join the cells. Each cadherin inserts into the plaque from the opposite side of the plasma membrane, partially crosses the intercellular space (the space between the cells), and connects to cadherins of an adjacent cell. In epithelial cells, adherens junctions often form extensive zones called adhesion belts because they encircle the cell similar to the way a belt encircles your waist. Adherens junctions help epithelial surfaces resist separation during various contractile activities, as when food moves through the intestines.

1.Epithelial tissue

covers body surfaces and lines hollow organs, body cavities, and ducts; it also forms glands. This tissue allows the body to interact with both its internal and external environments.or epithelium (plural is epithelia) consists of cells arranged in continuous sheets, in either single or multiple layers. Because the cells are closely packed and are held tightly together by many cell junctions, there is little intercellular space between adjacent plasma membranes. Epithelial tissue is arranged in two general patterns in the body: (1) covering and lining various surfaces and (2) forming the secreting portions of glands. Functionally, epithelial tissue protects, secretes (mucus, hormones, and enzymes), absorbs (nutrients in the gastrointestinal tract), and excretes (various substances in the urinary tract). The various surfaces of covering and lining epithelial cells often differ in structure and have specialized functions. The apical (free) surface of an epithelial cell faces the body surface, a body cavity, the lumen (interior space) of an internal organ, or a tubular duct that receives cell secretions (Figure 4.4). Apical surfaces may contain cilia or microvilli. The lateral surfaces of an epithelial cell, which face the adjacent cells on either side, may contain tight junctions, adherens junctions, desmosomes, and/or gap junctions. The basal surface of an epithelial cell is opposite the apical surface. The basal surfaces of the deepest layer of epithelial cells adhere to extracellular materials such as the basement membrane. Hemidesmosomes in the basal surfaces of the deepest layer of epithelial cells anchor the epithelium to the basement membrane (described next). In discussing epithelia with multiple layers, the term apical layer refers to the most superficial layer of cells, and the basal layer is the deepest layer of cells.

Desmosomes

desmo- = band) contain plaque and have transmembrane glycoproteins (cadherins) that extend into the intercellular space between adjacent cell membranes and attach cells to one another (Figure 4.2c). However, unlike adherens junctions, the plaque of desmosomes does not attach to microfilaments. Instead, a desmosome plaque attaches to elements of the cytoskeleton known as intermediate filaments, which consist of the protein keratin. The intermediate filaments extend from desmosomes on one side of the cell across the cytosol to desmosomes on the opposite side of the cell. This structural arrangement contributes to the stability of the cells and tissue. These spot weld-like junctions are common among the cells that make up the epidermis (the outermost layer of the skin) and among cardiac muscle cells in the heart. Desmosomes prevent epidermal cells from separating under tension and cardiac muscle cells from pulling apart during contraction.

Nervous Tissue

detects changes in a variety of conditions inside and outside the body and responds by generating electrical signals called nerve action potentials (nerve impulses) that activate muscular contractions and glandular secretions. Despite the awesome complexity of the nervous system, nervous tissue consists of only two principal types of cells: neurons and neuroglia. Neurons (NOO-rons; neuro- = nerve), or nerve cells, are sensitive to various stimuli. They convert stimuli into electrical signals called nerve action potentials (nerve impulses) and conduct these action potentials to other neurons, to muscle tissue, or to glands. Most neurons consist of three basic parts: a cell body and two kinds of cell processes—dendrites and axons (Table 4.10). The cell body contains the nucleus and other organelles. Dendrites (dendr- = tree) are tapering, highly branched, and usually short cell processes (extensions). They are the major receiving or input portion of a neuron. The axon (axo- = axis) of a neuron is a single, thin, cylindrical process that may be very long. It is the output portion of a neuron, conducting nerve impulses toward another neuron or to some other tissue.

Dense connective tissue

is a second type of connective tissue proper that contains more fibers, which are thicker and more densely packed, but have considerably fewer cells than loose connective tissue. There are three types: dense regular connective tissue, dense irregular connective tissue, and elastic connective tissue

Muscular tissue

is composed of cells specialized for contraction and generation of force. In the process, muscular tissue generates heat that warms the body. consists of elongated cells called muscle fibers or myocytes that can use ATP to generate force. As a result, muscular tissue produces body movements, maintains posture, and generates heat. It also provides protection. Based on location and certain structural and functional features, muscular tissue is classified into three types: skeletal, cardiac, and smooth

Ground Substance

is the component of a connective tissue between the cells and fibers. The ground substance may be fluid, semifluid, gelatinous, or calcified. It supports cells, binds them together, stores water, and provides a medium for exchange of substances between the blood and cells. It plays an active role in how tissues develop, migrate, proliferate, and change shape, and in how they carry out their metabolic functions. Ground substance contains water and an assortment of large organic molecules, many of which are complex combinations of polysaccharides and proteins.

LYMPH

is the extracellular fluid that flows in lymphatic vessels. It is a liquid connective tissue that consists of several types of cells in a clear liquid extracellular matrix that is similar to blood plasma but with much less protein. The composition of lymph varies from one part of the body to another. For example, lymph leaving lymph nodes includes many lymphocytes, a type of white blood cell, in contrast to lymph from the small intestine, which has a high content of newly absorbed dietary lipids. The details of lymph are considered in Chapter 22.

bone, cartilage, and the cornea of the eye contain

keratan sulfate

Gap Junctions

membrane proteins called connexins form tiny fluid-filled tunnels called connexons that connect neighboring cells (Figure 4.2e). The plasma membranes of gap junctions are not fused together as in tight junctions but are separated by a very narrow intercellular gap (space). Through the connexons, ions and small molecules can diffuse from the cytosol of one cell to another, but the passage of large molecules such as vital intracellular proteins is prevented. The transfer of nutrients, and perhaps wastes, takes place through gap junctions in avascular tissues such as the lens and cornea of the eye. Gap junctions allow the cells in a tissue to communicate with one another. In a developing embryo, some of the chemical and electrical signals that regulate growth and cell differentiation travel via gap junctions. Gap junctions also enable nerve or muscle impulses to spread rapidly among cells, a process that is crucial for the normal operation of some parts of the nervous system and for the contraction of muscle in the heart, gastrointestinal tract, and uterus.

BLOOD TISSUE

one of the liquid connective tissues has a liquid extracellular matrix called blood plasma and formed elements. Blood plasma is a pale yellow fluid that consists mostly of water with a wide variety of dissolved substances—nutrients, wastes, enzymes, plasma proteins, hormones, respiratory gases, and ions. Suspended in the blood plasma are formed elements—red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes) (Table 4.8). Red blood cells transport oxygen to body cells and remove some carbon dioxide from them. White blood cells are involved in phagocytosis, immunity, and allergic reactions. Platelets (PLĀT-lets) participate in blood clotting

Epithelial Membranes Mucous Membranes

or mucosa (mū-KŌ-sa) lines a body cavity that opens directly to the exterior. Mucous membranes line the entire digestive, respiratory, and reproductive tracts, and much of the urinary tract. They consist of a lining layer of epithelium and an underlying layer of connective tissue

Serous Membranes(SĒR-us = watery

or serosa lines a body cavity that does not open directly to the exterior (thoracic or abdominal cavities), and it covers the organs that are within the cavity. Serous membranes consist of areolar connective tissue covered by mesothelium (simple squamous epithelium) (Figure 4.9b). You will recall from Chapter 1 that serous membranes have two layers: The layer attached to and lining the cavity wall is called the parietal layer (pa-RĪ-e-tal; pariet- = wall); the layer that covers and adheres to the organs within the cavity is the visceral layer (viscer- = body organ) (see Figure 1.10a). The mesothelium of a serous membrane secretes serous fluid, a watery lubricant that allows organs to glide easily over one another or to slide against the walls of cavities. Recall from Chapter 1 that the serous membrane lining the thoracic cavity and covering the lungs is the pleura. The serous membrane lining the heart cavity and covering the heart is the pericardium. The serous membrane lining the abdominal cavity and covering the abdominal organs is the peritoneum.

Cutaneous Membrane

or skin covers the entire surface of the body and consists of a superficial portion called the epidermis and a deeper portion called the dermis (Figure 4.9c). The epidermis consists of keratinized stratified squamous epithelium, which protects underlying tissues. The dermis consists of dense irregular connective tissue and areolar connective tissue. Details of the cutaneous membrane are presented in Chapter 5.

A covering of dense irregular connective tissue called the

perichondrium (per′-i-KON-drē-um; peri- = around) surrounds the surface of most cartilage and contains blood vessels and nerves and is the source of new cartilage cells. Since cartilage has no blood supply, it heals poorly following an injury. The cells and collagen-embedded extracellular matrix of cartilage form a strong, firm material that resists tension (stretching), compression (squeezing), and shear (pushing in opposite directions). The chondroitin sulfate in the extracellular matrix is largely responsible for cartilage's resilience. Because of these properties, cartilage plays an important role as a support tissue in the body. It is also a precursor to bone, forming almost the entire embryonic skeleton. Though bone gradually replaces cartilage during further development, cartilage persists after birth as the growth plates within bone that allow bones to increase in length during the growing years. Cartilage also persists throughout life as the lubricated articular surfaces of most joints. There are three types of cartilage: hyaline cartilage, fibrocartilage, and elastic cartilage

2.Connective tissue

protects and supports the body and its organs. Various types of connective tissues bind organs together, store energy reserves as fat, and help provide the body with immunity to disease-causing organisms.Connective tissue is one of the most abundant and widely distributed tissues in the body. In its various forms, connective tissue has a variety of functions. It binds together, supports, and strengthens other body tissues; protects and insulates internal organs; compartmentalizes structures such as skeletal muscles; serves as the major transport system within the body (blood, a fluid connective tissue); is the primary location of stored energy reserves (adipose, or fat, tissue); and is the main source of immune responses.

The four basic types of tissues in the human body contribute to homeostasis by

providing diverse functions including protection, support, communication among cells, and resistance to disease,

Hemidesmosomes(hemi- = half)

resemble desmosomes, but they do not link adjacent cells. The name arises from the fact that they look like half of a desmosome (Figure 4.2d). However, the transmembrane glycoproteins in hemidesmosomes are integrins rather than cadherins. On the inside of the plasma membrane, integrins attach to intermediate filaments made of the protein keratin. On the outside of the plasma membrane, the integrins attach to the protein laminin, which is present in the basement membrane (discussed shortly). Thus, hemidesmosomes anchor cells not to each other but to the basement membrane.