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4.1TYPES OF TISSUESOBJECTIVE•Name the four basic types of tissues that make up the human body, and state the characteristics ofeach.Body tissues can be classified into four basic types according to their structure and function (Figure4.1):1.Epithelial tissuescover body surfaces and line hollow organs, body cavities, and ducts; they also formsglands. This tissue allows the body to interact with both its internal and external environments.2.Connective tissuesprotect and support the body and its organs. Various types of connective tissuesbind organs together, store energy reserves as fat, and help provide the body with immunity to disease‐causing organisms.3.Muscular tissuesare composed of cells specialized for contraction and generation of force. In theprocess, muscular tissues generate heat that warms the body.4.Nervous tissuedetects changes in a variety of conditions inside and outside the body and responds bygenerating electrical signals called nerve action potentials (nerve impulses) that activate muscularcontractions and glandular secretions.Figure4.1Types of tissues.Each of the four types of tissues has different cells that vary in shape, structure, function, anddistribution.Kevin Somerville/Imagineering; Photo by Mark NielsenWhat are some key differences in function among the four tissue types?
Epithelial tissues and most types of connective tissues, except cartilage, bone, and blood, are more general innature and have a wide distribution in the body. These tissues are components of most body organs and have awide range of structure and function. We will look at epithelial tissues and connective tissues in some detail inthis chapter. The general features of bone tissue and blood will be introduced here, but their detailed discussionis presented in Chapters6and19, respectively. Similarly, the structure and function of muscular tissues andnervous tissue are introduced here and examined in detail in Chapters10and12, respectively.Normally, most cells within a tissue remain anchored to other cells or structures. Only a few cells, such asphagocytes, move freely through the body, searching for invaders to destroy. However, many cells migrateextensively during the growth and development process before birth.ExamplesAnatomy Overview: Epithelial TissuesAnatomy Overview: Connective TissuesAnatomy Overview: Muscle TissueAnatomy Overview: Nervous TissueCLINICALCONNECTIONBiopsyAbiopsy(BĪ‐op‐sē;bio‐ = life; ‐opsy= to view) is the removal of a sample of living tissue formicroscopic examination. This procedure is used to help diagnose many disorders, especially cancer, andto discover the cause of unexplained infections and inflammations. Both normal and potentially diseasedtissues are removed for purposes of comparison. Once the tissue samples are removed, either surgicallyor through a needle and syringe, they may be preserved, stained to highlight special properties, or cut intothin sections for microscopic observation. Sometimes a biopsy is conducted while a patient isanesthetized during surgery to help a physician determine the most appropriate treatment. For example, ifa biopsy of thyroid tissue reveals malignant cells, the surgeon can proceed immediately with the mostappropriate procedure.4.2CELL JUNCTIONSOBJECTIVE
•Describe the structure and functions of the five main types of cell junctions.Before looking more specifically at the types of tissues, we will first examine how cells are held together toform tissues. Most epithelial cells and some muscle and nerve cells are tightly joined into functional units.Celljunctionsare contact points between the plasma membranes of tissue cells. Here we consider the five mostimportant types of cell junctions: tight junctions, adherens junctions, desmosomes, hemidesmosomes, and gapjunctions (Figure4.2).Figure4.2Cell junctions.Most epithelial cells and some muscle and nerve cells contain cell junctions.Imagineering; Photo by MarkNielsenWhich type of cell junction functions in communication between adjacent cells?Tight JunctionsTight junctionsconsist of weblike strands of transmembrane proteins that fuse together the outer surfaces ofadjacent plasma membranes to seal off passageways between adjacent cells (Figure4.2a). Cells of epithelialtissues that line the stomach, intestines, and urinary bladder have many tight junctions. They inhibit thepassage of substances between cells and prevent the contents of these organs from leaking into the blood orsurrounding tissues.Adherens JunctionsAdherens junctions(ad‐HER‐ens) containplaque(PLAK), a dense layer of proteins on the inside of theplasma membrane that attaches both to membrane proteins and to microfilaments of the cytoskeleton(Figure4.2b). Transmembrane glycoproteins calledcadherinsjoin the cells. Each cadherin inserts into theplaque from the opposite side of the plasma membrane, partially crosses the intercellular space (the spacebetween the cells), and connects to cadherins of an adjacent cell. In epithelial cells, adherens junctions oftenform extensive zones calledadhesion beltsbecause they encircle the cell similar to the way a belt encirclesyour waist. Adherens junctions help epithelial surfaces resist separation during various contractile activities, aswhen food moves through the intestines.Desmosomes
Like adherens junctions,desmosomes(DEZ‐mō‐sōms;desmo‐ = band) contain plaque and havetransmembrane glycoproteins (cadherins) that extend into the intercellular space between adjacent cellmembranes and attach cells to one another (Figure4.2c). However, unlike adherens junctions, the plaque ofdesmosomes does not attach to microfilaments. Instead, a desmosome plaque attaches to elements of thecytoskeleton known as intermediate filaments, which consist of the protein keratin. The intermediate filamentsextend from desmosomes on one side of the cell across the cytosol to desmosomes on the opposite side of thecell. This structural arrangement contributes to the stability of the cells and tissue. These spot‐weld‐likejunctions are common among the cells that make up the epidermis (the outermost layer of the skin) and amongcardiac muscle cells in the heart. Desmosomes prevent epidermal cells from separating under tension andcardiac muscle cells from pulling apart during contraction.HemidesmosomesHemidesmosomes(hemi‐ = half) resemble desmosomes, but they do not link adjacent cells. The name arisesfrom the fact that they look like half of a desmosome (Figure4.2d). However, the transmembraneglycoproteins in hemidesmosomes areintegrinsrather 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 proteinlaminin,which is present in the basement membrane (discussed shortly).Thus, hemidesmosomes anchor cells not to each other but to the basement membrane.Gap JunctionsAtgap junctions, membrane proteins calledconnexinsform tiny fluid‐filled tunnels calledconnexonsthatconnect neighboring cells (Figure4.2e). The plasma membranes of gap junctions are not fused together as intight junctions but are separated by a very narrow intercellular gap (space). Through the connexons, ions andsmall molecules can diffuse from the cytosol of one cell to another, but the passage of large molecules such asvital intracellular proteins is prevented. The transfer of nutrients, and perhaps wastes, takes place through gapjunctions in avascular tissues such as the lens and cornea of the eye. Gap junctions allow the cells in a tissue tocommunicate with one another. In a developing embryo, some of the chemical and electrical signals thatregulate growth and cell differentiation travel via gap junctions. Gap junctions also enable nerve or muscleimpulses to spread rapidly among cells, a process that is crucial for the normal operation of some parts of thenervous system and for the contraction of muscle in the heart, gastrointestinal tract, and uterus.4.3COMPARISON BETWEEN EPITHELIAL ANDCONNECTIVE TISSUESOBJECTIVE•State the main differences between epithelial and connective tissues.
Before examining epithelial tissues and connective tissues in more detail, let's compare these two widelydistributed tissues (Figure4.3). Major structural differences between an epithelial tissue and a connectivetissue are immediately obvious under a light microscope. The first obvious difference is the number of cells inrelation to the extracellular matrix (the substance between cells). In an epithelial tissue many cells are tightlypacked together with little or no extracellular matrix, whereas in a connective tissue a large amount ofextracellular material separates cells that are usually widely scattered. The second obvious difference is that anepithelial tissue has no blood vessels, whereas most connective tissues have significant networks of bloodvessels. Another key difference is that epithelial tissues almost always form surface layers and are not coveredby another tissue. An exception is the epithelial lining of blood vessels where blood constantly passes over theepithelium. While these key structural distinctions account for some of the major functional differencesbetween these tissue types, they also lead to a common bond. Because epithelial tissues lack blood vessels andform surfaces, they are always found immediately adjacent to blood‐vessel‐rich connective tissues, whichenable them to make the exchanges with blood necessary for the delivery of oxygen and nutrients and theremoval of wastes that are critical processes for their survival and function.Figure4.3Comparison between epithelial tissues and connective tissues.The ratio of cells to extracellular matrix is a major difference between epithelial tissues and connectivetissues.Photo by Mark NielsenWhat relationship between epithelial tissues and connective tissues is important for thesurvival and function of epithelial tissues?4.4EPITHELIAL TISSUESOBJECTIVES•Describe the general features of epithelial tissues.•List the location, structure, and function of each different type of epithelial tissues.
Anepithelial tissue(ep‐i‐THĒ‐lē‐al) orepithelium(plural isepithelia) consists of cells arranged incontinuous sheets, in either single or multiple layers. Because the cells are closely packed and are held tightlytogether by many cell junctions, there is little intercellular space between adjacent plasma membranes.Epithelial tissues form coverings and linings throughout the body. They are rarely covered by another tissue,so they always have a free surface. Epithelial tissues have three major functions. They serve as (1) selectivebarriers that limit or aid the transfer of substances into and out of the body; (2) secretory surfaces that releaseproducts produced by the cells onto their free surfaces; and (3) protective surfaces that resist the abrasiveinfluences of the environment.The various surfaces of epithelial cells often differ in structure and have specialized functions. Theapical(free) surfaceof an epithelial cell faces the body surface, a body cavity, the lumen (interior space) of aninternal organ, or a tubular duct that receives cell secretions (Figure4.4). Apical surfaces may contain cilia ormicrovilli. Thelateral surfacesof an epithelial cell, which face the adjacent cells on either side, may containtight junctions, adherens junctions, desmosomes, and/or gap junctions. Thebasal surfaceof an epithelial cellis opposite the apical surface. The basal surfaces of the deepest layer of epithelial cells adhere to extracellularmaterials such as the basement membrane. Hemidesmosomes in the basal surfaces of the deepest layer ofepithelial cells anchor the epithelium to the basement membrane (described next). In discussing epithelia withmultiple layers, the termapical layerrefers to the most superficial layer of cells, and thebasal layeris thedeepest layer of cells.4.5CONNECTIVE TISSUESOBJECTIVES•Describe the general features of connective tissues.•Describe the structure, location, and function of the various types of connective tissues.Connective tissuesare one of the most abundant and widely distributed tissues in the body. In their variousforms, connective tissues have a variety of functions. They bind together, support, and strengthen other bodytissues; protect and insulate internal organs; compartmentalize structures such as skeletal muscles; serve as themajor transport system within the body (blood, a fluid connective tissue); are the primary locations of storedenergy reserves (adipose, or fat, tissue); and are the main source of immune responses.ExamplesAnatomy Overview: Connective TissuesGeneral Features of Connective TissuesConnective tissues consist of two basic elements: extracellular matrix and cells. A connectivetissue'sextracellular matrix(MĀ‐triks) is the material located between its widely spaced cells. Theextracellular matrix consists ofprotein fibersandground substance,the material between the cells and thefibers. The extracellular fibers are secreted by the connective tissue cells and account for many of thefunctional properties of the tissue in addition to controlling the surrounding watery environment via specificproteoglycan molecules (described shortly). The structure of the extracellular matrix determines much of thetissue's qualities. For instance, in cartilage, the extracellular matrix is firm but pliable. The extracellular matrixof bone, by contrast, is hard and inflexible.Recall that, in contrast to epithelial tissues, connective tissues do not usually occur on body surfaces. Also,unlike epithelial tissues, connective tissues usually are highly vascular; that is, they have a rich blood supply.
Exceptions include cartilage, which is avascular, and tendons, with a scanty blood supply. Except for cartilage,connective tissues, like epithelial tissues, are supplied with nerves.Connective Tissue CellsEmbryonic cells called mesenchymal cells give rise to the cells of connective tissues. Each major type ofconnective tissue contains an immature class of cells with a name ending in ‐blast,which means “to bud orsprout.” These immature cells are calledfibro blastsin loose and dense connective tissue (describedshortly),chondroblastsin cartilage, andosteoblastsin bone. Blast cells retain the capacity for cell division andsecrete the extracellular matrix that is characteristic of the tissue. In cartilage and bone, once the extracellularmatrix is produced, the immature cells differentiate into mature cells with names ending in ‐cyte,namely,chondrocytes and osteocytes. Mature cells have reduced capacities for cell division and extracellular matrixformation and are mostly involved in monitoring and maintaining the extracellular matrix.The types of cells in connective tissues vary according to the type of tissue and include the following(Figure4.8):1.Fibroblasts(FĪ‐brō‐blasts;fibro‐ = fibers) are large, flat cells with branching processes. They arepresent in all the general connective tissues, and usually are the most numerous. Fibroblasts migratethrough the connective tissues, secreting the fibers and certain components of the ground substance ofthe extracellular matrix.2.Macrophages(MAK‐rō‐fā‐jez;macro‐ = large; ‐phages= eaters) develop frommonocytes,a typeof white blood cell. Macrophages have an irregular shape with short branching projections and arecapable of engulfing bacteria and cellular debris by phagocytosis.Fixed macrophagesreside in aparticular tissue; examples include alveolar macrophages in the lungs or splenic macrophages in thespleen.Wandering macrophageshave the ability to move throughout the tissue and gather at sites ofinfection or inflammation to carry on phagocytosis.3.Plasma cellsare small cells that develop from a type of white blood cell called aB lymphocyte. Plasmacells secrete antibodies, proteins that attack or neutralize foreign substances in the body. Thus, plasmacells are an important part of the body's immune response. Although they are found in many places inthe body, most plasma cells reside in connective tissues, especially in the gastrointestinal andrespiratory tracts. They are also abundant in the salivary glands, lymph nodes, spleen, and red bonemarrow.4.Mast cellsare abundant alongside the blood vessels that supply connective tissue. They producehistamine, a chemical that dilates small blood vessels as part of the inflammatory response, the body'sreaction to injury or infection. In addition, researchers have recently discovered that mast cells can bindto, ingest, and kill bacteria.5.Adipocytes(AD‐i‐pō‐sīts), also called fat cells oradiposecells, are connective tissue cells that storetriglycerides (fats). They are found deep to the skin and around organs such as the heart and kidneys.6.White blood cellsare not found in significant numbers in normal connective tissues. However, inresponse to certain conditions they migrate from blood into connective tissues. Forexample,neutrophilsgather at sites of infection, andeosinophilsmigrate to sites of parasitic invasionsand allergic responses.Figure4.8Representative cells and fibers present in connective tissues.Fibroblasts are usually the most numerous connective tissue cells.Imagineering; Photo by Mark NielsenWhat is the function of fibroblasts?
Connective Tissue Extracellular MatrixEach type of connective tissue has unique properties, based on the specific extracellular materials between thecells. The extracellular matrix consists of two major components: (1) the ground substance and (2) the fibers.Ground SubstanceAs noted earlier, theground substanceis 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 activerole in how tissues develop, migrate, proliferate, and change shape, and in how they carry out their metabolicfunctions.Ground substance contains water and an assortment of large organic molecules, many of which are complexcombinations of polysaccharides and proteins. The polysaccharides include hyaluronic acid, chondroitinsulfate, dermatan sulfate, and keratan sulfate. Collectively, they are referred to asglycosaminoglycans(glī‐kōs‐a‐mē′‐nō‐GLĪ‐kans) orGAGs.Except for hyaluronic acid, the GAGs are associated with proteinscalledproteoglycans(prō‐tē‐ō‐GLĪ‐kans). The proteoglycans form a core protein and the GAGs projectfrom the protein like the bristles of a brush. One of the most important properties of GAGs is that they trapwater, making the ground substance more jellylike.Hyaluronic acid(hī′‐a‐loo‐RON‐ik) is a viscous, slippery substance that binds cells together, lubricatesjoints, and helps maintain the shape of the eyeballs. White blood cells, sperm cells, and some bacteriaproducehyaluronidase,an enzyme that breaks apart hyaluronic acid, thus causing the ground substance ofconnective tissue to become more liquid. The ability to produce hyaluronidase helps white blood cells movemore easily through connective tissues to reach sites of infection and aids penetration of an oocyte by a spermcell during fertilization. It also accounts for the rapid spread of bacteria through connectivetissues.Chondroitin sulfate(kon‐DROY‐tin) provides support and adhesiveness in cartilage, bone, skin,and blood vessels. The skin, tendons, blood vessels, and heart valves containdermatan sulfate;bone,cartilage, and the cornea of the eye containkeratan sulfate.Also present in the ground substanceareadhesion proteins,which are responsible for linking components of the ground substance to one anotherand to the surfaces of cells. The main adhesion protein of connective tissues isfibronectin,which binds toboth collagen fibers (discussed shortly) and ground substance, linking them together. Fibronectin also attachescells to the ground substance.CLINICAL CONNECTIONChondroitin Sulfate, Glucosamine, and JointDiseaseIn recent years,chondroitin sulfateandglucosamine(a proteoglycan) have been used as nutritionalsupplements either alone or in combination to promote and maintain the structure and function of jointcartilage, to provide pain relief from osteoarthritis, and to reduce joint inflammation. Although thesesupplements have benefited some individuals with moderate to severe osteoarthritis, the benefit is minimalin lesser cases. More research is needed to determine how they act and why they help some people and notothers.FibersThree types offibersare embedded in the extracellular matrix between the cells: collagen fibers, elastic fibers,and reticular fibers (Figure4.8). They function to strengthen and support connective tissues.Collagen fibers(KOL‐a‐jen;colla= glue) are very strong and resist pulling forces (tension), but they arenot stiff, which allows tissue flexibility. The properties of different types of collagen fibers vary from tissue totissue. For example, the collagen fibers found in cartilage and bone form different associations withsurrounding molecules. As a result of these associations, the collagen fibers in cartilage are surrounded bymore water molecules than those in bone, which gives cartilage a more cushioning effect. Collagen fibers often