Tissues of the Body

Despite the amazing variety of intracellular structures, no single cell can carry out all the processes of the mature human body. Instead, cells assemble into the larger units we call tissues. The cells in tissues are held together by specialized connections called cell junctions and by other support structures. Tissues range in complexity from simple tissues containing only one cell type, such as the lining of blood vessels, to complex tissues containing many cell types and extensive extracellular material, such as connective tissue. The cells of most tissues work together to achieve a common purpose.

The study of tissue structure and function is known as histology {histos, tissue}. Histologists describe tissues by their physical features: (1) the shape and size of the cells, (2) the arrangement of the cells in the tissue (in layers, scattered, and so on), (3) the way cells are connected to one another, and (4) the amount of extracellular material present in the tissue. There are four primary tissue types in the human body: epithelial, connective, muscle, and neural, or nerve. Before we consider each tissue type specifically, let’s examine how cells link together to form tissues.

Extracellular Matrix Has Many Functions

Extracellular matrix (usually just called matrix) is extracellular material that is synthesized and secreted by the cells of a tissue. For years, scientists believed that matrix was an inert substance whose only function was to hold cells together. However, experimental evidence now shows that the extracellular matrix plays a vital role in many physiological processes, ranging from growth and development to cell death. A number of disease states are associated with overproduction or disruption of extracellular matrix, including chronic heart failure and the spread of cancerous cells throughout the body (metastasis).

The composition of extracellular matrix varies from tissue to tissue, and the mechanical properties, such as elasticity and flexibility, of a tissue depend on the amount and consistency of the tissue’s matrix. Matrix always has two basic components: proteoglycans and insoluble protein fibers. Proteoglycans are glycoproteins, which are proteins covalently bound to polysaccharide chains [here]. Insoluble protein fibers such as collagen, fibronectin, and laminin provide strength and anchor cells to the matrix. Attachments between the extracellular matrix and proteins in the cell membrane or the cytoskeleton are ways cells communicate with their external environment.

The amount of extracellular matrix in a tissue is highly variable. Nerve and muscle tissue have very little matrix, but the connective tissues, such as cartilage, bone, and blood, have extensive matrix that occupies as much volume as their cells. The consistency of extracellular matrix can vary from watery (blood and lymph) to rigid (bone).

Cell Junctions Hold Cells Together to Form Tissues

During growth and development, cells form cell-cell adhesions that may be transient or that may develop into more permanent cell junctions. Cell adhesion molecules (CAMs) are membrane-spanning proteins responsible both for cell junctions and for transient cell adhesions (TBL. 3.3). Cell-cell or cell-matrix adhesions mediated by CAMs are essential for normal growth and development. For example, growing nerve cells creep across the extracellular matrix with the help of nerve-cell adhesion molecules, or NCAMs. Cell adhesion helps white blood cells escape from the circulation and move into infected tissues, and it allows clumps of platelets to cling to damaged blood vessels. Because cell adhesions are not permanent, the bond between those CAMs and matrix is weak.

Table 3.3 Major Cell Adhesion Molecules (CAMs)

Name Examples
Cadherins Cell-cell junctions such as adherens junctions and desmosomes. Calcium-dependent.
Integrins Primarily found in cell-matrix junctions. These also function in cell signaling.
Immunoglobulin superfamily CAMs NCAMs (nerve-cell adhesion molecules). Responsible for nerve cell growth during nervous system development.
Selectins Temporary cell-cell adhesions.

Stronger cell junctions can be grouped into three broad categories by function: communicating junctions, occluding junctions {occludere, to close up}, and anchoring junctions (Fig. 3.8). In animals, the communicating junctions are gap junctions. The occluding junctions of vertebrates are tight junctions that limit movement of materials between cells. The three major types of junctions are described next.

  1. Gap junctions are the simplest cell-cell junctions (Fig.  3.8b). They allow direct and rapid cell-to-cell communication through cytoplasmic bridges between adjoining cells. Cylindrical proteins called connexins interlock to create passageways that look like hollow rivets with narrow channels through their centers. The channels are able to open and close, regulating the movement of small molecules and ions through them.

    Gap junctions allow both chemical and electrical signals to pass rapidly from one cell to the next. They were once thought to occur only in certain muscle and nerve cells, but we now know they are important in cell-to-cell communication in many tissues, including the liver, pancreas, ovary, and thyroid gland.

  2. Tight junctions are occluding junctions that restrict the movement of material between the cells they link (Fig. 3.8c). In tight junctions, the cell membranes of adjacent cells partly fuse together with the help of proteins called claudins and occludins, thereby making a barrier. As in many physiological processes, the barrier properties of tight junctions are dynamic and can be altered depending on the body’s needs. Tight junctions may have varying degrees of “leakiness.”

    Tight junctions in the intestinal tract and kidney prevent most substances from moving freely between the external and internal environments. In this way, they enable cells to regulate what enters and leaves the body. Tight junctions also create the so-called blood-brain barrier that prevents many potentially harmful substances in the blood from reaching the extracellular fluid of the brain.

  3. Anchoring junctions (Fig. 3.8d) attach cells to each other (cell-cell anchoring junctions) or to the extracellular matrix (cell-matrix anchoring junctions). In vertebrates, cell-cell anchoring junctions are created by CAMs called cadherins, which connect with one another across the intercellular space. Cell-matrix junctions use CAMs called integrins. Integrins are membrane proteins that can also bind to signal molecules in the cell’s environment, transferring information carried by the signal across the cell membrane into the cytoplasm.

Anchoring junctions contribute to the mechanical strength of the tissue. They have been compared to buttons or zippers that tie cells together and hold them in position within a tissue. Notice how the interlocking cadherin proteins in Figure 3.8d resemble the teeth of a zipper.

The protein linkage of anchoring cell junctions is very strong, allowing sheets of tissue in skin and lining body cavities to resist damage from stretching and twisting. Even the tough protein fibers of anchoring junctions can be broken, however. If you have shoes that rub against your skin, the stress can shear the proteins connecting the different skin layers. When fluid accumulates in the resulting space and the layers separate, a blister results.

Tissues held together with anchoring junctions are like a picket fence, where spaces between the pickets (the cells) allow materials to pass from one side of the fence to the other. Movement of materials between cells is known as the paracellular pathway. In contrast, tissues held together with tight junctions are more like a solid brick wall: Very little can pass from one side of the wall to the other between the bricks.

Cell-cell anchoring junctions take the form of either adherens junctions or desmosomes. Adherens junctions link actin fibers in adjacent cells together, as shown in the micrograph in Figure 3.8e. Desmosomes {desmos, band + soma, body} attach to intermediate filaments of the cytoskeleton. Desmosomes are the strongest cell-cell junctions. In electron micrographs they can be recognized by the dense glycoprotein bodies, or plaques, that lie just inside the cell membranes in the region where the two cells connect (Fig. 3.8d, e). Desmosomes may be small points of contact between two cells (spot desmosomes) or bands that encircle the entire cell (belt desmosomes).

There are also two types of cell-matrix anchoring junctions. Hemidesmosomes {hemi-, half} are strong junctions that anchor intermediate fibers of the cytoskeleton to fibrous matrix proteins such as laminin. Focal adhesions tie intracellular actin fibers to different matrix proteins, such as fibronectin.

The loss of normal cell junctions plays a role in a number of diseases and in metastasis. Diseases in which cell junctions are destroyed or fail to form can have disfiguring and painful symptoms, such as blistering skin. One such disease is pemphigus, a condition in which the body attacks some of its own cell junction proteins (www.pemphigus.org).

The disappearance of anchoring junctions probably contributes to the metastasis of cancer cells throughout the body. Cancer cells lose their anchoring junctions because they have fewer cadherin molecules and are not bound as tightly to neighboring cells. Once a cancer cell is released from its moorings, it secretes protein-digesting enzymes known as proteases. These enzymes, especially those called matrix metalloproteinases (MMPs), dissolve the extracellular matrix so that escaping cancer cells can invade adjacent tissues or enter the bloodstream. Researchers are investigating ways of blocking MMP enzymes to see if they can prevent metastasis.

Now that you understand how cells are held together into tissues, we will look at the four different tissue types in the body: (1) epithelial, (2) connective, (3) muscle, and (4) neural.

Concept Check

  1. Name the three functional categories of cell junctions.

  2. Which type of cell junction:

    • (a) restricts movement of materials between cells?

    • (b) allows direct movement of substances from the cytoplasm of one cell to the cytoplasm of an adjacent cell?

    • (c) provides the strongest cell-cell junction?

    • (d) anchors actin fibers in the cell to the extracellular matrix?

Epithelia Provide Protection and Regulate Exchange

The epithelial tissues, or epithelia {epi-, upon + thele-, nipple; singular epithelium}, protect the internal environment of the body and regulate the exchange of materials between the internal and external environments (Fig. 3.9). These tissues cover exposed surfaces, such as the skin, and line internal passageways, such as the digestive tract. Any substance that enters or leaves the internal environment of the body must cross an epithelium.

Some epithelia, such as those of the skin and mucous membranes of the mouth, act as a barrier to keep water in the body and invaders such as bacteria out. Other epithelia, such as those in the kidney and intestinal tract, control the movement of materials between the external environment and the extracellular fluid of the body. Nutrients, gases, and wastes often must cross several different epithelia in their passage between cells and the outside world.

Another type of epithelium is specialized to manufacture and secrete chemicals into the blood or into the external environment. Sweat and saliva are examples of substances secreted by epithelia into the environment. Hormones are secreted into the blood.

Structure of Epithelia

Epithelia typically consist of one or more layers of cells connected to one another, with a thin layer of extracellular matrix lying between the epithelial cells and their underlying tissues (Fig. 3.9c). This matrix layer, called the basal lamina {bassus, low; lamina, a thin plate}, or basement membrane, is composed of a network of collagen and laminin filaments embedded in proteoglycans. The protein filaments hold the epithelial cells to the underlying cell layers, just as cell junctions hold the individual cells in the epithelium to one another.

The cell junctions in epithelia are variable. Physiologists classify epithelia either as “leaky” or “tight,” depending on how easily substances pass from one side of the epithelial layer to the other. In a leaky epithelium, anchoring junctions allow molecules to cross the epithelium by passing through the gap between two adjacent epithelial cells. A typical leaky epithelium is the wall of capillaries (the smallest blood vessels), where all dissolved molecules except for large proteins can pass from the blood to the interstitial fluid by traveling through gaps between adjacent epithelial cells.

In a tight epithelium, such as that in the kidney, adjacent cells are bound to each other by tight junctions that create a barrier, preventing substances from traveling between adjacent cells. To cross a tight epithelium, most substances must enter the epithelial cells and go through them. The tightness of an epithelium is directly related to how selective it is about what can move across it. Some epithelia, such as those of the intestine, have the ability to alter the tightness of their junctions according to the body’s needs.

Types of Epithelia

Structurally, epithelial tissues can be divided into two general types: (1) sheets of tissue that lie on the surface of the body or that line the inside of tubes and hollow organs and (2) secretory epithelia that synthesize and release substances into the extracellular space. Histologists classify sheet epithelia by the number of cell layers in the tissue and by the shape of the cells in the surface layer. This classification scheme recognizes two types of layering—simple (one cell thick) and stratified (multiple cell layers) {stratum, layer + facere, to make}—and three cell shapes—squamous {squama, flattened plate or scale}, cuboidal, and columnar. However, physiologists are more concerned with the functions of these tissues, so instead of using the histological descriptions, we will divide epithelia into five groups according to their function.

There are five functional types of epithelia: exchange, transporting, ciliated, protective, and secretory (Fig. 3.10). Exchange epithelia permit rapid exchange of materials, especially gases. Transporting epithelia are selective about what can cross them and are found primarily in the intestinal tract and the kidney. Ciliated epithelia are located primarily in the airways of the respiratory system and in the female reproductive tract. Protective epithelia are found on the surface of the body and just inside the openings of body cavities. Secretory epithelia synthesize and release secretory products into the external environment or into the blood.

Figure 3.9b shows the distribution of these epithelia in the systems of the body. Notice that most epithelia face the external environment on one surface and the extracellular fluid on the other. One exception is the endocrine glands and a second is the epithelium lining the circulatory system.

Exchange Epithelia

The exchange epithelia are composed of very thin, flattened cells that allow gases (CO2 and O2) to pass rapidly across the epithelium. This type of epithelium lines the blood vessels and the lungs, the two major sites of gas exchange in the body. In capillaries, gaps or pores in the epithelium also allow molecules smaller than proteins to pass between two adjacent epithelial cells, making this a leaky epithelium (Fig. 3.10a). Histologists classify thin exchange tissue as simple squamous epithelium because it is a single layer of thin, flattened cells. The simple squamous epithelium lining the heart and blood vessels is also called the endothelium.

Transporting Epithelia

The transporting epithelia actively and selectively regulate the exchange of nongaseous materials, such as ions and nutrients, between the internal and external environments. These epithelia line the hollow tubes of the digestive system and the kidney, where lumens open into the external environment [here]. Movement of material from the external environment across the epithelium to the internal environment is called absorption. Movement in the opposite direction, from the internal to the external environment, is called secretion.

Transporting epithelia can be identified by the following characteristics (Fig. 3.10b):

  1. Cell shape. Cells of transporting epithelia are much thicker than cells of exchange epithelia, and they act as a barrier as well as an entry point. The cell layer is only one cell thick (a simple epithelium), but cells are cuboidal or columnar.

  2. Membrane modifications. The apical membrane, the surface of the epithelial cell that faces the lumen, has tiny finger-like projections called microvilli that increase the surface area available for transport. A cell with microvilli has at least 20 times the surface area of a cell without them. In addition, the basolateral membrane, the side of the epithelial cell facing the extracellular fluid, may also have folds that increase the cell’s surface area.

  3. Cell junctions. The cells of transporting epithelia are firmly attached to adjacent cells by moderately tight to very tight junctions. This means that to cross the epithelium, material must move into an epithelial cell on one side of the tissue and out of the cell on the other side.

  4. Cell organelles. Most cells that transport materials have numerous mitochondria to provide energy for transport processes [discussed further in Chapter 5]. The properties of transporting epithelia differ depending on where in the body the epithelia are located. For example, glucose can cross the epithelium of the small intestine and enter the extracellular fluid but cannot cross the epithelium of the large intestine.

The transport properties of an epithelium can be regulated and modified in response to various stimuli. Hormones, for example, affect the transport of ions by kidney epithelium. You will learn more about transporting epithelia when you study the kidney and digestive systems.

Ciliated Epithelia

Ciliated epithelia are nontransporting tissues that line the respiratory system and parts of the female reproductive tract. The surface of the tissue facing the lumen is covered with cilia that beat in a coordinated, rhythmic fashion, moving fluid and particles across the surface of the tissue (Fig.  3.10c). Injury to the cilia or to their epithelial cells can stop ciliary movement. For example, smoking paralyzes the ciliated epithelium lining the respiratory tract. Loss of ciliary function contributes to the higher incidence of respiratory infection in smokers, when the mucus that traps bacteria can no longer be swept out of the lungs by the cilia.

Protective Epithelia

The protective epithelia prevent exchange between the internal and external environments and protect areas subject to mechanical or chemical stresses. These epithelia are stratified tissues, composed of many stacked layers of cells (Fig. 3.10d). Protective epithelia may be toughened by the secretion of keratin {keras, horn}, the same insoluble protein abundant in hair and nails. The epidermis {epi, upon + derma, skin} and linings of the mouth, pharynx, esophagus, urethra, and vagina are all protective epithelia.

Because protective epithelia are subjected to irritating chemicals, bacteria, and other destructive forces, the cells in them have a short life span. In deeper layers, new cells are produced continuously, displacing older cells at the surface. Each time you wash your face, you scrub off dead cells on the surface layer. As skin ages, the rate of cell turnover declines. Retinoids, a group of chemicals derived from vitamin A, speed up cell division and surface shedding so treated skin develops a more youthful appearance.

Secretory Epithelia

Secretory epithelia are composed of cells that produce a substance and then secrete it into the extracellular space. Secretory cells may be scattered among other epithelial cells, or they may group together to form a multicellular gland. There are two types of secretory glands: exocrine and endocrine.

Exocrine glands release their secretions to the body’s external environment {exo-, outside + krinein, to secrete}. This may be onto the surface of the skin or onto an epithelium lining one of the internal passageways, such as the airways of the lung or the lumen of the intestine (Fig. 3.10e). In effect, an exocrine secretion leaves the body. This explains how some exocrine secretions, like stomach acid, can have a pH that is incompatible with life [Fig.  2.9, here].

Most exocrine glands release their products through open tubes known as ducts. Sweat glands, mammary glands in the breast, salivary glands, the liver, and the pancreas are all exocrine glands.

Exocrine gland cells produce two types of secretions. Serous secretions are watery solutions, and many of them contain enzymes. Tears, sweat, and digestive enzyme solutions are all serous exocrine secretions. Mucous secretions (also called mucus) are sticky solutions containing glycoproteins and proteoglycans. Some exocrine glands contain more than one type of secretory cell, and they produce both serous and mucous secretions. For example, the salivary glands release mixed secretions.

Goblet cells, shown in Figure 3.10e, are single exocrine cells that produce mucus. Mucus acts as a lubricant for food to be swallowed, as a trap for foreign particles and microorganisms inhaled or ingested, and as a protective barrier between the epithelium and the environment.

Unlike exocrine glands, endocrine glands are ductless and release their secretions, called hormones, into the body’s extracellular compartment (Fig. 3.9b). Hormones enter the blood for distribution to other parts of the body, where they regulate or coordinate the activities of various tissues, organs, and organ systems. Some of the best-known endocrine glands are the pancreas, the thyroid gland, the gonads, and the pituitary gland. For years, it was thought that all hormones were produced by cells grouped together into endocrine glands. We now know that isolated endocrine cells occur scattered in the epithelial lining of the digestive tract, in the tubules of the kidney, and in the walls of the heart.

Figure 3.11 shows the epithelial origin of endocrine and exocrine glands. During embryonic development, epithelial cells grow downward into the supporting connective tissue. Exocrine glands remain connected to the parent epithelium by a duct that transports the secretion to its destination (the external environment). Endocrine glands lose the connecting cells and secrete their hormones into the bloodstream.

FIG. 3.11 Development of endocrine and exocrine glands

Concept Check

  1. List the five functional types of epithelia.

  2. Define secretion.

  3. Name two properties that distinguish endocrine glands from exocrine glands.

  4. The basal lamina of epithelium contains the protein fiber laminin. Are the overlying cells attached by focal adhesions or hemidesmosomes?

  5. You look at a tissue under a microscope and see a simple squamous epithelium. Can it be a sample of the skin surface? Explain.

  6. A cell of the intestinal epithelium secretes a substance into the extracellular fluid, where it is picked up by the blood and carried to the pancreas. Is the intestinal epithelium cell an endocrine or an exocrine cell?

Connective Tissues Provide Support and Barriers

Connective tissues, the second major tissue type, provide structural support and sometimes a physical barrier that, along with specialized cells, helps defend the body from foreign invaders such as bacteria. The distinguishing characteristic of connective tissues is the presence of extensive extracellular matrix containing widely scattered cells that secrete and modify the matrix (Fig. 3.12). Connective tissues include blood, the support tissues for the skin and internal organs, and cartilage and bone.

Structure of Connective Tissue

The extracellular matrix of connective tissue is a ground substance of proteoglycans and water in which insoluble protein fibers are arranged, much like pieces of fruit suspended in a gelatin salad. The consistency of ground substance is highly variable, depending on the type of connective tissue (Fig. 3.12a). At one extreme is the watery matrix of blood, and at the other extreme is the hardened matrix of bone. In between are solutions of proteoglycans that vary in consistency from syrupy to gelatinous. The term ground substance is sometimes used interchangeably with matrix.

Connective tissue cells lie embedded in the extracellular matrix. These cells are described as fixed if they remain in one place and as mobile if they can move from place to place. Fixed cells are responsible for local maintenance, tissue repair, and energy storage. Mobile cells are responsible mainly for defense. The distinction between fixed and mobile cells is not absolute, because at least one cell type is found in both fixed and mobile forms.

Extracellular matrix is nonliving, but the connective tissue cells constantly modify it by adding, deleting, or rearranging molecules. The suffix -blast {blastos, sprout} on a connective tissue cell name often indicates a cell that is either growing or actively secreting extracellular matrix. Fibroblasts, for example, are connective tissue cells that secrete collagen-rich matrix. Cells that are actively breaking down matrix are identified by the suffix -clast {klastos, broken}. Cells that are neither growing, secreting matrix components, nor breaking down matrix may be given the suffix -cyte, meaning “cell.” Remembering these suffixes should help you remember the functional differences between cells with similar names, such as the osteoblast, osteocyte, and osteoclast, three cell types found in bone.

In addition to secreting proteoglycan ground substance, connective tissue cells produce matrix fibers. Four types of fiber proteins are found in matrix, aggregated into insoluble fibers. Collagen {kolla, glue + -genes, produced} is the most abundant protein in the human body, almost one-third of the body’s dry weight. Collagen is also the most diverse of the four protein types, with at least 12 variations. It is found almost everywhere connective tissue is found, from the skin to muscles and bones. Individual collagen molecules pack together to form collagen fibers, flexible but inelastic fibers whose strength per unit weight exceeds that of steel. The amount and arrangement of collagen fibers help determine the mechanical properties of different types of connective tissues.

Three other protein fibers in connective tissue are elastin, fibrillin, and fibronectin. Elastin is a coiled, wavy protein that returns to its original length after being stretched. This property is known as elastance. Elastin combines with the very thin, straight fibers of fibrillin to form filaments and sheets of elastic fibers. These two fibers are important in elastic tissues such as the lungs, blood vessels, and skin. As mentioned earlier, fibronectin connects cells to extracellular matrix at focal adhesions. Fibronectins also play an important role in wound healing and in blood clotting.

Types of Connective Tissue

Figure 3.12b compares the properties of different types of connective tissue. The most common types are loose and dense connective tissue, adipose tissue, blood, cartilage, and bone. By many estimates, connective tissues are the most abundant of the tissue types as they are a component of most organs.

Loose connective tissues (Fig. 3.13a) are the elastic tissues that underlie skin and provide support for small glands. Dense connective tissues (irregular and regular) provide strength or flexibility. Examples are tendons, ligaments, and the sheaths that surround muscles and nerves. In these dense tissues, collagen fibers are the dominant type. Tendons (Fig. 3.13c) attach skeletal muscles to bones. Ligaments connect one bone to another. Because ligaments contain elastic fibers in addition to collagen fibers, they have a limited ability to stretch. Tendons lack elastic fibers and so cannot stretch.

Cartilage and bone together are considered supporting connective tissues. These tissues have a dense ground substance that contains closely packed fibers. Cartilage is found in structures such as the nose, ears, knee, and windpipe. It is solid, flexible, and notable for its lack of blood supply. Without a blood supply, nutrients and oxygen must reach the cells of cartilage by diffusion. This is a slow process, which means that damaged cartilage heals slowly.

The fibrous extracellular matrix of bone is said to be calcified because it contains mineral deposits, primarily calcium salts, such as calcium phosphate (Fig. 3.13b). These minerals give the bone strength and rigidity. We examine the structure and formation of bone along with calcium metabolism later [Chapter 23].

Adipose tissue is made up of adipocytes, or fat cells. An adipocyte of white fat typically contains a single enormous lipid droplet that occupies most of the volume of the cell (Fig. 3.13e). This is the most common form of adipose tissue in adults.

Brown fat is composed of adipose cells that contain multiple lipid droplets rather than a single large droplet. This type of fat has been known for many years to play an important role in temperature regulation in infants. Until recently it was thought to be almost completely absent in adults. However, modern imaging techniques such as combined CT and PET scans have revealed that adults do have brown fat [discussed in more detail in ­Chapter 22].

Blood is an unusual connective tissue that is characterized by its watery extracellular matrix called plasma. Plasma consists of a dilute solution of ions and dissolved organic molecules, including a large variety of soluble proteins. Blood cells and cell fragments are suspended in the plasma (Fig. 3.13d), but the insoluble protein fibers typical of other connective tissues are absent. [We discuss blood in Chapter 16.]

Concept Check

  1. What is the distinguishing characteristic of connective tissues?

  2. Name four types of protein fibers found in connective tissue matrix and give the characteristics of each.

  3. Name six types of connective tissues.

  4. Blood is a connective tissue with two components: plasma and cells. Which of these is the matrix in this connective tissue?

  5. Why does torn cartilage heal more slowly than a cut in the skin?

Muscle and Neural Tissues Are Excitable

The third and fourth of the body’s four tissue types—muscle and neural—are collectively called the excitable tissues because of their ability to generate and propagate electrical signals called action potentials. Both of these tissue types have minimal extracellular matrix, usually limited to a supportive layer called the external lamina. Some types of muscle and nerve cells are also notable for their gap junctions, which allow the direct and rapid conduction of electrical signals from cell to cell.

Muscle tissue has the ability to contract and produce force and movement. The body contains three types of muscle tissue: cardiac muscle in the heart; smooth muscle, which makes up most internal organs; and skeletal muscle. Most skeletal muscles attach to bones and are responsible for gross movement of the body. [We discuss muscle tissue in more detail in Chapter 12.]

Neural tissue has two types of cells. Neurons, or nerve cells, carry information in the form of chemical and electrical signals from one part of the body to another. They are concentrated in the brain and spinal cord but also include a network of cells that extends to virtually every part of the body. Glial cells, or neuroglia, are the support cells for neurons. [We discuss the anatomy of neural tissue in Chapter 8.] A summary of the characteristics of the four tissue types can be found in Table 3.4.

Table 3.4 Characteristics of the Four Tissue Types

Epithelial Connective Muscle Nerve
Matrix Amount Minimal Extensive Minimal Minimal
Matrix Type Basal lamina Varied—protein fibers in ground substance that ranges from liquid to gelatinous to firm to calcified External lamina External lamina
Unique Features No direct blood supply Cartilage has no blood supply Able to generate electrical signals, force, and movement Able to generate electrical signals
Surface Features of Cells Microvilli, cilia N/A N/A N/A
Locations Covers body surface; lines cavities and hollow organs, and tubes; secretory glands Supports skin and other organs; cartilage, bone, and blood Makes up skeletal muscles, hollow organs, and tubes Throughout body; concentrated in brain and spinal cord
Cell Arrangement and Shapes Variable number of layers, from one to many; cells flattened, cuboidal, or columnar Cells not in layers; usually randomly scattered in matrix; cell shape irregular to round Cells linked in sheets or elongated bundles; cells shaped in elongated, thin cylinders; heart muscle cells may be branched Cells isolated or networked; cell appendages highly branched and/or elongated