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The major cavities of the body have membranes that line both the internal surfaces of the cavities and the external surfaces of some of the viscera housed within those cavities. We discuss these membranes here because they consist of an epithelial sheet and an underlying connective tissue layer. The four types of body membranes are mucous, serous, cutaneous, and synovial membranes. The two principal kinds of internal membranes are mucous and serous membranes. A mucous membrane, also called a mucosa (myū-kō′să), lines body passageways and compartments that eventually open to the external environment; these include the digestive, respiratory, reproductive, and urinary tracts. Mucous membranes perform absorptive, protective, and/or secretory functions. A mucous membrane is composed of an epithelium and underlying connective tissue called the lamina propria. Often, it is covered with a thin layer of mucus derived from goblet cells, multicellular mucous glands, or both. The mucus prevents the underlying layer of cells from drying out (a process called desiccation), provides lubrication, and traps bacteria and foreign particles to prevent them from invading the body.

A serous membrane, also termed a serosa (se-rō′să), is composed of a simple squamous epithelium called mesothelium and a thin underlying layer of loose connective tissue. The mesothelium is so named because it is derived from mesoderm. Serous membranes produce a thin, watery serous fluid, or transudate (tranz′yū-dāt; trans = across, sudo = to sweat), which is derived from blood plasma. Serous membranes are composed of two parts: a parietal layer that lines the body cavity and a visceral layer that covers organs. The parietal and visceral layers are in close contact; a thin layer of serous fluid between them reduces the friction between their opposing surfaces. Examples of serous membranes include the pericardium, the peritoneum, and the pleura. The largest body membrane is the cutaneous (kyū-tā′nē-ŭs; cutis = skin) membrane, more commonly called the skin. The cutaneous membrane is composed of a keratinized stratified squamous epithelium (called the epidermis) and a layer of connective tissue (termed the dermis) upon which the epithelium rests (see sections 5.2 and 5.3). It differs from the other membranes discussed so far in that it is relatively dry. Its many functions include protecting internal organs and preventing water loss. Some joints of the skeletal system are lined by a synovial (si-nō′vē-ăl; syn = together, ovum = egg) membrane, which is composed of a well-vascularized areolar, fibrous, or adipose connective tissue under a superficial highly cellular lining. Some of the cells secrete a synovial fluid that reduces friction in the joint cavity and distributes nutrients to the cartilage on the joint surfaces of the bone.

Fat is a primary energy store for the body. The amount of stored fat fluctuates as the adipose cells either increase their amount of stored fat (called lipogenesis) or decrease their amount of stored fat (called lipolysis). But although there is a constant turnover of the stored fat, an equilibrium is usually reached, and the amount of stored fat and the number of adipocytes are normally quite stable in an individual. Although adipocytes cannot divide, mesenchymal cells can provide additional fat cells if the body has excess nutrients. Thus, even after a surgical procedure to reduce the amount of body fat, such as liposuction, the mesenchymal stem cells may replace adipocytes to store excess fat in the body. Reticular connective tissue contains a meshwork of reticular fibers, fibroblasts, and leukocytes (table 4.7c). This connective tissue forms the stroma of many lymphatic organs, such as the spleen, thymus, lymph nodes, and bone marrow.

Dense regular connective tissue has collagen fibers that are packed tightly and aligned parallel to an applied force (table 4.8a). This tissue type is found in tendons and ligaments, where stress is applied in a single direction. Dense regular connective tissue has few blood vessels, and thus it takes a long time to heal following injury, because a rich blood supply is necessary for good healing. In dense irregular connective tissue, individual bundles of collagen fibers extend in all directions in a scattered meshwork. These bundles of collagen fibers appear in clumps throughout the tissue, rather than arranged in parallel as seen in dense regular connective tissue (table 4.8b). Dense irregular connective tissue provides support and resistance to stress in multiple directions.

The cells in nonkeratinized stratified squamous epithelium remain alive all the way to its apical surface, and they are kept Page 91moist with secretions such as saliva or mucus. These cells lack keratin. Because all of the cells are still alive, the flattened nuclei characteristic of squamous cells are visible even in the most superficial cells (table 4.3b). Nonkeratinized stratified squamous epithelium lines the vagina, the oral cavity (mouth), part of the pharynx (throat), the esophagus, and the anus. A stratified cuboidal epithelium contains two or more layers of cells, and the apical cells tend to be cuboidal in shape (table 4.3c). This type of epithelium forms the walls of the larger ducts of most exocrine glands, such as the sweat glands in the skin. Although the function of stratified cuboidal epithelium is mainly protective, it also serves to strengthen the wall of gland ducts.

A transitional epithelium varies in appearance, depending upon whether it is in a relaxed or a stretched state (table 4.3e). In a relaxed state, the basal cells appear almost cuboidal, and the apical cells are large and rounded. During stretching, the transitional epithelium thins, and the apical cells continue to flatten, becoming almost squamous. In this distended state, it may be difficult to distinguish a transitional epithelium from a squamous epithelium. However, one distinguishing feature of transitional epithelium is the presence of a handful of binucleated (double-nucleus-containing) cells. This epithelium lines the urinary bladder, an organ that changes shape as it fills with urine. It also lines the ureters and the proximal part of the urethra. Transitional epithelium permits stretching and ensures that toxic urine does not seep into the underlying tissues and structures of these organs

Epithelial cells are strongly bound together by specialized connections in the plasma membranes of their lateral surfaces called intercellular junctions. There are four types of junctions: tight junctions, adhering junctions, desmosomes, and gap junctions (figure 4.1b). Each of these types of junctions has a specialized structure. A tight junction, also called a zonula (zō′nyū-lă) occludens ("occluding belt"), encircles epithelial cells near their apical surface and completely attaches each cell to its neighbors. Plasma membrane proteins among neighboring cells fuse, so the apical surfaces of the cells are tightly connected everywhere around the cell. This seals off the intercellular space and prevents substances from passing between the epithelial cells. The tight junction forces almost all materials to move through, rather than between, the epithelial cells in order to cross the epithelium. Thus, epithelia control whatever enters and leaves the body. For example, in the small intestine, tight junctions prevent digestive enzymes that degrade molecules from moving between epithelial cells into underlying connective tissue.

An adhering junction, also called a zonula adherens ("adhesion belt"), is formed completely around the cell. This type of junction occurs when extensive zones of microfilaments extend from the cytoplasm into the plasma membrane, forming a supporting and strengthening belt within the plasma membrane that completely encircles the cell immediately adjacent to all of its neighbors. Typically, adhering junctions are located deep to the tight junctions; the anchoring of the microfilament proteins within this belt provides the only means of junctional support for the apical surface of the cell. The ultra-strong tight junctions are needed only near the apical surface and not along the entire length of the cell. Once neighboring cells are fused together by the tight junctions near the apical surface, the adhering junctions support the apical surface and provide for a small space between neighboring cells in the direction of the basal surface. Thus, the junction affords a passageway between cells for materials that have already passed through the apical surface of the epithelial cell and can then exit through the membranes on the lateral surface and continue their journey toward the basement membrane.

Reticular Connective Tissue Structure Ground substance is gel-like liquid; scattered arrangement of reticular fibers, fibroblasts, and leukocytes Function Provides supportive framework for spleen, lymph nodes, thymus, bone marrow

Areolar connective tissue is found nearly everywhere in the body. It surrounds nerves, blood vessels, and individual muscle cells. It is also a major component of the subcutaneous layer deep to the skin. Adipose connective tissue (commonly known as "fat") is a loose connective tissue composed primarily of cells called adipocytes (table 4.7b). Adipocytes (white fat) usually range from 70 μm to 120 μm in diameter. In life, adipocytes are filled with one lipid droplet. On a histology slide, the lipid has been extracted during preparation, so all that is left is the plasma membrane of the adipocyte, with the nucleus pushed to the side of a round, clear space looking much like a ring.

Mast cells. These small, mobile cells contain a granule-filled cytoplasm. They are usually found close to blood vessels; they secrete heparin to inhibit blood clotting, and histamine to dilate blood vessels and increase blood flow. Loose connective tissue contains relatively fewer cells and protein fibers than dense connective tissue. The protein fibers in loose connective tissue are loosely arranged rather than tightly packed together. Usually, this tissue occupies the spaces between and around organs. Loose connective tissues support the overlying epithelia and provide cushioning around organs, support and surround blood vessels and nerves, store lipids, and provide a medium for the diffusion of materials. Thus, loose connective tissues act as the body's "packing material." There are three types of loose connective tissue: areolar connective tissue, adipose connective tissue, and reticular connective tissue

As mentioned previously, the three types of protein fibers in connective tissue proper are collagen fibers, elastic fibers, and reticular fibers. Fibroblasts synthesize the components of all three fiber types, and then secrete these protein subunits into the interstitial fluid. The subunits combine or aggregate within the matrix and form the completed fiber. Collagen (kol′lă-jen; koila = glue, gen = producing) fibers are long, unbranched, "cablelike" extracellular fibers composed of the protein collagen. They are strong, flexible, and resistant to stretching. Collagen forms about 25% of the body's protein, making it the most abundant protein in the body. In fresh tissue, collagen fibers appear white, and thus they are often called white fibers. In tissue sections stained with hematoxylin and eosin to give contrast, they appear Page 100pink. In tissue sections, collagen forms coarse, sometimes wavy bundles. The parallel structure and arrangement of collagen bundles in tendons and ligaments allows them to withstand enormous forces in one direction.

Physical protection. Epithelial tissues protect both exposed and internal surfaces from dehydration, abrasion, and destruction by physical, chemical, or biological agents.

Because epithelial tissues are located at all free surfaces in the body, they exhibit distinct structural specializations. An epithelium rests on a layer of connective tissue and adheres firmly to it which secures the epithelium in place and prevents it from tearing. Between the epithelium and the underlying connective tissue is a thin extracellular layer called the basement membrane. The basement membrane can be seen as a single layer beneath epithelium using the light microscope (figure 4.1a). However, it is shown to consist of three layers when examined using an electron microscope: the lamina lucida, the lamina densa, and the reticular lamina. The two laminae closest to the epithelium contain collagen fibers as well as specific proteins and carbohydrates some of which are secreted by the epithelial cells. Cells in the underlying connective tissue secrete the reticular lamina, which contains protein fibers and carbohydrates. Together, these components of the basement membrane strengthen the attachment and form a selective molecular barrier between the epithelium and the underlying connective tissue.

Elastic cartilage is so named because it contains numerous elastic fibers in its matrix (table 4.9c). The higher concentration of elastic fibers in this cartilage causes it to appear yellow in fresh sections. The chondrocytes of elastic cartilage are almost indistinguishable from those of hyaline cartilage. They are typically closely Page 104packed and surrounded by only a small amount of extracellular matrix. The elastic fibers are both denser and more highly branched in the central region of the extracellular matrix, where they form a weblike mesh around the chondrocytes within the lacunae. These fibers ensure that elastic cartilage is extremely resilient and flexible. Elastic cartilage is surrounded by a perichondrium.

Bone connective tissue (or osseous connective tissue) makes up the mass of most of the body structures referred to as "bones." Bone is more solid than cartilage and provides greater support. Section 6.3 provides a detailed description of the histology of bone connective tissue. About one-third of the dry weight of bone is composed of organic components (collagen fibers and different protein-carbohydrate molecules), and two-thirds consists of inorganic components (a mixture of calcium salts, primarily calcium phosphate). Bone derives its remarkable properties from its combination of components: Its organic portions provide some flexibility and tensile strength, and its inorganic portions provide compressional strength. The minerals are deposited onto the collagen fibers, resulting in a structure that is strong and durable but not brittle. Almost all bone surfaces (except for the surfaces of the joints of long bones) are covered by a dense irregular connective tissue called the periosteum (per′ē-os′tē-ŭm; osteon = bone), which is similar to the perichondrium of cartilage.

Structure Compact bone: Calcified matrix arranged in osteons (concentric lamellae arranged around a central canal containing blood vessels) Spongy bone: Lacks the organization of compact bone; contains macroscopic spaces; bone arranged in a meshwork pattern Function Supports soft structures; protects vital organs; provides levers for movement; stores calcium and phosphorus. Spongy bone is the site of hemopoiesis. Location Bones of the body

Bone serves a variety of functions. Bones provide levers for movement when the muscles attached to them contract, and they protect soft tissues and vital body organs. The hard matrix of bone stores important minerals, such as calcium and phosphorus. Finally, many areas of spongy bone contain hemopoietic (hē′mō-poy-et′ik; hemat = blood) cells, which form reticular connective tissue that is responsible for producing blood cells (a process called hemopoiesis). Thus, the connective tissue that produces our blood cells is stored within our spongy bone. Fluid Connective Tissue There are two types of fluid connective tissue: blood and lymph. Blood is a fluid connective tissue composed in part of cells and cell fragments called formed elements. These formed elements are erythrocytes (red blood cells), leukocytes (white blood cells), and platelets (table 4.11). The erythrocytes transport oxygen and carbon dioxide between the lungs and the body tissues, while some leukocytes mount an adaptive immune response and others respond to foreign pathogens such as bacteria, viruses, fungi, and parasites. Platelets are involved in blood clotting.

Although the types of connective tissue are diverse, all of them share three basic components: cells, protein fibers, and ground substance (figure 4.8). Their diversity is due to varying proportions of these components as well as to differences in the types and amounts of protein fibers. Each type of connective tissue contains specific types of cells. For example, connective tissue proper contains fibroblasts, fat contains adipocytes, cartilage contains chondrocytes, and bone contains Page 96osteocytes. Most connective tissue cells are not in direct contact with each other, but are scattered throughout the tissue. This differs markedly from epithelial tissue, whose cells crowd closely together with little to no extracellular matrix surrounding them.

Both the cells and the protein fibers reside within a material called ground substance. This nonliving material is produced by the connective tissue cells. It primarily consists of protein and carbohydrate molecules and variable amounts of water. The ground substance may be viscous (as in blood), semisolid (as in cartilage), or solid (as in bone). Together, the ground substance and the protein fibers form an extracellular matrix. Most connective tissues are composed primarily of an extracellular matrix, with relatively small proportions of cells.

Classification of Epithelia. Two criteria are used to classify epithelia: the number of cell layers and the shape of the cell at the apical surface. (a) An epithelium is simple if it is one cell layer thick, and stratified if it has two or more layers of cells. (b) Epithelial cell shapes include squamous (thin, flattened cells), cuboidal (cells about as tall as they are wide), and columnar (cells taller than they are wide). A simple squamous epithelium consists of a single layer of flattened cells (table 4.2a). When viewed "en face," the irregularly shaped cells Page 85display a spherical to oval nucleus, and they appear tightly bound together. Each squamous cell resembles a fried egg, with the nucleus representing the yolk. This epithelium is extremely delicate and highly specialized to allow rapid movement of molecules and ions across its surface by membrane transport processes (see section 2.3c). Simple squamous epithelium is found only in protected regions where moist surfaces reduce friction and abrasion. For example, in the lining of the lung air sacs (alveoli), the thin epithelium is well suited for the exchange of oxygen and carbon dioxide between the blood and inhaled air. This type of epithelium is also found lining the lumen (inside space) of blood vessel walls, where it allows for rapid exchange of nutrients and waste between the blood and the interstitial fluid surrounding the blood vessels.

Epithelia are also classified by the shape of the cell at the apical surface. In a simple epithelium, all of the cells display the same shape. However, in a stratified epithelium a difference in shape can be seen between cells in the basal layer and those within the apical layer. Figure 4.2b shows the three common cell shapes seen in epithelia: squamous, cuboidal, and columnar. (Note: All cells in this figure appear hexagonal when looking at their apical surface, or "en face"; thus these terms describe the cells' shapes when viewed laterally, or from the side.) Squamous (skwā′mŭs; squamosus = scaly) cells are flat, wide, and somewhat irregular in shape. The nucleus looks like a flattened disc. The cells are arranged like irregular, flattened floor tiles. Cuboidal (kū-boy′dăl; kybos = cube, eidos = resemblance) cells are about as tall as they are wide. The cells do not resemble perfect "cubes," because their edges are rounded. The cell nucleus is spherical and located within the center of the cell. Columnar (kō-lŭm′năr; columna = column) cells are slender and taller than they are wide. The cells look like a group of hexagonal columns aligned next to each other. Each cell nucleus is oval and usually oriented lengthwise and located in the basal region of the cell.

A desmosome (dez′mō-sōm; desmos = a band, soma = body), also called a macula adherens ("adhering spot"), is like a button or snap between adjacent epithelial cells. Each cell contributes half of the complete desmosome. It is a small region that holds cells together and provides resistance to mechanical stress at a single point, but it does not totally encircle the cell. In contrast to tight junctions, which encircle the cell to secure it to its neighbors everywhere around its periphery, the desmosome attaches a cell to its neighbors only at potential stress points. The neighboring cells are separated by a small space that is spanned by a fine web of protein filaments. These filaments anchor into a thickened protein plaque located at the internal surface of the plasma membrane. On the cytoplasmic side of each plaque, intermediate Page 83filaments of the cytoskeleton penetrate the plaque to extend throughout the cell the support and strength supplied between the cells by the desmosome. The basal cells of some epithelial tissue exhibit structures called hemidesmosomes, half-desmosomes that anchor them to the underlying basement membrane.

Epithelia may be classified as either simple or stratified (figure 4.2a). A simple epithelium is one cell layer thick, and all of the epithelial cells are in direct contact with the basement membrane. A simple epithelium is found in areas where stress is minimal and where filtration, absorption, or secretion is the primary function. Examples of these locations include the linings of the air sacs in the lungs, intestines, and blood vessels. A pseudostratified (sū′do-strat′i-fīd; pseudes = false, stratum = layer) epithelium appears layered (stratified) because the cells' nuclei are distributed at different levels between the apical and basal surfaces. Although all of these epithelial cells are attached to the basement membrane, some of them do not reach its apical surface. Because all the cells are attached to the basement membrane, we have classified pseudostratified epithelium as a type of simple epithelium. A ciliated pseudostratified epithelium lines the nasal cavity and the respiratory passageways.

Elastic connective tissue has branching elastic fibers and more fibroblasts than loose connective tissue in addition to packed collagen fibers (table 4.8c). The elastic fibers provide resilience and the ability to deform and then return to normal shape. Examples of structures composed of elastic connective tissue are the vocal cords, the suspensory ligament of the penis, and some ligaments of the spinal column. Elastic connective tissue also is present as wavy sheets in the walls of large and medium arteries.

Hyaline (hī′ă-lin, -lēn; hyalos = glass) cartilage is the most common type of cartilage and also the weakest. It provides its support through flexibility and resilience. Hyaline cartilage is surrounded by perichondrium. It is named for its clear, glassy appearance under the microscope. The chondrocytes within their lacunae are irregularly scattered throughout the extracellular matrix (table 4.9a). However, the collagen within the matrix is not readily seen by light microscopy. If the hyaline cartilage tissue is stained by hematoxylin and eosin and then examined under the microscrope, the tissue resembles carbonated grape soda, where the lacunae represent the bubbles in the soda.

(a) Simple glands have unbranched ducts, whereas (b) compound glands have ducts that branch. These glands also exhibit different forms: Tubular glands have secretory cells in a space with a uniform diameter, acinar glands have secretory cells arranged in saclike acini, and tubuloacinar glands have secretory cells in both acinar and tubular regions. Secretion Types Exocrine glands are classified by the nature of their secretions as serous glands, mucous glands, or mixed glands. Serous (sē′rŭs; serum = whey) glands produce and secrete a nonviscous, watery fluid, such as sweat, milk, tears, or digestive juices. This fluid carries wastes (sweat) to the surface of the skin, nutrients (milk) to a nursing infant, or digestive enzymes from the pancreas to the lumen of the small intestine. Mucous (myū′kŭs) glands secrete mucin, which forms mucus when mixed with water. Mucous glands occur in such places as the roof of the oral cavity and the surface of the tongue. Mixed glands, such as the two pairs of salivary glands inferior to the oral cavity, contain both serous and mucous cells, and produce a mixture of the two types of secretions.

Merocrine (mer′ō-krin; meros = share) glands package their secretions in structures called secretory vesicles and release their secretion by exocytosis. The cells remain intact and are not damaged in any way by producing the secretion. Lacrimal (tear) glands, salivary glands, sweat glands, the exocrine glands of the pancreas, and the gastric glands of the stomach are examples of merocrine glands. Some merocrine glands are also called eccrine glands, to denote a type of sweat gland in the skin that is not connected to a hair follicle (see section 5.5c). Holocrine (hol′ō-krin; holos = whole) glands are formed from cells that accumulate a product and then the entire cell disintegrates. Thus, a holocrine secretion is a viscous mixture of cell fragments and the product the cell synthesized prior to its destruction. The ruptured, dead cells are continuously replaced by other epithelial cells undergoing mitosis. The oil-producing glands (sebaceous glands) in the skin are an example of holocrine glands. (So the oily secretion you feel on your skin is actually composed of ruptured, dead cells!) Apocrine (ap′ō-krin; apo = away from or off) glands are composed of cells that accumulate their secretory products within the apical portion of their cytoplasm. Secretion occurs when the cell's apical portion pinches off, releasing cytoplasmic content. Thereafter, the cell repairs itself in order to repeat its secretory activity. Mammary glands and ceruminous glands are apocrine glands.

Glands are classified as either endocrine or exocrine, depending upon whether they have a duct connecting the secretory cells to the surface of an epithelium. Multicellular exocrine glands are composed of numerous cells that work together to produce a secretion and secrete it onto the surface of an epithelium. The gland often consists of acini (as′i-nī; sing., as′i-nŭs; acinus = grape), sacs that produce the secretion, and one or more smaller ducts, which merge to eventually form a larger duct that transports the secretion to the epithelial surface (figure 4.5). Acini are the secretory portions, and ducts are the conducting portions of these glands.

Multicellular exocrine glands may be classified according to three criteria: (1) form and structure (morphology), which is considered an anatomic classification; (2) type of secretion; and (3) method of secretion. The latter two are considered physiologic classifications. Exocrine glands are also classified according to the shape or organization of their secretory portions. If the secretory portion and the duct are of uniform diameter, the gland is called tubular. If the secretory cells form an expanded sac, the gland is called acinar (as′i-năr). Finally, a gland with both secretory tubules and secretory acini is called a tubuloacinar gland. Figure 4.6 shows the several types of exocrine glands as classified by morphology.

Two classes of cells form the connective tissue proper: resident cells and wandering cells Page 99(table 4.5). Resident cells are stationary cells permanently contained within the connective tissue. They help support, maintain, and repair the extracellular matrix. Wandering cells continuously move throughout the connective tissue and are involved in immune protection and repair of damaged extracellular matrix. The number of wandering cells at any given moment varies depending on local conditions.

Resident Cells Maintain and repair extracellular matrix; store materials Fibroblasts Abundant, large, relatively flat cells, often with tapered ends Produce fibers and ground substance of the extracellular matrix Adipocytes Fat cells with a single large lipid droplet; cellular components pushed to one side

Neurons are specialized to detect stimuli, process information quickly, and rapidly transmit electrical impulses from one region of the body to another. Each neuron has a prominent cell body, or soma, that houses the nucleus and most other organelles. The cell body is the "head" that controls the rest of the cell and produces proteins for the cell. Extending from the cell body are branches called nerve cell processes. The short, branched processes are dendrites (den′drītēs; dendrites = relating to a tree), which receive incoming signals from other cells and transmit the information to the cell body. The long nerve cell process extending from a cell body is the axon (ak′son; axon = axis), which carries outgoing signals to other cells. Due to the length of an axon, neurons are usually the longest cells in the body; some are longer than a meter. Much of the nervous tissue in the body is concentrated in the brain and spinal cord, the control centers for the nervous system.

Simple squamous epithelia that line closed internal body cavities and all circulatory structures have special names. The simple squamous epithelium that lines the lumen of the blood and lymphatic vessels and the heart and its chambers is termed endothelium (en′dō-thē′lē-ŭm; endon = within, thele = nipple). Mesothelium (mez′ō-thē′lē-ŭm; mesos = middle) is the simple squamous epithelium of the serous membrane (discussed in section 1.4e) that lines the internal walls of the pericardial, pleural, and peritoneal cavities as well as the external surfaces of the organs within those cavities. Mesothelium gets its name from the primary germ layer mesoderm, from which it is derived. Simple Cuboidal Epithelium A simple cuboidal epithelium consists of a single layer of cells that are as tall as they are wide (table 4.2b). A spherical nucleus is located in the center of the cell. A simple cuboidal epithelium functions primarily for absorption and secretion. It forms the walls of kidney tubules, where it participates in the reabsorption of nutrients, ions, and water that are filtered out of the blood plasma. It also forms the ducts of exocrine glands, which secrete materials. Simple cuboidal epithelium covers the surface of the ovary and also lines the follicles of the thyroid gland.

Muscle tissue is composed of specialized cells (fibers) that respond to stimulation from the nervous system by undergoing internal changes that cause them to shorten. As muscle tissue shortens, it exerts physical forces on other tissues and organs to produce movement; these movements include voluntary motion of body parts, blood circulation, respiratory activities, propulsion of materials through the digestive tract, and waste elimination. To perform these functions, muscle cells are very different from typical cells with respect to their cellular organization, cellular organelles, and other properties.

Skeletal muscle tissue is composed of cylindrical muscle cells called muscle fibers (table 4.12a). Individual skeletal muscle cells are slender and often long (sometimes the length of the entire muscle). Such long cells need more than one nucleus to control and carry out all cellular functions, so each skeletal muscle fiber is multinucleated; some contain hundreds of nuclei. These multiple nuclei form when smaller embryonic muscle cells fuse early in the development of the skeletal muscle fiber. The nuclei in skeletal muscle fibers are located at the edge of the cell (called the periphery), immediately internal to the plasma membrane.

Cardiac muscle tissue is confined to the thick middle layer of the heart wall (called the myocardium). Microscopically, cardiac muscle tissue resembles skeletal muscle in that both contain visible striations (table 4.12b). However, several obvious cellular differences distinguish the two types. First, the typical cardiac muscle cell is much shorter than a typical skeletal muscle fiber. Second, a cardiac muscle cell contains only one or two centrally located nuclei. Third, the cardiac muscle cell often bifurcates (branches), thus resembling a Y in shape. Finally, cardiac muscle cells are connected by intercalated discs (in-ter′kă-lā-tĕd; intercalates = inserted between), which have strong desmosomes and gap junctions between the cells. The intercalated discs promote the rapid transport of an electrical activity through many cardiac muscle cells at once, allowing the entire muscle wall to contract as a unit. When you view cardiac muscle tissue under the microscope, the intercalated discs appear as thick, dark lines between the cells.

Smooth muscle tissue is so named because it lacks the striations seen in the other two types of muscle tissue, so the cells appear smooth (table 4.12c). Smooth muscle tissue is also called visceral muscle tissue because it is found in the walls of most viscera, such as the stomach, urinary bladder, and blood vessels. The contraction of smooth muscle helps propel and control the movement of material through these organs. Smooth muscle cells are fusiform (spindle-shaped), which means they are thick in the middle and tapered at their ends. The cells are also relatively short. Each cell has one centrally placed nucleus. Smooth muscle is considered involuntary because we do not have voluntary control over it. For example, you cannot voluntarily stop your stomach from digesting your food or your blood vessels from transporting your blood

Nervous tissue is sometimes termed neural tissue. It consists of cells called neurons (nū′ron), or nerve cells, and a larger number of different types of glial cells (or supporting cells) that support, protect, and provide a framework for neurons

Structure Contains neurons with rounded or stellate cell bodies and an axon and dendrites extending from the cell body; glial cells lack such extensive fibrous processes Function Neurons: Responsible for control; information processing, storage, and retrieval; internal communication Glial cells: Support and protect neurons Location Brain, spinal cord, nerves

The primary germ layer mesoderm forms all connective tissues. There are two types of embryonic connective tissue: mesenchyme and mucous connective tissue. In the developing embryo, mesenchyme (mes′eng-kīm; mesos = middle, enkyma = infusion) is the first type of connective tissue to emerge. It has star-shaped (stellate) or spindle-shaped mesenchymal cells dispersed within a gel-like ground substance that contains fine, immature protein fibers (table 4.4a). In fact, there is proportionately more ground substance than mesenchymal cells in this type of embryonic connective tissue. Mesenchyme is the source of all other connective tissues. Adult connective tissues often house numerous mesenchymal (stem) cells that support the repair of the tissue following damage or injury.

The connective tissue types present after birth are classified into three broad categories: connective tissue proper, supporting connective tissue, and fluid connective tissue. Figure 4.9 provides an overview of these tissue types and the subcategories within them, each of which is described in detail next.

Epithelial tissue Cellular, polar, attached, avascular, Epithelial (ep′i-thē′lē-ăl; epi = upon, thēlē = nipple) tissue covers or lines every body surface and all body cavities; thus it forms both the external and internal lining of many organs, and it constitutes the majority of glands. An epithelium (pl., epithelia) is composed of one or more layers of closely packed cells between two compartments having different components. There is little to no extracellular matrix between epithelial cells; additionally, no blood vessels penetrate an epithelium. Cellularity. Epithelial tissue is composed almost entirely of cells. The cells of an epithelium are bound closely together by different types of intercellular junctions. A minimal amount of extracellular matrix separates the cells. Polarity. Every epithelium has an apical (ap′i-kăl) surface (free or top surface), which is exposed either to the external environment or to some internal body space. Lateral surfaces have intercellular junctions (see section 4.1c). Additionally, each epithelium has a basal (bā′săl) surface (fixed or bottom surface) where the epithelium is attached to the underlying connective tissue.

Tissues are groups of similar cells and extracellular products that carry out a common function, such as providing protection or facilitating body movement. The study of tissues and their relationships within organs is called histology. There are four principal types of tissues in the body: epithelial tissue, connective tissue, muscle tissue, and nervous tissue. Tissues are formed from the three primary germ layers (ectoderm, mesoderm, and endoderm). The four tissue types vary in terms of the structure and function of their specialized cells, as well as the presence of an extracellular matrix (mā′triks; matrix = womb) that not only is produced by the cells but also surrounds them. The extracellular matrix is composed of varying amounts of water, protein fibers, and dissolved molecules (e.g., glucose, oxygen). Its consistency ranges from fluid to quite solid. Epithelial, muscle, and nervous tissues have relatively little extracellular matrix. In contrast, connective tissue types contain varying amounts of extracellular matrix that differ in the volume of space occupied, the relative amounts of the extracellular matrix components, and the consistency (fluid to solid) of the extracellular matrix. A discussion of body membranes (see section 4.3) immediately follows the connective tissue section because these structures are composed of an epithelial sheet and an underlying connective tissue layer.