Histologists recognize four basic types of tissue:
1. Epithelial tissue , which covers exposed surfaces, lines internal passageways and chambers, and forms glands.
2. Connective tissue , which fills internal spaces, provides structural support for other tissues, transports materials within the body, and stores energy reserves.
3. Muscle tissue , which is specialized for contraction, includes the skeletal muscles of the body, as well as the muscle of the heart and the muscular lining of hollow organs.
4. Neural tissue , which carries information from one part of the body to another in the form of electrical impulses.
Epithelial tissue includes epithelia and glands . Epithelia are layers of cells that cover internal or external surfaces. Glands are structures that produce fluid secretions; they are either attached to or derived from epithelia.
Epithelia cover every exposed surface of the body. Important characteristics:
Functions of Epithelial Tissue Epithelia perform four essential functions:
1. Epithelia protect exposed and internal surfaces from abrasion, dehydration, and destruction by chemical or biological agents
2. Control Permeability. Any substance that enters or leaves your body has to cross an epithelium. Some epithelia are relatively impermeable; others are easily crossed by compounds as large as proteins.
3. Provide Sensation. Extremely sensitive to stimulation, because they have a large sensory nerve supply. Neuroepithelium is an epithelium that is specialized to perform a particular sensory function; neuroepithelia contain sensory cells that provide the sensations of smell, taste, sight, equilibrium, and hearing.
4. Produce Specialized Secretions. Individual gland cells are typically scattered among other cell types in an epithelium. In a glandular epithelium , most or all of the epithelial cells produce secretions.
Specialized epithelial cells generally possess a strong polarity; one common type of epithelial polarity is shown below.
Many epithelial cells that line internal passageways have microvilli on their exposed surfaces. Just a few may be present, or microvilli may carpet the entire surface. Microvilli are especially abundant on epithelial surfaces where absorption and secretion take place, such as along portions of the digestive and urinary tracts.
As shown above, a typical ciliated cell contains about 250 cilia that beat in a coordinated fashion. The ciliated epithelium that lines the respiratory tract moves mucus up from the lungs and toward the throat. The sticky mucus traps inhaled particles, including dust, pollen, and pathogens; the ciliated epithelium carries the mucus and the trapped debris to the throat, where they can be swallowed. Injury to the cilia or to the epithelial cells, most commonly by abrasion or exposure to toxic compounds such as the nicotine in cigarette smoke, can stop ciliary movement and block the protective flow of mucus.
Maintaining the Integrity of Epithelia
An epithelium must form a complete cover or lining to be effective as a barrier. Three factors help maintain the physical integrity of an epithelium: (1) intercellular connections, (2) attachment to the basal lamina, and (3) epithelial maintenance and repair.
Cells in an epithelium are firmly attached to one another, and the epithelium as a unit is attached to extracellular fibers of the basal lamina.
Large areas of opposing cell membranes are interconnected by transmembrane proteins called cell adhesion molecules (CAMs) , which bind to each other and to extracellular materials.
For example, CAMs on the attached base of an epithelium help bind the cell to the underlying basal lamina.
The membranes of adjacent cells may also be bonded by intercellular cement.
Cell junctions are specialized areas of the cell membrane that attach a cell to another cell or to extracellular materials.
The three most common types of cell junctions are (1) tight
junctions, (2) gap junctions, and (3) desmosomes.
At a tight junction , the lipid portions of the two cell membranes are tightly bound together by interlocking membrane proteins.
When the epithelium lines a tube, such as the intestinal tract, the apical surfaces of the epithelial cells are exposed to the space inside the tube, a passageway called the lumen.
Tight junctions effectively isolate the contents of the lumen from the basolateral surfaces of the cell. For example, tight junctions near the apical surfaces of cells that line the digestive tract keep enzymes, acids, and wastes in the lumen from reaching the basolateral surfaces and digesting or otherwise damaging the underlying tissues and organs.
Some epithelial functions require rapid intercellular communication. At a gap junction, two cells are held together by interlocking membrane proteins called connexons . Because these are channel proteins, they leave a narrow passageway that lets small molecules and ions pass from cell to cell. Gap junctions are common among epithelial cells, where the movement of ions helps coordinate functions such as the beating of cilia. Gap junctions in cardiac muscle tissue and smooth muscle tissue are essential to the coordination of muscle cell contractions.
At a desmosome, CAMs and proteoglycans link the opposing cell membranes. Desmosomes are very strong and can resist stretching and twisting.
Desmosomes are abundant between cells in the superficial layers of the skin. As a result, damaged skin cells are usually lost in sheets rather than as individual cells.
Attachment to the Basal Lamina
Not only do epithelial cells hold onto one another, but they also remain firmly connected to the rest of the body. The basal surface of each epithelium is attached to a special two–part basal lamina .
Epithelial Maintenance and Repair
Epithelial cells lead hard lives, for they are exposed to disruptive enzymes, toxic chemicals, pathogenic bacteria, and mechanical abrasion. An epithelial cell may last just a day or two before it is shed or destroyed. The only way the epithelium can maintain its structure over time is by the continual division of stem cells . Most stem cells are located near the basal lamina, in a relatively protected location.
Classification of Epithelia Categories based on (1) the cell shape, and (2) the number of cell layers between the base and the exposed surface of the epithelium.
Three cell shapes are identified: squamous, cuboidal , and columnar .
In sectional view, squamous cells appear thin and flat, cuboidal cells look like little boxes, and columnar cells are tall and relatively slender rectangles.
There are two choices for the number of cell layers: simple or stratified .
If only one layer of cells covers the basal lamina, that
layer is a simple epithelium . Simple epithelia are located only in
protected areas inside the body. They line internal compartments and
passageways, including the ventral body cavities, the heart chambers, and blood
Simple epithelia are also characteristic of regions in which secretion or absorption occurs, such as the lining of the intestines and the gas–exchange surfaces of the lungs. In these places, thinness is an advantage, for it reduces the time required for materials to cross the epithelial barrier.
In a stratified epithelium , several layers of cells cover the basal lamina. Stratified epithelia are generally located in areas that need protection from mechanical or chemical stresses, such as the surface of the skin and the lining of the mouth.
The cells in a squamous epithelium are thin, flat, and somewhat irregular in shape, like pieces of a puzzle
The cells look like fried eggs laid side by side.
A simple squamous epithelium is the body's most delicate type of epithelium. This type of epithelium is located in protected regions where absorption or diffusion takes place or where a slick, slippery surface reduces friction. Examples are the respiratory exchange surfaces (alveoli) of the lungs, the lining of the ventral body cavities, and the lining of the heart and blood vessels.
Special names have been given to the simple squamous epithelia that line chambers and passageways that do not communicate with the outside world. The simple squamous epithelium that lines the ventral body cavities is a mesothelium. The pleura, peritoneum, and pericardium each contain a superficial layer of mesothelium. The simple squamous epithelium lining the inner surface of the heart and all blood vessels is an endothelium.
A stratified squamous epithelium is generally located where mechanical stresses are severe. The cells form a series of layers, like a stack of plywood sheets or a ream of paper. The surface of the skin and the lining of the mouth, esophagus, and anus are areas where this type of epithelium protects against physical and chemical attacks. On exposed body surfaces, where mechanical stress and dehydration are potential problems, apical layers of epithelial cells are packed with filaments of the protein keratin . As a result, superficial layers are both tough and water resistant; the epithelium is said to be keratinized . A nonkeratinized stratified squamous epithelium resists abrasion, but will dry out and deteriorate unless kept moist. Nonkeratinized stratified squamous epithelia are situated in the oral cavity, pharynx, esophagus, anus, and vagina.
The cells of a cuboidal epithelium resemble hexagonal boxes. (In typical sectional views they appear square.). A simple cuboidal epithelium provides limited protection and occurs where secretion or absorption takes place. Such an epithelium lines portions of the kidney tubules.
In the pancreas and salivary glands, simple cuboidal epithelia secrete enzymes and buffers and are found lining portions of the ducts that discharge those secretions. The thyroid gland contains chambers, called thyroid follicles , that are lined by a cuboidal secretory epithelium. Thyroid hormones are stored in the follicles and released as needed into the bloodstream.
Stratified cuboidal epithelia are relatively rare; they are located along the ducts of sweat glands and in the larger ducts of the mammary glands. A transitional epithelium is situated in regions of the urinary system, such as the urinary bladder.
In a typical sectional view, columnar epithelial cells appear rectangular.
A simple columnar epithelium typically occurs where absorption or secretion is under way, such as in the small intestine. In the stomach and large intestine, the secretions of simple columnar epithelia protect against chemical stresses.
Portions of the respiratory tract contain a pseudostratified columnar epithelium , a columnar epithelium that includes several types of cells with varying shapes and functions.
Pseudostratified columnar epithelial cells typically possess
cilia. Epithelia of this type line most of the nasal cavity, the trachea
(windpipe), the bronchi, and portions of the male reproductive tract.
Stratified columnar epithelia are relatively rare, providing protection along portions of the pharynx, epiglottis, anus, and urethra, as well as along a few large excretory ducts.
Many epithelia contain gland cells that are specialized for secretion. Collections of epithelial cells (or structures derived from epithelial cells) that produce secretions are called glands . They range from scattered cells to complex glandular organs. Some of these glands, called endocrine glands , release their secretions into the interstitial fluid. Others, known as exocrine glands , release their secretions into passageways called ducts that open onto the epithelial surface.
An endocrine gland produces endocrine secretions , which are released directly into the surrounding interstitial fluid. These secretions, also called hormones, enter the bloodstream for distribution throughout the body. Examples of endocrine glands include the thyroid gland and the pituitary gland.
Exocrine glands produce exocrine secretions that are discharged onto an epithelial surface. Most exocrine secretions reach the surface through tubular ducts, which empty onto the skin surface or onto an epithelium lining an internal passageway that communicates with the exterior. Enzymes entering the digestive tract, perspiration on the skin, tears in the eyes, and milk produced by mammary glands are examples of exocrine secretions delivered to epithelial surfaces by ducts.
Exocrine glands exhibit several different methods of secretion; therefore, they are classified by their mode and type of secretion, as well as by the structure of the gland cells and associated ducts.
Modes of Secretion A glandular epithelial cell releases its secretions by (1) merocrine secretion, (2) apocrine secretion, or (3) holocrine secretion. In merocrine secretion the product is released from secretory vesicles by exocytosis.
This is the most common mode of secretion. One type of merocrine secretion, mucin , mixes with water to form mucus , an effective lubricant, a protective barrier, and a sticky trap for foreign particles and microorganisms. The mucous secretions of merocrine glands coat passageways in the digestive and respiratory tracts. In the skin, merocrine sweat glands produce the watery perspiration that helps cool you on a hot day.
Apocrine secretion involves the loss of cytoplasm, as well as the secretory product. The apical portion of the cytoplasm becomes packed with secretory vesicles and is then shed. Milk production in the mammary glands involves a combination of merocrine and apocrine secretions.
Merocrine and apocrine secretions leave a cell relatively intact and able to continue secreting.
Holocrine secretion destroys the gland cell. During holocrine secretion, the entire cell becomes packed with secretory products and then bursts, releasing the secretion, but killing the cell. Sebaceous glands, associated with hair follicles, produce an oily hair coating by means of holocrine secretion.
Types of Secretions Exocrine glands are also categorized by the types of secretion produced:
1. Serous glands secrete a watery solution that contains enzymes. The parotid salivary glands are serous glands.
2. Mucous glands secrete mucins that hydrate to form mucus. The sublingual salivary glands and the submucosal glands of the small intestine are mucous glands.
3. Mixed exocrine glands contain more than one type of gland cell and may produce two different exocrine secretions, one serous and the other mucous. The submandibular salivary glands are mixed exocrine glands.
4–3 Connective Tissues
Connective tissue connects the epithelium to the rest of the body. Other connective tissues include bone, fat, and blood, as well as tissues that provide structure, store energy reserves, and transport materials throughout the body. Connective tissues vary widely in appearance and function, but they all share three basic components: (1) specialized cells, (2) extracellular protein fibers, and (3) a fluid known as ground substance . The extracellular fibers and ground substance together constitute the matrix , which surrounds the cells. Whereas cells make up the bulk of epithelial tissue, the matrix typically accounts for most of the volume of connective tissues.
Connective tissues are situated throughout the body, but are never exposed to the outside environment. Many connective tissues are highly vascular (that is, they have many blood vessels) and contain sensory receptors that provide pain, pressure, temperature, and other sensations.
Functions of connective tissue:
Classification of Connective Tissues
The three general categories of connective tissue are connective tissue proper, fluid connective tissues, and supporting connective tissues
1. Connective tissue proper includes those connective tissues with many types of cells and extracellular fibers in a syrupy ground substance. For example, both adipose tissue (fat) and tendons are connective tissue proper, but they have very different structural and functional characteristics. Connective tissue proper is divided into loose connective tissues and dense connective tissues on the basis of the relative proportions of cells, fibers, and ground substance.
2. Fluid connective tissues have distinctive populations of cells suspended in a watery matrix that contains dissolved proteins. Two types exist: blood and lymph .
3. Supporting connective tissues differ from connective tissue proper in having a less diverse cell population and a matrix containing much more densely packed fibers. Supporting connective tissues protect soft tissues and support the weight of part or all of the body. The two types of supporting connective tissues are cartilage and bone. The matrix of bone is said to be calcified , because it contains mineral deposits (primarily calcium salts) that provide rigidity.
Connective Tissue Proper
Connective tissue proper contains extracellular fibers, a viscous (syrupy) ground substance, and a varied cell population. Some of the cells, including fibroblasts, adipocytes , and mesenchymal cells , are involved with local maintenance, repair, and energy storage. These cells are permanent residents of the connective tissue. Other cells, including macrophages, mast cells, lymphocytes, plasma cells , and microphages , defend and repair damaged tissues. These cells are not permanent residents; they migrate through healthy connective tissues and aggregate at sites of tissue injury.
The number of cells and cell types in a tissue at any moment varies with localconditions.
Components of Connective Tissue Proper:
• Fibroblasts are the most abundant permanent residents of connective tissue proper and are the only cells that are always present in it. Fibroblasts secrete hyaluronan (a polysaccharide derivative) and proteins. In connective tissue proper, extracellular fluid, hyaluronan, and proteins interact to form the proteoglycans that make ground substance viscous. Each fibroblast also secretes protein subunits that interact to form large extracellular fibers. In addition to fibroblasts, connective tissues proper may contain several other types of cells:
Connective Tissue Fibers Three types of fibers occur in connective tissue: collagen, reticular , and elastic . Fibroblasts form all three by secreting protein subunits that interact in the matrix.
1. Collagen fibers are long, straight, and unbranched. They are the most common fibers in connective tissue proper. Each collagen fiber consists of a bundle of fibrous protein subunits, wound together like the strands of a rope. Like a rope, a collagen fiber is flexible, but it is stronger than steel when pulled from either end. Tendons , which connect skeletal muscles to bones, consist almost entirely of collagen fibers. Typical ligaments are similar to tendons, but they connect one bone to another. Tendons and ligaments can withstand tremendous forces. Uncontrolled muscle contractions or skeletal movements are more likely to break a bone than to snap a tendon or a ligament.
2. Reticular fibers contain the same protein subunits as do collagen fibers, but arranged differently. Thinner than collagen fibers, reticular fibers form a branching, interwoven framework that is tough yet flexible. Because they form a network rather than share a common alignment, reticular fibers resist forces applied from many directions. This interwoven network, called a stroma , stabilizes the relative positions of the functional cells, or parenchyma of organs such as the liver. Reticular fibers also stabilize the positions of an organ's blood vessels, nerves, and other structures, despite changing positions and the pull of gravity.
3. Elastic fibers contain the protein elastin . Elastic fibers are branched and wavy. After stretching, they will return to their original length. Elastic ligaments are dominated by elastic fibers. They are relatively rare, but have important functions, such as interconnecting vertebrae.
Ground Substance fills the spaces between cells and surrounds connective tissue fibers. In connective tissue proper, ground substance is clear, viscous, and colorless. Its typical consistency is like that of maple syrup, due to the presence of proteoglycans and glycoproteins. Ground substance is dense enough that bacteria have trouble moving through it–imagine swimming in molasses.
Marfan's syndrome is an inherited condition caused by the production of an abnormal form of fibrillin , a glycoprotein that is important to the strength and elasticity of connective tissues. Because most organs contain connective tissues, the effects of this defect are widespread. The most visible sign of Marfan's syndrome involves the skeleton; most individuals with the condition are tall and have abnormally long limbs and fingers. The most serious consequences involve the cardiovascular system; roughly 90 percent of people with Marfan's syndrome have structural abnormalities in their cardiovascular system. The most dangerous possibility is that the weakened elastic connective tissues in the walls of major arteries, such as the aorta, may burst, causing a sudden, fatal loss of blood.
Embryonic Connective Tissues
Mesenchyme , or embryonic connective tissue , is the first connective tissue to appear in a developing embryo. Mesenchyme contains an abundance of star–shaped stem cells.
gives rise to all other connective tissues. Mucous
connective tissue, or Wharton's jelly, is a loose connective
tissue found in many parts of the embryo, including the umbilical cord.
Adults have neither form of embryonic connective tissue. However, many adult connective tissues contain scattered mesenchymal stem cells that can assist in tissue repair after an injury.
Loose connective tissues are the "packing materials" of the body. They fill spaces between organs, cushion and stabilize specialized cells in many organs, and support epithelia. These tissues surround and support blood vessels and nerves, store lipids, and provide a route for the diffusion of materials. Loose connective tissues include mucous connective tissue in embryos and areolar tissue, adipose tissue , and reticular tissue in adults.
Areolar Tissue is the
least specialized connective tissue in adults. It may contain all the cells and
fibers of any connective tissue proper in a very loosely organized array. The ground
substance syrupy fluid absorbs shocks. Because its fibers are loosely
organized, areolar tissue can distort without damage. The presence of elastic
fibers makes it resilient, so areolar tissue returns to its original shape
after external pressure is relieved.
Areolar tissue forms a layer that separates the skin from deeper structures. In addition to providing padding, the elastic properties of this layer allow a considerable amount of independent movement. Thus, if you pinch the skin of your arm, you will not affect the underlying muscle. Conversely, contractions of the underlying muscle do not pull against your skin; as the muscle bulges, the areolar tissue stretches. Because this tissue has an extensive blood supply, the areolar tissue layer under the skin is a common injection site for drugs.
Adipose Tissue The distinction between areolar tissue and fat, or adipose tissue , is somewhat arbitrary. Adipocytes account for most of the volume of adipose tissue, but only a fraction of the volume of areolar tissue.
Adipose tissue provides padding, absorbs shocks, acts as an insulator to slow heat loss through the skin, and serves as packing or filler around structures. Adipose tissue is common under the skin of the flanks, buttocks, and breasts. It fills the bony sockets behind the eyes, surrounds the kidneys, and is common beneath the mesothelial lining of the pericardial and abdominal cavities. Most of the adipose tissue in the body is called white fat , because it has a pale, yellow–white color.
In infants and young children, the adipose tissue between the shoulder blades, around the neck, and possibly elsewhere in the upper body is highly vascularized, and the individual adipocytes contain numerous mitochondria. Together, these characteristics give the tissue a deep, rich color from which the name brown fat is derived. When these cells are stimulated by the nervous system, lipid breakdown accelerates. The cells do not capture the energy that is released. Instead, it is absorbed by the surrounding tissues as heat. The heat warms the circulating blood, which distributes the heat throughout the body. In this way, an infant can increase metabolic heat generation by 100 percent very quickly. In adults, who have little if any brown fat, body temperature is elevated primarily by shivering, which involves rapidly oscillating contractions within large skeletal muscles. These contractions consume energy and generate heat that warms the body. Shivering is not an effective mechanism for infants, because their skeletal muscles are relatively small, weak, and poorly controlled.
are metabolically active cells; their lipids are constantly being broken down
and replaced. When nutrients are scarce, adipocytes deflate like collapsing
balloons. Often, such deflation occurs during a weight–loss program. Because
the cells are not killed but merely reduced in size, the lost weight can easily
be regained in the same areas of the body. In adults, adipocytes are incapable
of dividing. The number of fat cells in peripheral tissues is established in
the first few weeks of a newborn's life, perhaps in response to the amount of
fats in the diet. However, that is not the end of the story, because loose
connective tissues also contain mesenchymal cells. If circulating–lipid levels
are chronically elevated, the mesenchymal cells will divide, giving rise to
cells that differentiate into fat cells. As a result, areas of areolar tissue
can become adipose tissue in times of nutritional plenty, even in adults. In
the procedure known as liposuction ,
unwanted adipose tissue is surgically removed. Because adipose tissue can
regenerate through the differentiation of mesenchymal cells, liposuction
provides only a temporary solution to the problem of excess weight.
Reticular Tissue Organs such as the spleen and liver contain reticular tissue , in which reticular fibers create a complex three–dimensional stroma. The stroma supports the parenchyma (functional cells) of these organs. This fibrous framework is also found in the lymph nodes and bone marrow. Fixed macrophages and fibroblasts are associated with the reticular fibers, but these cells are seldom visible, because the organs are dominated by specialized cells with other functions.
Dense Connective Tissues
Most of the volume of dense connective tissues is occupied by fibers. Dense connective tissues are often called collagenous tissues , because collagen fibers are the dominant type of fiber in them. The body has two types of dense connective tissues: (1) dense regular connective tissue and (2) dense irregular connective tissue.
In dense regular connective tissue , the collagen fibers are parallel to each other, packed tightly, and aligned with the forces applied to the tissue. Tendons are cords of dense regular connective tissue that attach skeletal muscles to bones .
The collagen fibers run along the longitudinal axis of the tendon and transfer the pull of the contracting muscle to the bone. Ligaments resemble tendons, but connect one bone to another or stabilize the positions of internal organs. An aponeurosis is a tendinous sheet that attaches a broad, flat muscle to another muscle or to several bones of the skeleton. It can also stabilize the positions of tendons and ligaments. Aponeuroses are associated with large muscles of the lower back and abdomen and with the tendons and ligaments of the palms of the hands and the soles of the feet.
In contrast, the fibers in dense irregular connective tissue form an interwoven meshwork in no consistent pattern. These tissues strengthen and support areas subjected to stresses from many directions. A layer of dense irregular connective tissue gives skin its strength. Cured leather (animal skin) is an excellent illustration of the interwoven nature of this tissue. Except at joints, dense irregular connective tissue forms a sheath around cartilages (the perichondrium ) and bones (the periosteum ). Dense irregular connective tissue also forms a thick fibrous layer called a capsule , which surrounds internal organs such as the liver, kidneys, and spleen and which encloses the cavities of joints.
Elastic tissue is a dense regular connective tissue dominated by elastic fibers. Elastic ligaments , which are almost completely dominated by elastic fibers, help stabilize the positions of the vertebrae of the spinal column.
Fluid Connective Tissues
Blood and lymph are connective tissues with distinctive collections of cells. The fluid matrix that surrounds the cells also includes many types of suspended proteins that do not form insoluble fibers under normal conditions.
In blood , the watery matrix is called plasma . Plasma contains blood cells and fragments of cells, collectively known as formed elements.
are three types of formed elements: (1) red blood cells, (2) white blood cells,
and (3) platelets.
A single cell type, the red blood cell , or erythrocyte, accounts for almost half the volume of blood and is the reason we associate the color red with blood. Red blood cells are responsible for the transport of oxygen and, to a lesser degree, of carbon dioxide in the blood.
Plasma also contains small numbers of white blood cells , or leukocytes. White blood cells include the phagocytic microphages ( neutrophils and eosinophils ), basophils, lymphocytes , and monocytes . White blood cells are important components of the immune system, which protects the body from infection and disease.
The third type of formed element in blood consists not of whole cells, but of tiny membrane–enclosed packets of cytoplasm called platelets . These cell fragments, which contain enzymes and special proteins, function in the clotting response that seals breaks in the endothelial lining.
Extracellular fluid includes three major subdivisions: plasma, interstitial fluid , and lymph . Plasma is normally confined to the vessels of the circulatory system, and contractions of the heart keep it in motion.
The major difference between plasma and interstitial fluid is that plasma contains numerous suspended proteins that are too large to pass into the interstitial fluid.
Lymph forms as interstitial fluid enters lymphatic vessels , small passageways that return it to the cardiovascular system. As fluid passes along the lymphatic vessels, cells of the immune system monitor the composition of the lymph and respond to signs of injury or infection. The number of cells in lymph may vary, but ordinarily 99 percent of them are lymphocytes. The rest are primarily macrophages or microphages. This recirculation of fluid from the cardiovascular system, through the interstitial fluid, to the lymph, and then back to the cardiovascular system is a continuous process that is essential to homeostasis. It helps eliminate local differences in the levels of nutrients, wastes, or toxins, maintains blood volume, and alerts the immune system to infections that may be underway in peripheral tissues.
Supporting Connective Tissues
Cartilage and bone are called supporting connective tissues because they provide a strong framework that supports the rest of the body. In these connective tissues, the matrix contains numerous fibers and, in some cases, deposits of insoluble calcium salts.
The matrix of cartilage is a firm gel. Cartilage cells, or chondrocytes, are the only cells in the cartilage matrix. They occupy small chambers known as lacunae. Unlike other connective tissues, cartilage is avascular, so all exchange of nutrients and waste products must occur by diffusion through the matrix. Blood vessels do not grow into cartilage because chondrocytes produce a chemical that discourages their formation.
A cartilage is generally set apart from surrounding tissues by a fibrous perichondrium.
Types of Cartilage The body contains three major types of cartilage: hyaline cartilage, elastic cartilage, and fibrocartilage.
1. Hyaline cartilage is the most common type of cartilage. Except inside joint cavities, hyaline cartilage is covered by a dense perichondrium.
The matrix of hyaline cartilage contains closely packed collagen fibers, making it tough but somewhat flexible. Because the fibers are not in large bundles and do not stain darkly, they are not always apparent in light microscopy. Examples in adults include the connections between the ribs and the sternum; the nasal cartilages and the supporting cartilages along the conducting passageways of the respiratory tract; and the articular cartilages , which cover opposing bone surfaces within many joints, such as the elbow and knee.
2. Elastic cartilage contains numerous elastic fibers that make it extremely resilient and flexible. These cartilages usually have a yellowish color on gross dissection. Elastic cartilage forms the external flap (the auricle , or pinna ) of the outer ear, the epiglottis, an airway to the middle ear cavity (the auditory tube ), and small cartilages in the larynx (the cuneiform cartilages ).
3. Fibrocartilage has little ground substance, and its matrix is dominated by densely interwoven collagen fibers, making this tissue extremely durable and tough. Fibrocartilaginous pads lie between the spinal vertebrae, between the pubic bones of the pelvis, and around or in a few joints and tendons. In these positions, fibrocartilage resists compression, absorbs shocks, and prevents damaging bone–to–bone contact. Cartilage heals poorly, and damaged fibrocartilage in joints such as the knee can interfere with normal movements.
Several complex joints, including the knee, contain both
hyaline cartilage and fibrocartilage. The hyaline cartilage covers bony
surfaces, and fibrocartilage pads in the joint prevent contact between bones
during movement. Injuries to these joints can produce tears in the
fibrocartilage pads, and the tears do not heal. Eventually, joint mobility is
severely reduced. Surgery generally produces only a temporary or incomplete
Recent advances in tissue culture have enabled researchers to grow fibrocartilage in the laboratory. Chondrocytes removed from the knees of injured dogs are cultured in an artificial framework of collagen fibers. Eventually, they produce masses of fibrocartilage that can be inserted into the damaged joints. Over time, the pads change shape and grow, restoring normal joint function. In the future, this technique may be used to treat severe joint injuries in humans.
Bone, or Osseous tissue
Roughly two–thirds of the matrix of bone consists of a mixture of calcium salts, primarily calcium phosphate, with lesser amounts of calcium carbonate. The rest of the matrix is dominated by collagen fibers. By themselves, calcium salts are hard, but rather brittle. Collagen fibers are stronger, but relatively flexible. In bone, the minerals are organized around the collagen fibers. The result is a strong, somewhat flexible combination that is highly resistant to shattering. In its overall properties, bone can compete with the best steel–reinforced concrete.
Lacunae in the matrix contain osteocytes, or bone cells. The lacunae are
typically organized around blood vessels that branch through the bony matrix.
Although diffusion cannot occur through the hard matrix, osteocytes communicate
with the blood vessels and with one another by means of slender cytoplasmic
extensions. These extensions run through long, slender passageways in the
matrix. Called canaliculi,
these passageways form a branching network for the exchange of materials between
blood vessels and osteocytes.
Except in joint cavities, where they are covered by a layer
of hyaline cartilage, bone surfaces are sheathed by a periosteum, a layer composed of fibrous (outer) and
cellular (inner) layers. The periosteum assists in the attachment of a bone to
surrounding tissues and to associated tendons and ligaments. Unlike cartilage,
bone undergoes extensive remodeling throughout life, and complete repairs can
be made even after severe damage has occurred. Bones also respond to the
stresses placed on them, growing thicker and stronger with exercise and
becoming thin and brittle with inactivity.
A membrane is a physical barrier. Each consists of an epithelium supported by connective tissue. Four such membranes occur in the body: (1) mucous membranes , (2) serous membranes , (3) the cutaneous membrane , and (4) synovial membranes .
Mucous membranes , or mucosae, line passageways and chambers, including the digestive, respiratory, reproductive, and urinary tracts, that communicate with the exterior.
The epithelial surfaces of these passageways must be kept
moist to reduce friction and, in many cases, facilitate absorption or
secretion. The epithelial surfaces are lubricated either by mucus, produced by
goblet cells or multicellular glands, or by exposure to fluids such as urine or
semen. The areolar tissue component of a mucous membrane is called the lamina propria.
Many mucous membranes are lined by simple epithelia that perform absorptive or secretory functions, such as the simple columnar epithelium of the digestive tract. Other types of epithelia may be involved, however. For example, a stratified squamous epithelium is part of the mucous membrane of the mouth, and the mucous membrane along most of the urinary tract has a transitional epithelium.
Serous membranes line the sealed, internal subdivisions of the ventral body cavity–cavities that are not open to the exterior. These membranes consist of a mesothelium supported by areolar tissue. As you may recall from Chapter 1 , the three types of serous membranes are (1) the pleura , which lines the pleural cavities and covers the lungs; (2) the peritoneum , which lines the peritoneal cavity and covers the surfaces of the enclosed organs; and (3) the pericardium , which lines the pericardial cavity and covers the heart.
Each serous membrane can be divided into a parietal portion , which lines the inner surface of the cavity, and an opposing visceral portion , or serosa , which covers the outer surfaces of visceral organs.
The primary function of any serous membrane is to minimize friction between the opposing parietal and visceral surfaces. Tissue fluids continuously diffuse onto the exposed surface, keeping it moist and slippery.
The fluid formed on the surfaces of a serous membrane is called a transudate. In healthy individuals, the total volume of transudate is extremely small, just enough to prevent friction between the walls of the cavities and the surfaces of internal organs. But after an injury or in certain disease states, the volume of transudate may increase dramatically, complicating existing medical problems or producing new ones.
The Cutaneous Membrane
The cutaneous membrane , or skin, covers the surface of the body. It consists of a stratified squamous epithelium and a layer of areolar tissue reinforced by underlying dense connective tissue. In contrast to serous and mucous membranes, the cutaneous membrane is thick, relatively waterproof, and usually dry.
Adjacent bones often interact at joints, or articulations . At an articulation, the two articulating bones are very close together if not in contact. Joints that permit significant amounts of movement are complex structures. Such a joint is surrounded by a fibrous capsule, and the ends of the articulating bones lie within a joint cavity filled with synovial fluid. The synovial fluid is produced by a synovial membrane , which lines the joint cavity.
Even though the adjacent ends of the bones are covered by a smooth layer of articular cartilage, the surfaces must be lubricated to keep friction from damaging the opposing surfaces. The necessary lubrication is provided by the synovial fluid, which is similar in composition to the ground substance in loose connective tissues. Synovial fluid circulates from the areolar tissue into the joint cavity and percolates through the articular cartilages, providing oxygen and nutrients to the chondrocytes. Joint movement is important in stimulating the formation and circulation of synovial fluid: if a synovial joint is immobilized for long periods, the articular cartilages and the synovial membrane undergo degenerative changes.
Problems with Serous Membranes
Infection and chronic irritation, can cause the abnormal buildup of fluid in a ventral body cavity.
Pleuritis , or pleurisy , is an inflammation of the pleural cavities. At first the membranes become dry, and the opposing membranes may scratch against one another, producing a sound known as a pleural rub . Friction between opposing layers of serous membranes may promote the formation of adhesions –fibrous connections that lock the membranes together and eliminate the friction. Adhesions also severely restrict the movement of the affected organ or organs and may compress blood vessels or nerves. However, adhesions seldom form between the serous membranes of the pleural cavities. More commonly, continued inflammation and rubbing lead to a gradual increase in fluid production to levels well above normal. Fluid then accumulates in the pleural cavities, producing a condition known as pleural effusion . Pleural effusion is also caused by heart conditions that elevate the pressure in blood vessels of the lungs. As fluids build up in the pleural cavities, the lungs are compressed, making breathing difficult. The combination of severe pleural effusion and heart disease can be lethal.
Pericarditis is an inflammation of the pericardium. This condition typically leads to pericardial effusion , an abnormal accumulation of the fluid in the pericardial cavity. When sudden or severe, the fluid buildup can seriously reduce the efficiency of the heart and restrict blood flow through major vessels.
Peritonitis , an inflammation of the peritoneum, can follow infection of, or injury to, the peritoneal lining. Peritonitis is a potential complication of any surgical procedure in which the peritoneal cavity is opened. Liver disease, kidney disease, or heart failure can cause an increase in the rate of fluid movement through the peritoneal lining. Ascites, the accumulation of peritoneal fluid, creates a characteristic abdominal swelling.
4–5 The Connective Tissue Framework of the Body
Layers of connective tissue connect the organs within the dorsal and ventral body cavities with the rest of the body. These layers (1) provide strength and stability, (2) maintain the relative positions of internal organs, and (3) provide a route for the distribution of blood vessels, lymphatic vessels, and nerves. Fasciae are connective tissue layers and wrappings that support and surround organs. We can divide the fasciae into three types of layers: the superficial fascia, the deep fascia, and the subserous fascia.
1. The superficial fascia , or subcutaneous layer is also termed the hypodermis. This layer of areolar tissue and fat separates the skin from underlying tissues and organs, provides insulation and padding, and lets the skin and underlying structures move independently.
2. The deep fascia consists of dense irregular connective tissue. The organization of the fibers resembles that of plywood: All the fibers in one layer run in the same direction, but the orientation of the fibers changes from layer to layer. This arrangement helps the tissue resist forces applied from many directions. The tough capsules that surround most organs, including the kidneys and the organs in the thoracic and peritoneal cavities, are bound to the deep fascia. The perichondrium around cartilages, the periosteum around bones and the ligaments that interconnect them, and the connective tissues of muscle, including tendons, are also connected to the deep fascia. The dense connective tissue components are interwoven. For example, the deep fascia around a muscle blends into the tendon, whose fibers intermingle with those of the periosteum. This arrangement creates a strong, fibrous network and ties structural elements together.
3. The subserous fascia is a layer of areolar tissue that lies between the deep fascia and the serous membranes that line body cavities. Because this layer separates the serous membranes from the deep fascia, movements of muscles or muscular organs do not severely distort the delicate lining.
4–6 Muscle Tissue
Muscle tissue is specialized for contraction. Muscle cells possess organelles and properties distinct from those of other cells. There are three types of muscle tissue: (1) skeletal muscle , which forms the large skeletal muscles responsible for gross body movements and locomotion; (2) cardiac muscle , found only in the heart and responsible for the circulation of blood; and (3) smooth muscle , found in the walls of visceral organs and a variety of other locations, where it provides elasticity, contractility, and support.
Skeletal muscle tissue contains very large muscle cells–up to 0.3 meter (1 ft) or more in length. Because the individual muscle cells are relatively long and slender, they are usually called muscle fibers . Each muscle fiber is described as multinucleate , because, instead of having a single nucleus, it has several hundred distributed just inside the cell membrane.
Skeletal muscle fibers are incapable of dividing, but new
muscle fibers are produced through the divisions of satellite cells , stem cells that
persist in adult skeletal muscle tissue. As a result, skeletal muscle tissue
can at least partially repair itself after an injury.
As noted in Chapter 3 , the cytoskeleton contains actin and myosin filaments. In skeletal muscle fibers, however, these filaments are organized into repeating groups that give the cells a striated, or banded, appearance. The striations , or bands, are readily apparent in light micrographs. Skeletal muscle fibers do not usually contract unless stimulated by nerves, and the nervous system provides voluntary control over their activities. Thus, skeletal muscle is called striated voluntary muscle .
A skeletal muscle is an organ of the muscular system, and although muscle tissue predominates, it contains all four types of body tissue. Within a skeletal muscle, adjacent skeletal muscle fibers are tied together by collagen and elastic fibers that blend into the attached tendon or aponeurosis. The tendon or aponeurosis conducts the force of contraction, often to a bone of the skeleton. Thus, when the muscles contract, they pull on the attached bone, producing movement.
Cardiac Muscle Tissue
Cardiac muscle tissue is located only in the heart. A typical cardiac muscle cell, also known as a cardiocyte , or cardiac myocyte , is smaller than a skeletal muscle cell. A typical cardiac muscle cell has one centrally positioned nucleus, but some cardiocytes have as many as five. Prominent striations resemble those of skeletal muscle; the actin and myosin filaments are arranged the same way in both cell types.
Cardiac muscle cells form extensive connections with one another. The connections occur at specialized regions known as intercalated discs . At an intercalated disc, the membranes are locked together by desmosomes, intercellular cement, and gap junctions. As a result, cardiac muscle tissue consists of a branching network of interconnected muscle cells. The desmosomes and intercellular cement lock the cells together during a contraction. Ion movement through gap junctions helps coordinate the contractions of the cardiac muscle cells. Cardiac muscle tissue has a very limited ability to repair itself. Although some cardiac muscle cells do divide after an injury to the heart, the repairs are incomplete and some heart function is usually lost.
Cardiac muscle cells do not rely on nerve activity to start a contraction. Instead, specialized cardiac muscle cells called pacemaker cells establish a regular rate of contraction. Although the nervous system can alter the rate of pacemaker cell activity, it does not provide voluntary control over individual cardiac muscle cells. Therefore, cardiac muscle is called striated involuntary muscle .
Smooth Muscle Tissue
Smooth muscle tissue is located (1) in the walls of blood vessels, (2) around hollow organs such as the urinary bladder, and (3) in layers around the respiratory, circulatory, digestive, and reproductive tracts. A smooth muscle cell is a small, spindle–shaped cell with tapering ends and a single, oval nucleus. Smooth muscle cells can divide; hence, smooth muscle tissue can regenerate after an injury.
The actin and myosin filaments in smooth muscle cells are organized differently from those of skeletal and cardiac muscles. One result of this difference is that smooth muscle tissue has no striations. Smooth muscle cells may contract on their own, with gap junctions between adjacent cells coordinating the contractions of individual cells. The contraction of some smooth muscle tissue can be controlled by the nervous system, but contractile activity is not under voluntary control. Imagine the degree of effort that would be required to exert conscious control over the smooth muscles along the 8 m of digestive tract, not to mention the miles of blood vessels! Because the nervous system usually does not provide voluntary control over smooth muscle contractions, smooth muscle is known as nonstriated involuntary muscle .
4–7 Neural Tissue
Neural tissue , which is also known as nervous tissue or nerve tissue , is specialized for the conduction of electrical impulses from one region of the body to another. Ninety–eight percent of the neural tissue in the body is concentrated in the brain and spinal cord.
Neural tissue contains two basic types of cells: (1) neurons and (2) several kinds of supporting cells, collectively called neuroglia, or glial cells). Our conscious and unconscious thought processes reflect the communication among neurons in the brain. Such communication involves the propagation of electrical impulses, in the form of changes in the transmembrane potential. Information is conveyed both by the frequency and by the pattern of the impulses. Neuroglia support and repair neural tissue and supply nutrients to neurons.
The longest cells in your body are neurons, many of which are as much as a meter (39 in.) long. Most neurons cannot divide under normal circumstances, so they have a very limited ability to repair themselves after injury. A typical neuron has a large cell body with a large nucleus and a prominent nucleolus.
Extending from the cell body are many branching processes (projections or outgrowths) termed dendrites and one axon . The dendrites receive information, typically from other neurons, and the axon carries that information to other cells. Because axons tend to be very long and slender, they are also called nerve fibers . In Chapter 12 , we will further examine the properties of neural tissue.
4–8 Tissue Injuries and Aging
Inflammation and Regeneration
The restoration of homeostasis after a tissue has been injured involves two related processes: inflammation and repair. First, immediately after the injury, the area is isolated while damaged cells, tissue components, and any dangerous microorganisms are cleaned up. This phase, which coordinates the activities of several types of tissue, is called inflammation , or the inflammatory response . It produces several familiar sensations, including swelling, redness, warmth, and pain. An inflammation resulting from the presence of pathogens, such as harmful bacteria, is called an infection .
Second, the damaged tissues are replaced or repaired to restore normal function. The repair process is called regeneration .
First Phase: Inflammation
Trauma from impact, abrasion, distortion, chemical irritation, infection by pathogenic organisms (such as bacteria or viruses), and extreme temperatures (hot or cold)–can produce inflammation. Each of these stimuli kills cells, damages fibers, or injures the tissue in some other way.
Necrosis, the tissue degeneration that occurs after cells have been hurt or destroyed, begins several hours after the original injury. The damage is caused by lysosomal enzymes. Through widespread autolysis, lysosomes release enzymes that first destroy the injured cells and then attack surrounding tissues. The result may be an accumulation of debris, fluid, dead and dying cells, and necrotic tissue components collectively known as pus . An accumulation of pus in an enclosed tissue space is an abscess . These tissue changes trigger the inflammatory response by stimulating mast cells.
release a variety of chemicals. These chemicals, including histamine and
prostaglandins, trigger changes in local circulation. In response, the smooth
muscle tissue that surrounds local blood vessels relaxes, and the vessels dilate , or enlarge in diameter. This dilation
increases blood flow through the tissue, turning the region red and making it
warm to the touch. The combination of abnormal tissue conditions and chemicals
released by mast cells stimulates sensory nerve endings that produce sensations
of pain. At the same time, the chemicals released by mast cells make the
endothelial cells of local capillaries more permeable. Plasma, including blood
proteins, now diffuses into the injured tissue, so the area becomes swollen.
The increased blood flow accelerates the delivery of nutrients and oxygen and the removal of dissolved waste products and toxic chemicals. It also brings white blood cells to the region. These phagocytic cells migrate to the site of the injury and assist in defense and cleanup operations. Macrophages and microphages protect the tissue from infection and perform cleanup by engulfing both debris and bacteria.
Second Phase: Regeneration As
tissue conditions return to normal, fibroblasts
move into the necrotic area, laying down a network of collagen fibers that
stabilizes the injury site. This process produces a dense, collagenous
framework known as scar
tissue or fibrous tissue . Over time, scar tissue is usually
"remodeled" and gradually assumes a more normal appearance. The cell
population in the area gradually increases; some cells migrate to the site, and
others are produced by the division of mesenchymal stem cells.
Each organ has a different ability to regenerate after injury–an ability that can be directly linked to the pattern of tissue organization in the injured organ. Epithelia, connective tissues (except cartilage), and smooth muscle tissue usually regenerate well, whereas other muscle tissues and neural tissue regenerate relatively poorly if at all. The skin, which is dominated by epithelia and connective tissues, regenerates rapidly and completely after injury.
In contrast, damage to the heart is much more serious. Although the connective tissues of the heart can be repaired, the majority of damaged cardiac muscle cells are replaced only by fibrous tissue. The permanent replacement of normal tissue by fibrous tissue is called fibrosis. Fibrosis in muscle and other tissues may occur in response to injury, disease, or aging.
Aging and Tissue Repair
Epithelia get thinner and connective tissues more fragile.
Individuals bruise easily and bones become brittle; joint pain and broken bones
are common in the elderly. Because cardiac muscle cells and neurons cannot be
replaced, cumulative damage can
eventually cause major health problems, such as cardiovascular disease or a
deterioration in mental functioning.
For example, the chondrocytes of older individuals produce a slightly different form of proteoglycan than do those of younger people. This difference probably accounts for the thinner and less resilient cartilage of older people. In some cases, the tissue degeneration can be temporarily slowed or even reversed. The age–related reduction in bone strength, a condition called osteoporosis , typically results from a combination of inactivity, low dietary calcium levels, and a reduction in circulating sex hormones. A program of exercise, calcium supplements, and hormone replacement therapies can generally maintain healthy bone structure for many years.
Aging and Cancer Incidence
Cancer rates increase with age, and roughly 25 percent of
all people in the
Physicians who specialize in the study of disease processes are called pathologists. Diagnosis, rather than treatment, is usually the main focus of their activities. In their analyses, pathologists integrate anatomical and histological observations to determine the nature and severity of a disease. Physicians who specialize in the study of cancer are called oncologists.
The first abnormality to be observed is dysplasia, a change in the normal shape, size, and organization of tissue cells. Dysplasia is generally a response to chronic irritation or inflammation, and the changes are reversible. The normal trachea (windpipe) and its branches are lined by a pseudostratified ciliated columnar epithelium. The cilia move a mucous layer that traps foreign particles and moistens incoming air. The drying and chemical effects of smoking first paralyze the cilia, halting the movement of mucus. As mucus builds up, the individual coughs to dislodge it (the well–known "smoker's cough").
Epithelia and connective tissues may undergo more radical changes in structure, caused by the division and differentiation of stem cells. Metaplasia is a structural change that dramatically alters the character of the tissue. In our heavy–smoking example, over time the epithelial cells lose their cilia altogether.
As metaplasia progresses, the epithelial cells produced by stem cell divisions no longer differentiate into ciliated columnar cells. Instead, they form a stratified squamous epithelium
that provides greater resistance to drying and chemical irritation. This epithelium protects the underlying tissues more effectively, but it eliminates the moisturization and cleaning properties of the epithelium. Cigarette smoke will now have an even greater effect on more delicate portions of the respiratory tract. Fortunately, metaplasia is reversible, and the epithelium gradually returns to normal if the individual quits smoking.
In anaplasia, tissue organization breaks down. Tissue cells change size and shape, typically becoming unusually large or small. Anaplasia occurs in smokers who develop one form of lung cancer; the cells divide more frequently, but not all divisions proceed in the normal way. Many of the tumor cells have abnormal chromosomes. Unlike dysplasia and metaplasia, anaplasia is irreversible.