biol 140 Tissues: Living Communities

What is a gland?

A gland is an organ in an animal's body that synthesizes a substance for release of substances such as hormones or breast milk,

Major Cell Types

Although connective tissue contains a wide variety of cell types, they can be organized into two major categories: those that remain in the connective tissue, called fixed cells, and those that pass in and out of the connective tissue, called transient cells. Fixed cells remain in the connective tissue and are usually involved in the production and maintenance of the matrix. Transient cells, however, do not have a permanent residence in the tissue but move in and out of it as needed. Transient cells generally are involved in the repair and protection of the tissue.

What are the three basic shapes of epithelial cell?

Epithelial cells may be squamous, cuboidal, or columnar in shape and may be arranged in single or multiple layers.

what are the four primary types of tissues

Epithelial tissue Connective tissue Muscle tissue Nervous tissue

Elastic Cartilage

Histologically, elastic cartilage is similar to hyaline cartilage but contains a plethora of elastic fibers, which form dense, branching bundles that appear black microscopically (Figure 5-23). These fibers give elastic cartilage tremendous flexibility so that it can withstand repeated bending. Elastic cartilage is found in the epiglottis of the larynx and in the pinnae (external ears) of animals.

Homeostasis

Of the same Similar to Like Resembling

HOMEOSTASIS

Preservation of Dynamic equilibrium Dynamic - lively, energetic, actively Equilibrium - stability, balance Goal to maintain State of normal "Normalcy"

Epithelia perform vital functions in the bodies of animals.

Protects, covers, and lines • Filters biochemical substances • Absorbs nutrients • Provides sensory input • Manufactures secretions • Manufactures excretions

STASIS

Slowing Stoppage Not moving Equilibrium

where are goblet cells found

are situated in the epithelium of the conducting airways, often with their apical surfaces protruding into the lumen, a location which fits them for a rapid response to inhaled airway insults.

what the functions of Muscle tissue

enables movement

what the functions of connective tissue

provides support

Gross anatomy

the study of anatomic structures that can be seen with the naked eye, and includes learning the names and locations of bones, muscles, arteries, veins, and nerves.

surfaces of epithelial cells

vary depending on where the epithelium is located in the body and, more importantly, what role it plays in the function of the tissue. The epithelia that line blood vessels, for example, have smooth surfaces to allow the easy passage of blood cells. However other epithelia have irregular surfaces and may be covered with many fingerlike projections, called microvilli, or thousands of tiny hairs, called cilia

Histopathology

An Introduction Histopathology is the microscopic study of disease in tissues (histo- means tissue and pathology is the study of disease). The normal microanatomy of tissues is altered by pathologic disorders in many ways. For example, in mammary tissue that contains a malignant tumor, abnormally large, immature mammary cells may be evident microscopically. The nuclei of these cells are abnormally large, and many of the cells may be actively dividing. Other diseases, such as viral and bacterial infections, may cause cell death and create regions within the tissue that are dead or necrotic. Still other diseases may cause the abnormal accumulation of fluid, leading to a condition called edema, or may involve the accumulation of a waxlike glycoprotein called amyloid. Thus pathologic conditions may be evident microscopically as an increase or decrease in cell numbers, cell size, and changes in the cell's shape and in the architecture of the tissue's support structures. Although the pathologic nature of diseased tissue is often evident under microscopic examination, grossly the tissue may appear normal. For this reason, a definitive diagnosis often can be made only through the microscopic examination of tissue by taking a biopsy. Occasionally, aspirating cells from the tissue can lead to a diagnosis also. A biopsy is the removal of a small piece of tissue from an organ or mass. This may involve the insertion of a special kind of biopsy needle into the tissue, or it may involve cutting out, or excising, a piece of tissue with a scalpel. Some biopsy samples are obtained using special grasping attachments on the exploratory end of endoscopes; others may be acquired using a cookie-cutter type of instrument called a biopsy punch. Biopsy samples are harvested from the normal-abnormal tissue borders if possible. In addition, because the tissues are delicate, they should be cut with sharp instruments, such as scalpels, and maneuvered with tissue forceps, not dressing forceps, so that the microanatomy is not crushed. No matter how the sample is obtained, it must be handled with care and specially prepared before it can be examined. Samples should be sliced so that each piece is no thicker than 1 cm, and these should be pla

Loose Connective Tissue

Areolar Tissue. Areolar connective tissue is a beautiful tangle of randomly placed fibers and cells suspended in a thick, translucent ground substance (Figure 5-17). The tissue appears relaxed with a myriad of round and star-shaped cells placed among crisscrossing fibers. The predominant cell is the fibroblast, a large spindle-shaped cell that manufactures the elastic, reticular, and collagenous fibers found throughout the tissue. Areolar tissue acquires its name from the Latin areola, which means small, open space. Areolar tissue is the most common type of connective tissue and is found everywhere in the body. It acts generally as packing material to support and cushion organs and other delicate structures of the body. It surrounds every organ; forms the subcutaneous layer that connects skin to muscle; envelops blood vessels, nerves, and lymph nodes; and is present in all mucous membranes as the lamina propria and submucosa. It is supportive to body structures but is flexible and soft to enable organs the freedom to move within their position. Thus areolar tissue is moderately elastic but tears easily compared with the other types of connective tissue. The small, open spaces in areolar tissue are filled with a mixture of body fluids and ground substance. The ground substance is thick and is composed primarily of hyaluronic acid, which serves as a medium through which nutrients, gases, and waste can be easily transported to and from the bloodstream. In addition, the viscous texture of the ground substance is an effective barrier against most invading microorganisms, because it inhibits their movement through the tissue. Some white blood cells have developed the ability to produce hyaluronidase, an enzyme that liquefies the matrix and allows white blood cells to pass through with greater ease. This adaptation has improved the ability of white blood cells to perform their duties in loose connective tissue. Unfortunately, some microbes have also developed the ability to produce hyaluronidase, which facilitates the spread of infection throughout the tissue. During trauma or other pathologic states, the spaces in loose connective tissue can fill with an excessive amount of body fluid. This condition is called ede

Bone.

Bone or osseous connective tissue is the hardest and most rigid type of connective tissue (Figure 5-25). Its specialized matrix is a combination of organic collagen fibers and inorganic calcium salts, such as calcium phosphate and calcium carbonate. The calcium salts alone would render bone brittle, but when combined with collagen fibers, bone becomes more flexible and has greater strength. Despite the rigidity of its matrix, bone, unlike cartilage, is well vascularized. A central Haversian canal contains both a vascular and a nerve supply. In addition, tiny channels exist within the matrix that support the passage of blood vessels into deeper portions of the tissue. Bone cells, such as osteoblasts and osteoclasts, collaborate to remodel bone in response to the stresses placed on them. This involves a well-orchestrated combination of laying down new bone and taking away bone that is not needed. Osteoblasts, like other fibroblasts, manufacture the fibers that are part of the matrix. Although mature osteocytes reside individually in chambers called lacunae, they possess long, cellular extensions that pass through tiny threadlike channels called canaliculi, which radiate away from the lacunae. These chambers and canals are created as the osteoblasts surround themselves with the bony matrix they manufacture. Later they mature to become osteocytes. In this way, the cells not only create their own living spaces but also maintain connections with other cells. Bone forms the skeletal frame of animals. It protects vital organs, such as the brain and heart, and acts as a calcium reserve for the body. In addition, bone marrow is the site of blood cell production and fat storage. A discussion of the development, structure, and function of bone is found in Chapter 7.

Cardiac muscle

Cardiac muscle exists only in the heart and possesses the remarkable ability to contract even when neural input has been altered. Specialized pacemaker cells within the heart muscle supply the signal for the heart to contract at regular intervals. This input is entirely involuntary and is responsible for initiating the pumping force, which propels blood through blood vessels. Thank goodness we do not need to concentrate on keeping our hearts beating! As in smooth muscle, the cells of cardiac muscle are relatively small and contain only one nucleus. However, unlike smooth or skeletal muscle, cardiac muscle branches to form a complex network. The cardiac muscle cells are striated and are connected to one another at each end via a specialized intercellular junction called an intercalated disc. These discs occur only in cardiac muscle. Thus cardiac muscle is classified as an involuntary, striated tissue.

Specialized Connective Tissue Cartilage.

Cartilage is a tough, specialized connective tissue that is commonly called gristle. It is more rigid than dense connective tissue but is more flexible than bone (Figure 5-22). Cartilage is found in joints and helps to prevent the sensitive outer layers of bone from rubbing against one another. Because cartilage does not contain nerves, it can tolerate a great deal of compression without causing pain to the animal. (Imagine the compressive forces in the legs of an elephant!) In addition to joints, cartilage is found in the ear, nose, and vocal cords and forms a vital framework on which bone is formed in growing animals. Like other forms of connective tissue, cartilage is composed of cells and matrix. The cells, called chondrocytes, live in hollowed-out pockets in the matrix, called lacunae. The ground substance of the matrix is a firm gel containing two different types of glycosaminoglycans (chondroitin sulfate and hyaluronic acid) and an adhesion protein called chondronectin. It also contains an unexpectedly large amount of tissue fluid. The fluid is held within the matrix and is important in transporting nutrients to the chondrocytes. It also gives cartilage its flexible resiliency and its ability to withstand compression. Collagen fibers are most commonly found in the matrix, but elastic fibers are also present in varying amounts. Cartilage is avascular and therefore is very slow to heal. It receives its nutrition from a surrounding membrane, called the perichondrium, which is rich in tiny blood vessels. Nutrients diffuse from the perichondrium through the matrix to the chondrocytes. Therefore the chondrocytes that are farthest away from the perichondrium are potentially less well nourished than cells closer to it. For this reason, the thickness of cartilage is limited. Three types of cartilage that vary from one another on the basis of the type of fiber found in the matrix are hyaline cartilage, elastic cartilage, and fibrocartilage. Hyaline Cartilage. Hyaline cartilage is the most common type of cartilage found in the body. It is composed of closely packed collagen fibers that make it tough but more flexible than bone. Grossly, hyaline cartilage resembles blue-white, frosted, ground glass. It is found a

Fibers of Connective Tissue

Connective tissue contains three types of fiber: collagenous, reticular, and elastic. Although these fibers exist in all connective tissue, their proportions vary from one type of connective tissue to another. Collagenous fibers are by far the most commonly found in the body. Collagenous fibers are strong, thick strands composed of the structural protein collagen. Collagen fibers are organized into discrete bundles of long, parallel fibrils, which in turn are composed of bundled microfibrils. Because they possess tremendous tensile strength, enabling them to resist pulling forces, collagenous fibers are found in tendons and ligaments that are continually being pulled and stretched. When not under tension, collagenous fibers look wavy. The fiber itself is white, and the tissue it forms when the fibers are packed closely together is also white. Therefore it is not surprising that collagenous fibers are sometimes known as the white fibers. The density and arrangement of collagen fibers can vary depending on the function of the tissue as a whole. Collagenous connective tissue can range from loose, as in the loose connective tissue that surrounds and protects organs, to dense arrangements seen in tendons. The tissue forms when collagen proteins are secreted into the extracellular environment, where they are arranged into formation. If subjected to heat, collagen denatures and turns into a soft gel. This is why meat, which is rich with collagenous fibers, softens when cooked for long periods in soups and stews. At the same time, collagen can be fortified with tannic acid, as is evident in leather that has been strengthened by tanning. Reticular fibers, like collagenous fibers, are composed of collagen, but they are not thick. Instead, they are thin, delicate, and branched into complicated networks. Reticular fibers form a kind of "mist net" (rete is Latin for net) that provides support for highly cellular organs, such as endocrine glands, lymph nodes, spleen, bone marrow, and liver. Reticular fibers are also found around blood vessels, nerves, muscle fibers, and capillaries. Elastic fibers are composed primarily of the protein elastin. Like reticular fibers, elastic fibers are branched and form complex networks, bu

General Characteristics

Connective tissue is found everywhere in the body and represents the most abundant tissue type by weight. Some organ systems, such as the skeletal and integumentary systems, are composed almost exclusively of connective tissue, whereas others, such as the neurologic system, contain very little. Connective tissue is derived from mesoderm and, unlike epithelial tissue, is composed primarily of nonliving extracellular matrix. The matrix surrounds and separates the cells, and it provides important structural and nutritional support that enables connective tissue cells to exist farther apart than epithelial cells. In addition, unlike epithelial tissue, which has no direct blood supply, connective tissue is vascularized although the level of vascularity varies among different connective tissue types. Loose connective tissue and adipose connective tissue, for example, possess good blood supplies, whereas dense connective tissue is poorly vascularized. All connective tissue is composed of three distinct components: extracellular fibers, ground substance, and cells. The mixture of fiber and ground substance is called the extracellular matrix. Variations in the ground substance, fibers, and cellular components have given rise to a wide range of connective tissue types (Figure 5-16). Blood, tendon, fat, cartilage, and even bone are all examples of connective tissue, though their textures and appearances are different. Variations in the type of ground substance and in the type of fiber enable the tissue to take on many different qualities. It can be elastic and flexible, rigid, semisolid, or liquid. Blood, for example, is a highly cellular connective tissue with a liquid matrix containing relatively little fiber. In contrast, bone is composed of a solid calcified matrix. Tendon contains a matrix that is primarily fibrous with little ground substance. These variations give connective tissue the ability to withstand a wide range of forces, such as direct pressure, abrasion, and shearing forces that would destroy other tissue types.As with all living structures, form and function are intertwined. Thus the plethora of forms that characterize connective tissue give rise to a wide range of functions. In general, as its name im

Connective tissue proper

Connective tissue proper is the largest classification and contains every subtype of connective tissue except bone, cartilage, and blood. The two subclasses of connective tissue proper are loose connective tissue and dense connective tissue. Loose connective tissue includes areolar, adipose, and reticular tissue; dense connective tissue includes dense regular, dense irregular, and elastic tissue.

Dense fibrous connective tissue

Dense fibrous connective tissue is characterized by its densely packed arrangement of collagen fibers. Because little room is available for ground substance and cells, these are found in smaller quantities than in loose connective tissue. Nevertheless, as in loose connective tissue, fibroblasts can be found intermingled with fibers, where they play out their important role of manufacturing fibers and ground substance. The three major types of dense fibrous connective tissue are dense regular, dense irregular, and elastic.

Dense regular connective tissue

Dense regular connective tissue is composed of tightly packed, parallel collagen fibers (Figure 5-20). The fibers lie in the direction of the force that is exerted on them, thereby giving the overall tissue tremendous tensile strength, but only in one direction. Dense regular connective tissue is silvery or white. It is relatively avascular and therefore is very slow to heal, because restorative nutrients and building molecules have difficulty reaching the damaged tissue. Fibroblasts form rows along the crowded fibers and devote most of their energy to the manufacture of fibers. Little ground substance is produced.Dense connective tissue makes up the tendons that attach muscles to bone and the ligaments that hold bones together at joints. It also composes the broad, fibrous ribbons that sometimes cover muscles or connect them to other structures. In addition, dense connective tissue can be found in fascial sheets that cover muscles. These sheets are stacked into layers, one on top of another, but the direction of the fibers in one fascial layer may be different from the direction of the fibers in another layer. This helps to create an overall structure or fascia that can withstand forces from more than one direction. Dense irregular connective tissue is composed primarily of collagen fibers that are arranged in thicker bundles than those found in dense regular connective tissue (Figure 5-21). The fibers are interwoven randomly to form a single sheet that can withstand forces from many different directions. It is found in the dermis of the skin and in the fibrous coverings of organs such as the kidney, testes, liver, and spleen. It also forms the tough capsule of joints.

Membranes

Epithelial and connective tissue may be collaboratively linked to form membranes in the body. Membranes are thin, protective layers that line body cavities, separate organs, and cover surfaces. They are composed of a multicellular epithelial sheet that is bound to an underlying layer of connective tissue proper. Commonly, the epithelium is bathed in a wet solution of liquid mucus or in the case of the bladder, in urine. Four common types of epithelial membranes are mucous, serous, cutaneous, and synovial (Figures 5-27 and 5-28, Table 5-3).

epithelia share certain common characteristics.

Epithelial cells are polar, that is, they have a sense of direction relative to surrounding structures. Each epithelial cell has an apical surface and a basal surface, which are quite different from one another. The apical surface is the side of the cell that faces the lumen or body cavity, and the basal surface is the side of the cell that faces the underlying connective tissue. 2. Epithelial cells have lateral surfaces that are connected to neighboring cells by junctional complexes. These junctions bring the cells into close apposition to one another, leaving little room for extracellular matrix. The matrix that surrounds epithelia therefore exists in very small quantities, if at all. 3. All epithelial cells lack blood vessels or capillaries. They are avascular and rely on underlying connective tissue to provide oxygen and nutrients. 4. Although some epithelia lack nerves, such as those in the stomach, intestines, and cervix, most epithelial cells are innervated and provide valuable sensory input.

What is the difference between endocrine and exocrine glands? Can you give examples of each?

Examples of exocrine glands are sweat, salivary, sebaceous, mucous gland. An endocrine gland is a gland which secretes its products directly into the blood stream. Examples of endocrine glands are pituitary gland, ovaries, testes, thyroid gland, adrenal glands. Two principal types of glands exist: exocrine and endocrine. The key difference between the two types is that, whereas exocrine glands secrete substances into a ductal system to an epithelial surface, endocrine glands secrete products directly into the bloodstream

Fibrocartilage.

Fibrocartilage usually is found merged with hyaline cartilage and dense connective tissue (Figure 5-24). It contains thick bundles of collagen fibers, like hyaline cartilage, but it has fewer chondrocytes and lacks a perichondrium. Fibrocartilage is particularly well designed to take compression and therefore is found in the spaces between vertebrae of the spine, between bones in the pelvic girdle, and in the knee joint.

gland

Glands are classified further according to the way in which they secrete their products. How much of the cell is sacrificed in the act of secretion determines whether the gland is merocrine, apocrine, or holocrine (Figure 5-15). The majority of glands package their secretions into granular units and release them via exocytosis as they are manufactured. These glands are called merocrine glands because the secretory cells remain intact during the secretory process. The pancreas, sweat glands, and salivary glands are examples of merocrine glands. Secretion in apocrine glands involves the loss of the top part of the cell, called the apex of the secretory cell. The secretory cells in apocrine glands do not release their granules as they are manufactured. Instead, they store the granules until the apex of the cell is full. Then the cell pinches in two and releases the apex into the duct system. Later, the cell repairs the damage and repeats the process. Apocrine glands can be found in mammary tissue and are represented by some sweat glands.Like apocrine glands, holocrine glands also store granules in the secretory cells until they are needed. However, in holocrine glands, the entire secretory cell is destroyed in the act of releasing its secretory product. The subsequent degeneration of the cell allows the release of the granules. Holocrine secretion occurs principally in sebaceous glands. We can also categorize glands according to the type of secretion they produce. Serous secretions are watery and contain a high concentration of enzymes, whereas mucous secretions are thick, viscous, and composed of glycoproteins. Secretory cells that manufacture both types of secretion are common in the digestive and respiratory tracts. Mixed exocrine glands contain both mucous and serous components.

Endocrine Glands

Glands that do not have ducts or tubules and whose secretions are distributed throughout the body are called endocrine glands. They produce and secrete regulatory chemicals, known as hormones, into the bloodstream or the lymphatic system, where they are carried to many regions of the body. Endocrine glands are part of a complex, biochemical network known as the endocrine system. The pituitary gland in the brain and the adrenal gland near the kidney are examples of endocrine glands. The endocrine system and the glands it includes are discussed in detail in Chapter 11.With the exception of the goblet cell, exocrine glands possess ducts. They are more common than endocrine glands and act by discharging secretions via their ducts directly into nearby areas where they may, for example, cover cell surfaces or empty into body cavities. Unlike those of endocrine glands, the secretions of exocrine glands act locally and do not normally enter the circulation. A wide variety of exocrine glands are found in animals, including hepatoid, musk, sweat, and salivary glands. Other examples can be found in the liver and pancreas, where exocrine glands secrete bile and digestive enzymes, respectively. The pancreas possesses endocrine and exocrine properties because it is responsible for producing many hormones and for secreting digestive enzymes.

Major Components of Connective Tissue

Ground Substance Ranges from liquid to gel to solid Fibers Collagenous Reticular Elastic Cells Fixed cells Fibroblasts Adipocytes (fat cells) Reticular cells Wandering cells Mast cells Leukocytes (white blood cells) Macrophages (fixed and wandering)

Types of Connective Tissue As already mentioned, all connective tissue is made up of three major components

Ground substance • Cells • Fibers Many different types of connective tissue are formed by the variety of textures of ground substance, the number and type of cells, and the number and type of fibers present in the tissue. By varying the three major constituents, a wide range of connective tissue types is generated. In general, connective tissue is divided into two broad categories: connective tissue proper and specialized connective tissue.

Epithelial tissue is characterized as simple, stratified, or pseudostratified. What does this mean?

If there is only a single layer of epithelial cells, the tissue is classified as simple. If there is more than one layer of cells, the tissue is called stratified. Pseudostratified columnar epithelium is an epithelial layer that is not truly stratified.

Steps in the Process of Inflammation

Inflammation begins with a 5- to 10-minute period of vasoconstriction, followed by a sustained period of vasodilation. The initial constriction occurs in the small vessels of the injured tissue and aids in the control of hemorrhage. Histamine and heparin molecules subsequently are released from mast cells, which stimulate vasodilation and increase permeability of the capillaries. Blood flow to the area is increased, which in turn causes the clinical signs of heat and redness. It also increases the supplies of oxygen and nutrients to the active cells of the damaged tissue. 2. Fluid from plasma, composed of enzymes, antibodies, and proteins, pours into the affected area, causing swelling of the soft tissue structures. This swelling irritates delicate nerve endings and causes pain and tenderness in the affected area. 3. Clot formation begins to take place, which slows bleeding. The clot also helps to isolate the wound from the invasion of pathogens and helps to prevent bacteria and toxins from spreading to surrounding soft tissue structures. A clot first forms when platelets become sticky and clump together. Fibrinogen, which is found in large quantities in the swollen tissue, is converted to an insoluble protein called fibrin. The fibrin is woven into a netlike structure that surrounds the platelets and provides support and stability to the newly formed clot. It also forms a framework to support the movement of cells throughout the site. Clots that form on external surfaces, such as skin, eventually dry to become scabs. 4. Large cells such as macrophages and neutrophils, a type of white blood cell, move through blood vessels and squeeze through dilated capillaries to assist in the removal of debris and microinvaders. These phagocytic cells are short lived, however, and can function for only a few hours before dying. Pus, which is an accumulation of dead and degenerated neutrophils and macrophages, may therefore collect in the injured area. 5. With increased blood flow, histamine and heparin are dispersed, and their levels drop in the affected area. The decrease in these molecules causes the return of normal capillary size and permeability. When capillaries return to normal size, blood flow and fluid leakage int

Tissue Healing and Repair

Injuries occur in many ways. Animals may experience trauma from being hit by a car or from falling out a window. They may be bitten, scratched, or kicked by other animals and may experience broken bones and wounds that later become infected by pathogens. The body's initial response to these injuries is inflammation, a series of events that develop quickly to limit further damage and eliminate any harmful agents. Repair occurs more slowly and involves the organization of granulation tissue and the regeneration of lost tissue or the formation of scar tissue. Many of these processes occur simultaneously, making the injured area a busy workplace for cells. We will take a closer look at what is happening during the healing process. A summary of tissue repair is illustrated in Figure 5-31.

hemidesmosomes

Junctions that look like half of a desmosome are called

Elastic Connective Tissue.

Ligaments can stretch more than tendons because of the larger number of elastic fibers contained within them. The massive nuchal ligament in the neck of horses, for example, has a particularly high concentration of elastic fibers and is therefore extremely flexible, enabling horses to lower their heads for long periods while grazing. Dense connective tissue that is primarily composed of elastic fibers, rather than collagen fibers, is called elastic connective tissue. Elastic connective tissue is found in relatively few regions of the body, such as in the spaces between vertebrae in the backbone. It also occurs in regions of the body that require stretching, such as in the walls of arteries, stomach, large airways (bronchi), bladder, and regions of the heart. It lies beneath the transitional epithelium in the urinary tract and in the ligament suspending the penis. As its name implies, elastic connective tissue consists primarily of yellow elastic fibers. These fibers may be arranged in parallel or in an interwoven pattern with fibroblasts and collagenous fibers interspersed.

Mucous membranes,

Mucous membranes, or mucosae, are characterized by their position in the body, because they are always found lining the organs that have connections to the outside environment. These organs are part of the digestive, respiratory, urinary, and reproductive tracts and include the mouth, esophagus, stomach, intestines, colon, nasal passages, trachea, bladder, and uterus, to name a few. The epithelial layer in mucous membranes is usually composed of either stratified squamous or simple columnar epithelium, and it covers a layer of loose connective tissue called the lamina propria. Another connective tissue layer, called the submucosa, usually connects the mucosa to underlying structures. With the exception of the mucosae of the urinary tract, mucosae in general can produce large quantities of protective and lubricating mucus. Goblet cells or multicellular glands may be found throughout the tissue. These structures are responsible for the production and secretion of mucus, which consists primarily of water, electrolytes, and a protein called mucin. The mucus is slippery and therefore can decrease friction and assist with the passage of food or waste. Because of its rich supply of antibodies and viscous consistency, the mucus produced by the mucosae is also helpful in the entrapment and disposal of invading pathogens and foreign particles. This is particularly apparent in the nasal passages, where microorganisms and debris are inhaled and trapped by mucus. We may find, for example, accumulations of black debris in our noses after we have been near a sooty campfire. When we have a cold, we find that the amount of mucus produced and secreted by the mucosae increases and a runny nose develops. Some mucosae can absorb as well as secrete. For example, the epithelial layer in the intestine is specially designed for rapid and efficient transfer of nutrient molecules from the intestinal lumen to the underlying connective tissue and its blood supply. The mucous membranes therefore play an important role in monitoring and controlling what enters the body and form an important barrier between the outside environment and the delicate inner workings of underlying tissues. Their secretory and absorptive qualities make them parti

Multicellular exocrine glands

Multicellular exocrine glands are made up of two distinct components: a secretory unit in which secretions are produced by secretory cells and a duct that carries the secretion to the deposition site. In most glands the secretory unit is surrounded by connective tissue that is rich in blood vessels and nerve fibers. It not only nourishes the secretory unit but also provides structural support and may extend into the gland to form distinct lobes. In some exocrine glands the secretory unit is surrounded by contractile cells called myoepithelial cells that assist with the discharge of secretions into the glandular duct. The rate of secretion production and discharge is controlled by hormonal and nervous influences. We begin the classification of exocrine glands with examination of the glandular ducts (Table 5-1). If the main duct is unbranched, the gland is considered a simple gland. If the main duct is branched, the gland is called a compound gland. Next, we examine the secretory portions of glands. If the secretory cells form a long channel of even width, the gland is called a tubular gland (Figure 5-14). If the secretory unit forms a rounded sac, the gland is called an alveolar gland or an acinar gland. Glands with secretory units that possess both tubular and alveolar qualities are called tubuloalveolar or tubuloacinar (Figure 5-14).

Muscle cells

Muscle cells, or muscle fibers, are uniquely designed for contraction. The fibers are composed of specialized proteins called actin and myosin, which are arranged into microfilaments. Contraction, or shortening, of the muscle cell occurs when the microfilaments slide over one another, like the bars in an old-fashioned slide ruler. In this way, the cells change shape and can be made shorter or longer. As the muscles contract, they move the bones, blood, and soft tissue structures that are associated with them. Thus blood is circulated, legs are made to run, and food is moved slowly through the intestine. There are three types of muscle tissue: skeletal, smooth, and cardiac

Nervous Tissue

Nervous or neural tissue is uniquely designed to receive and transmit electrical and chemical signals throughout the body (Figure 5-30). It is found in the brain, spinal cord, and peripheral nerves and is composed primarily of two general cell types: neurons and supporting neuroglial cells.Neurons are the longest cells in the body and may reach up to a meter in length. They are composed of three primary parts: a cell body called a perikaryon, short cytoplasmic extensions called dendrites, and a long, single extension called an axon. The cell body contains the nucleus, which controls the metabolism of the cell. The dendrites receive impulses from other cells, whereas the axon conducts impulses away from the cell body. The neuron forms connections with many other tissues, such as muscle, viscera, glands, and other neurons. In this way, a complex network is formed that controls and regulates many body functions. The neuron is exquisitely sensitive to electrical and chemical changes in its environment and may respond by transmitting nerve impulses along its axon to other tissues. These electrical impulses, which carry information and instructions, are transmitted through conductive membranes on the neurons. Neuroglial cells are found in greater numbers in neural tissue than are neurons. They do not transmit impulses but rather serve to support the neurons. Some specialized types of neuroglial cells function to isolate the conductive membranes, others provide a supportive framework that helps bind the components of neural tissue together, and still others phagocytize or digest debris, or they help supply nutrients to neurons by connecting them to blood vessels. The gross anatomy and physiology of nervous tissue

HEALTH

Normal anatomy and physiology Disease = a change in structure and/or function Body health - interrelated structure and function System health Organ health Tissue health Cell health

Glands can be classified in many ways. For example, we can organize them based on the following factors:

Presence or absence of ducts (endocrine or exocrine). 2. Number of cells that compose them (unicellular or multicellular). 3. Shape of the secreting ducts (simple or compound). 4. Complexity of the glandular structure (tubular, acinar, or tubuloacinar). 5. Type of secretion they produce (mucoid or serous). 6. Manner in which the secretion is stored and discharged (merocrine, apocrine, or holocrine).

Pseudo- means "false," therefore pseudostratified columnar epithelium is an epithelial layer that is not truly stratified. The epithelial cells appear to be stratified because the nuclei are found at different levels across the length of the tissue layer. However, not all of the cells reach the luminal surface, so cells appear to be at different levels as though stratified. In reality, each cell forms a distinct attachment, however subtle, with the basement membrane. In this way, pseudostratified columnar epithelium forms a single layer and therefore is considered a simple epithelium (Figure 5-10).Most pseudostratified columnar epithelium is ciliated and is found in the respiratory tract and in portions of the male reproductive tract. In the trachea, for example, the epithelium is coated with a layer of mucus that is propelled by cilia across the luminal surface toward the mouth. This assists in preventing debris from entering the lungs. The mucus is also fortified with protective immunoglobulins, which are disease-fighting molecules that help to protect animals from pathogens (bacteria and viruses) that have been inhaled.

Pseudostratified Columnar Epithelium

The two main types of adipose tissue are white adipose tissue and brown adipose tissue. White adipose tissue is found throughout the body, particularly in the deep layers of the skin. Initially, white adipocytes resemble fibroblasts, but as they fill with lipid, the organelles and nuclei are pushed to one side and the cells become large spheres with eccentrically placed nuclei. As the cells swell, the cytosol is compressed into a thin, barely visible rim that surrounds the lipid droplet. Despite the compact condition of the cytoplasm, it continues to house all of the organelles normally found in cells. During tissue preparation for microscopic examination, the lipid content of the adipocyte is extracted, leaving a large unstained space in the center of the cell. This, combined with the densely cellular nature of adipose tissue, lends itself to the chicken-wire appearance that is evident microscopically. Brown adipose tissue is found in newborn animals and in animals that hibernate during the winter. It is a highly specialized form of adipose tissue and plays an important part in temperature regulation, because it is a site of heat production. In brown fat, as in white adipose tissue, the

Reticular connective tissue is composed of a complex, three-dimensional network of thin reticular fibers (Figure 5-19). It resembles areolar connective tissue in that it contains loosely arranged fibers and many fibroblasts suspended in a supportive ground substance. Unlike areolar connective tissue, however, reticular connective tissue contains only one type of fiber: reticular fibers. Together, the cellular and matrix components form a network called stroma, which constitutes the framework of several organs, such as the liver, spleen, lymph nodes, and bone marrow. Although reticular fibers are found throughout the body, reticular connective tissue is found in a limited number of sites.

Serous membranes

Serous membranes are also called serosae. They line the walls and cover the organs that fill closed body cavities, such as the chest cavity or thorax and the abdominal and pelvic cavities (see Figures 5-27 and 5-28). Serosa is characterized as a continuous sheet that is doubled over to form two layers with a narrow space in between. The portion of the membrane that lines the cavity wall is called the parietal layer, and the portion that covers the outer surfaces of the organs is called the visceral layer. The serosa is composed of a sheet of simple squamous epithelium bound to an underlying layer of loose connective tissue. This histologic organization allows a great deal of permeability and enables interstitial fluid to pass through the membrane into the narrow spaces between the serosal layers. In this way, serosal fluid is a transudate and, unlike the mucoid thick secretion of mucous membranes, is thin and watery. It contains electrolytes but no mucin. By coating the parietal and visceral layers, serosal fluid creates a moist and slippery surface, which reduces friction between adjacent organs and between the organs and the cavity wall. Transudates take on different names depending on where they are located in the body. A transudate in the thorax, for example, is called pleural fluid; in the abdomen, peritoneal fluid; and in the region around the heart, pericardial fluid. The consistency of serous fluid may vary and change in pathologic conditions. For example, if an animal fractures a rib, blood cells and fluid may leak from ruptured capillaries into the pleural space, creating a hemothorax. When cells, protein, and other solid material mix with serous fluid, it becomes denser than a transudate and is called an exudate. Normally, the amount of serous fluid found in body cavities is small, but during trauma or some pathologic conditions, such as in hemothorax, the amount of fluid may become excessive. When an abnormally large amount of fluid enters a body cavity, the fluid is known as an effusion. Ascites, for example, is the presence of an effusion in the peritoneal space of the abdominopelvic cavity and can be caused by a wide range of pathologic conditions, such as congestive heart failure, nephrosis, m

Number of layers of cells. If there is only a single layer of epithelial cells, the tissue is classified as simple. If there is more than one layer of cells, the tissue is called stratified. Simple epithelia provide little protection to the underlying connective tissue and therefore are found in protected areas of the body, such as internal compartments, ducts, vessels, and passageways. Stratified epithelia, on the other hand, are thicker and stronger and are found in areas of the body that are subjected to mechanical and chemical stress.

Shape of the cells. In cross section, epithelial cells may take on many shapes, such as squamous, cuboidal, and columnar. In stratified epithelium, many different cell shapes are visible within the same tissue, but the classification is based on the shape of the cell that resides on the exposed or luminal surface of the tissue. In stratified squamous epithelium, for example, cuboidal cells are visible near the basement membrane, but squamous cells are found at the luminal surface; therefore, the tissue is called stratified squamous, not stratified cuboidal.

Skeletal muscle

Skeletal muscle contains numerous large cells that may be a foot or more in length. Because of their large size and heavy metabolic requirements, the cells contain hundreds of nuclei and mitochondria needed to maintain cellular homeostasis. Skeletal muscle is responsible for an animal's ability to walk, run, kick, bite, and show facial expression. Unlike cardiac and smooth muscle, skeletal muscle is usually controlled through conscious effort and therefore is called voluntary muscle. In other words, the animal can control its movement through conscious thought. In addition, skeletal muscle cells are striated, or striped, because histologically they have alternating bands of light and dark across them. Thus skeletal muscle is referred to as voluntary, striated muscle. The cells of skeletal muscle are essentially fibers that are clustered into bundles and held together by loose connective tissue. The collagen fibers that surround the cells merge with the collagen fibers in tendons to attach muscle firmly to bone. Muscle cells are stimulated to contract by the action of nerve fibers attached to them, located throughout the entire muscle belly. If the nerves are damaged, the ability of the muscle to contract is impaired, and the muscle is said to be paretic, or paralyzed. In this way, all of the actions that an animal can normally control, such as walking, running, eating, and moving the head and arms, depend on a healthy nervous system as well as a healthy muscular system.

The Clinical Patient and Healing

The Clinical Patient and Healing Some tissue types heal more readily than others. Epithelial tissues such as skin and mucous membranes heal rapidly but smooth muscle and dense regular connective tissue have limited regenerative ability. Cardiac muscle and nervous tissue in the brain and spinal cord regenerate extremely slowly if at all and are often replaced by scar tissue. In addition, some patients heal more easily than other patients. Old, immunosuppressed, debilitated, or sick animals heal more slowly than young, healthy, well-nourished animals. In this way, the age, overall health, and nutrition of patients are important factors in the rate and extent of healing. This is why elective surgery is avoided in unhealthy animals and why intravenous nutrition may be used in critically ill patients. Some diseased or otherwise stressed animals, for example, may produce too much cortisol, which can inhibit the animal's ability to heal; incision sites may take weeks rather than days to close, and wounds from even superficial injuries may become chronic nightmares. In addition, some drugs, such as prednisone, can also delay healing if blood levels are high. Thus the clinician must consider the overall health and medical history of an animal before carrying out any procedure that subsequently requires the healing of tissue.

Ground Substance

The ground substance in connective tissue is an amorphous, homogeneous material that ranges in texture from a liquid or gel to a calcified solid. In soft connective tissues, it is composed of unbranched chains of glycoproteins called glycosaminoglycans (GAGs). The most commonly found GAG in connective tissue is hyaluronic acid combined with 2% protein. These large molecules help to orient the formation of fibers within the tissue. Ground substance is the medium through which cells exchange nutrients and waste with the bloodstream. It acts as a shock-absorbing cushion and helps to protect the more delicate cells that it envelops. In addition, its thick texture serves as an effective obstacle for invading microorganisms, although some microbes have developed the ability to produce the enzyme hyaluronidase, which degrades hyaluronic acid and enables the microbe to move with greater ease through the tissue.

fixed cell

The most noteworthy fixed cell is the fibroblast. These are large, irregularly shaped cells that manufacture and secrete both the fibers and the ground substance characteristic of their particular matrix. Fibroblasts can reproduce and are metabolically very active. Each type of connective tissue is characterized by a predominant fibroblast. For example, cartilage contains chondroblasts, bone contains osteoblasts, and connective tissue contains fibroblasts. As the cells mature and the matrix is formed, the cells adopt a less active role. When this occurs, the name of the cell adopts the suffix -cyte, for example chondrocyte, osteocyte, or fibrocyte, depending on the tissue in which they are found. If additional matrix is required later, the cells can convert back to the -blast form. Fat cells are found throughout connective tissue and are known as adipose cells or adipocytes. As young cells, adipocytes resemble fibroblasts, but as they mature, they fill with lipid and become swollen, with their nuclei pushed to one side. When adipocytes cluster into groups, they become a tissue in their own right, known as adipose tissue. Adipose tissue is found throughout the body but is particularly evident under the skin (particularly on the ventrum between the hind legs in cats), behind the eyes, around the kidneys, and in the omentum of the abdominal cavity. Reticular cells are flat, star-shaped cells with long, outreaching arms that touch other cells, forming netlike connections throughout the tissue they compose. The function of reticular cells is debated, but most agree that they are involved in the immune response and in the manufacture of reticular fibers. It is not surprising therefore that reticular cells are found primarily in tissues that are part of the immune system, such as lymph nodes, spleen, and bone marrow.

what type of secretion do sebaceous glands use

The normal function of sebaceous glands is to produce and secrete sebum, a group of complex oils including triglycerides and fatty acid breakdown products, wax esters, squalene, cholesterol esters and cholesterol. Sebum lubricates the skin to protect against friction and makes it more impervious to moisture.

unicellular exocrine gland

The only example of a unicellular exocrine gland is the ductless goblet cell, so named because of its resemblance to a drinking goblet (Figure 5-13). The goblet cell is a modified columnar epithelial cell and is found interspersed among the columnar cells of the respiratory and digestive tracts and in the conjunctiva of the eye. Goblet cells secrete mucin: a thick, sticky mixture of glycoproteins and proteoglycans that when mixed with water becomes mucus. The mucus functions in two ways: it helps protect the apical surface of the epithelial layer, and it assists with the entrapment of microorganisms and foreign particles.

Blood.

The red fluid that passes through vessels and that carries nutrient molecules and gases throughout the body is the most atypical connective tissue (Figure 5-26). The liquid component of blood is called plasma and constitutes the matrix. The fibrous component of the matrix is an array of protein molecules suspended in solution and visible only when blood clots. Blood is rich with a variety of cell types, such as erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes, also known as platelets. Blood is discussed in greater detail in Chapter 12.

Wandering Cells

There are many types of wandering cell that move in and out of connective tissue as needed. In this section, three common types of wandering cells are discussed: leukocytes, mast cells, and macrophages. Commonly known as white blood cells, leukocytes are found in blood and move into connective tissue in large numbers during times of infection. Although they are relatively large and round compared to red blood cells, they can squeeze through the walls of tiny blood vessels to enter the surrounding tissue. This process is called diapedesis. Leukocytes are important members of the defensive immune system. There are five different types of leukocyte, but most protect the body by engulfing and digesting invading microbes. Other kinds, however, defend against infection by manufacturing antibodies that attach to microbes and destroy them. Mast cells are oval cells that are easily identified by the large number of dark-staining granules stored in the cytoplasm. These granules contain histamine and heparin, potent biochemicals that initiate an inflammatory response when released into the tissue. Histamine increases blood flow to the area by making the capillaries leaky, and heparin prevents blood from clotting and ensures that the pathways for increased blood flow remain open. Mast cells tend to be found near blood vessels, where they can release their contents directly into the bloodstream and where they can most effectively guard against foreign proteins or microbes. When stimulated by the presence of these invaders, mast cells burst open, releasing hundreds of stored granules. This begins the complex events of allergic and inflammatory reactions, a process discussed in greater detail later in this chapter. Macrophages are massive, irregularly shaped phagocytizing scavengers that may be either fixed or transient in connective tissue. They engulf microbes, dead cells, and debris that are subsequently digested in the macrophage's lysosomes. Mobile macrophages are drawn to sites of infection or inflammation, where they move aggressively through the affected area to engulf microinvaders. In this way, they are an important part of the immune system and help tissues fight infection. Macrophages are given different names d

CONTRIBUTING FACTORS OF HOMEOSTASIS

Thermoregulation - Ability to maintain body temperature within a range Acid-Base Balance - Ability to keep the body in pH balance Cell health and effectively functioning kidneys and lungs essential Fluid Balance - State in which the volume of body water and electrolytes are in balance Normal distribution of fluids of extracellular (1/3) and intracellular (2/3) compartments ~60% of an animal's body weight Calcium Levels - Bone health, assists with intra and extracellular fluid concentration,maintaining a normal heartbeat, initiation of neuromuscular and metabolic activities Glucose Concentrations - Energy source, fuel the animal body

Epithelial cells are organized into tightly packed groups that form sheets of tissue.

These sheets may be composed of either a single layer or multiple layers of cells, depending on where they are located in the body. Although the size and shape of the cells vary,

Transitional epithelium

Transitional epithelium has the remarkable ability to stretch. It is found in regions of the body that are required to expand and contract as part of their normal function. Thus transitional epithelium is found in portions of the urinary tract where great changes in volume occur, such as the urinary bladder, ureters, urethra, and calyxes of the kidney. The histologic appearance of transitional epithelia varies, depending on how much it is stretched. For example, in an empty bladder the epithelium is thick, multilayered, and has rounded, domelike cells on the luminal surface. When the bladder is filled, greater pressure is applied to the epithelial layer, making it stretch and thin out. The extent to which the membrane stretches depends on how full the bladder is and how much force is applied to the epithelium. As epithelia stretch, they may thin out from six to three cell layers, and the apical cells become flattened and squamous. The ability of transitional cells to change shape in the urine-holding tissues allows greater volumes of urine to be transported, stored, and excreted (Figure 5-11).In addition to its ability to stretch, transitional epithelium forms a leak-proof membrane that prevents the diffusion of potentially scalding urine into the delicate environment of the abdominal cavity.

Injuries occur in many ways. Animals may experience trauma from being hit by a car or from falling out a window. They may be bitten, scratched, or kicked by other animals and may experience broken bones and wounds that later become infected by pathogens. The body's initial response to these injuries is inflammation, a series of events that develop quickly to limit further damage and eliminate any harmful agents. Repair occurs more slowly and involves the organization of granulation tissue and the regeneration of lost tissue or the formation of scar tissue. Many of these processes occur simultaneously, making the injured area a busy workplace for cells. We will take a closer look at what is happening during the healing process. A summary of tissue repair is illustrated in Figure 5-31.

Whenever a tissue is injured, it causes an immediate inflammatory response. The affected area becomes red, swollen, hot, and tender. Sometimes there is decreased function of the injured body part. Inflammation is the body's attempt to isolate the area, limit the damage caused by the injury, and prevent further damage. Note that inflammation does not imply infection. Infection is inflammation caused by viruses, bacteria, and fungi. Injuries such as chemical burns, broken bones, and pulled muscles do not necessarily involve the invasion of these microorganisms. Inflammation therefore is a nonspecific reaction to injury or disease. The inflammatory process is the same regardless of the type of disease or injury. The extent of inflammation, however, depends on the type of tissue involved and on the severity of the injury or illness.

Regeneration or Fibrosis Epithelialization and Scar Tissue

While organization is occurring, epithelial cells around the wound edges actively divide to lay down a new layer of epithelial tissue over the granulation tissue. This process is called epithelialization. Connections between the scab and the thickening epithelial layer are weakened, and the scab subsequently falls off. Fibroblasts in the granulation tissue continue to manufacture collagen fibers and ground substance, which are used to replace lost tissue and bridge the wound. Slowly, the granulation tissue is completely replaced by fibrous scar tissue, which contracts and assists in pulling the wound closed. When epithelialization is complete, the underlying scar may or may not be visible, depending on the severity of the injury and on the extent of scar formation. Although scar tissue is strong, it is less flexible than normal tissue and cannot perform the function of the damaged tissue. With time, scar tissue shrinks, but its presence can still have detrimental effects on the organ as a whole. For example, if scar tissue forms in the wall of the heart, it can interrupt electrical pathways, weaken contractile capability, and result in decreased cardiac function. Similarly, if it occurs in the wall of the intestine or esophagus, it can decrease the diameter of the lumen and lead to obstruction or occlusion. For this reason, dogs that have undergone esophageal surgery to remove a foreign body are likely to have a repeated episode. The site of the first incision in the esophagus often heals with a thick, fibrous scar, which narrows the esophageal lumen and increases the possibility that another foreign body, such as a piece of bone or chew toy, will once again become lodged. In the abdominal and thoracic cavities, healing is often associated with the formation of fibrous adhesions and tags, which cover organs and form connections between multiple structures. Reentry into the abdomen to repeat a surgical procedure or to correct a complication therefore can be more difficult because of the formation of adhesions. Adhesions can reduce the visibility of important structures. They can restrict normal shifting of bowel loops and can bind organs to the body wall or to the omentum. In addition, adhesions can be painful

Organization: The Formation of Granulation Tissue

Wound repair begins soon after the injury occurs and continues while dead cells and debris are removed from the area. In wounds that are infected, neutrophils and macrophages play a particularly critical role in the healing process, because they are responsible for phagocytizing and disposing of invasive microorganisms. The presence of pathogens inhibits healing. As macrophages work to clear debris, a new, bright pink tissue, called granulation tissue, forms beneath the overlying blood clot or scab. Granulation tissue is composed of a layer of collagen fibers that have been manufactured by fibroblasts. It is richly infiltrated with small permeable capillaries that have branched off from existing capillaries in the deeper layers of the damaged tissue. These tiny new vessels push up into the bed of collagen fibers and provide rich supplies of nutrients and oxygen to the hard-working fibroblasts, macrophages, and neutrophils. Grossly, the capillaries appear to be minute granules and therefore account for the name. Granulation tissue produces bacterium-inhibiting substances, which make it highly resistant to infection. In some cases, granulation tissue becomes too thick and stands out above the epithelial layer. This is known as proud flesh and may be surgically cut down to facilitate closure of the epithelial layer. Proud flesh is commonly seen in horses that have sustained skin wounds.

Classifications

Wound repair may be classified as first or second intention, depending on the mechanism of healing and the proximity of the wound edges. Wounds that heal via first intention are those in which the edges of the wound are held in close apposition. These wounds may be superficial scratches or wounds that have been sutured or held closed with special bandages. During first-intention healing, the skin forms a primary union without the formation of granulation tissue or significant scarring. Second-intention healing occurs in wounds in which the edges are separated from one another, in which granulation tissue forms to close the gap, and in which scarring results

Simple squamous epithelia

are delicate and thin. They are often found lining surfaces involved in the passage of either gas or liquid, for example in the inner lining of the lung, where oxygen is absorbed and carbon dioxide released, and in the filtration membranes of kidneys, where water and other small molecules are excreted as urine (Figure 5-4). The fragile nature of simple squamous epithelium requires that it occur only in protected regions of the body, such as in the lining of the chest and abdominal cavities. Because simple squamous epithelia are flat and smooth, they are important in reducing friction and are found in the lining of blood and lymphatic vessels. Simple squamous epithelia have been given special names depending on where they are located in the body. For example, the epithelium that lines the pleural (chest), pericardial (around the heart), and peritoneal (abdominal) cavities is called mesothelium. The epithelium that lines blood and lymphatic vessels is called endothelium. A single layer of flattened, hexagon-shaped cells.

glandular epithelia

cells may occur as individuals, such as the goblet cells found in the intestine, or they may occur as organized glands, such as those found in the pancreas.

Stratified squamous epithelium

consists of various cell layers (Figure 5-7). It occurs in regions of the body that are subject to mechanical and chemical stresses, such as the linings of the mouth, esophagus, vagina, and rectum. The epithelial cells that make up the outer surface are continually being worn away or sheared off, but they are replaced at an equal rate by cells from deeper layers. Cuboidal cells form the base of stratified squamous epithelium. They are attached to the basement membrane and are continually dividing to keep up with the cell losses from the luminal surface. As the young cuboidal cells mature, they are progressively pushed to the surface, away from the nutrient sources provided by the underlying connective tissue. During this movement, the cells lose their cytoplasm and nuclei, take on a squamous shape, and eventually become paperlike sheets that slough.

what the functions of nervous tissue

controls work

what the functions of epithelial tissue

covers and lines

Simple columnar epithelia are

elongated and closely packed together, making the epithelia relatively thick and more protective than the simple squamous and cuboidal epithelia (Figure 5-6). The nuclei are not centrally located, as they are in cuboidal cells, but rather are aligned in a row at the base of the cell near the basement membrane. Simple columnar epithelia line the length of the gastrointestinal tract from the stomach to the rectum. Like simple cuboidal, they are associated with absorption and secretion and are found in many excretory ducts, as well as in the digestive tract. Two types of cell make up the gut lining. The most numerous is the absorptive cell, whose apical surface is blanketed by dense microvilli that maximize absorption by increasing surface contact with the nutrient-filled lumen. The other cell is called a goblet cell because of its wineglass shape. Goblet cells manufacture and store lubricating mucus that is secreted onto the luminal surfaces of the epithelia. Some less common epithelia are covered with cilia on their apical surfaces. These cells are called simple ciliated columnar epithelia, and they line the uterine tubes and respiratory tracts.

A tight junction

formed by the fusion of the outermost layers of the plasma membranes of adjoining cells.

Cells that are connected by

gap junctions are linked by tubular channel proteins called connexons

Stratified cuboidal epithelium

generally occurs as two layers of cuboidal cells and is found primarily along large excretory ducts, such as those of sweat glands, mammary glands, and salivary glands (Figure 5-8). This type of epithelium is important in protecting the delicate tissues in deeper layers.

how are Epithelial cells are held together

in many ways. Their lateral surfaces, for example, are wavy and fit together like pieces of a jigsaw puzzle.Between the plasma membranes of adjacent cells are matrix-filled channels, which transport nutrients from underlying connective tissue. These passages act as distribution routes for biologic supplies and as elimination routes for waste. The plasma membranes of epithelial cells are joined to form specialized attachments, called junctional complexes, that give epithelial tissue surprising strength, even though the attachments only involve a small portion of the cell membrane. Three major types of cellular junction found between epithelial cells are tight junctions, desmosomes, and gap junctions

A gland

is a cell or group of cells that have the ability to manufacture and discharge a secretion. Secretions are specialized protein molecules that are produced in the rough endoplasmic reticulum, packaged into granules by the Golgi apparatus, and discharged from the cell. Thus typical glandular epithelial cells are recognized by their prominent endoplasmic reticulum, Golgi apparatus, and secretory granules. Some of the secretions produced by glandular epithelia are used locally, whereas others are needed in distant regions of the body. During embryonic development, multicellular glands form from an infolding layer of epithelial cells. Initially, these invaginations form ducts and tubules that maintain contact with the surface epithelium. In the course of development, some of the glands lose the ducts and become separated from the parent epithelial sheet (Figure 5-12). In this way, glands are derived from epithelium.

Simple cuboidal epithelium

is composed of a single layer of cubic cells (Figure 5-5). On microscopic examination, their round, dark-staining nuclei are seen to be aligned in a single row that resembles a string of pearls. Like simple squamous epithelium, simple cuboidal epithelium provides little protection from abrasion. Therefore it occurs in sheltered regions of the body, where secretion and absorption take place. It is found on the surface of ovaries, in the secretory portions of glands, such as the thyroid, and in the lining of the ducts of the liver, pancreas, kidney, and salivary glands. Some simple cuboidal epithelia in kidney tubules are covered with microvilli, attesting to their absorptive function. Others are smooth surfaced and associated with secretory glands.Simple cuboidal epithelium plays an important role in both endocrine and exocrine tissues. Exocrine ducts lined with simple cuboidal epithelium, for example, carry saliva from the salivary gland to the oral cavity, and enzymes secreted by the pancreas are transported to the duodenum. In addition the thyroid gland, an endocrine structure, contains chambers lined by a single row of cuboidal cells and secretes the hormone thyroxine, which is carried throughout the body via the bloodstream.

Smooth muscle

is composed of small, spindle-shaped cells that lack striations or bands and therefore appear smooth. Like skeletal muscle, smooth muscle may be stimulated to contract by the action of nerves but, unlike skeletal muscles, the contractions cannot be consciously controlled. Smooth muscle is therefore called nonstriated, involuntary muscle. It is found in the walls of hollow organs, such as blood vessels, urinary bladder, uterus, intestines, and stomach and is also found in exocrine glands and along the respiratory tract. It is responsible for peristalsis in the gastrointestinal tract, for the constriction of blood vessels, and for the emptying of the bladder. Because smooth muscle cells are relatively small, they require only one centrally located nucleus.

Stratified columnar epithelium

is rare and is found only in select parts of the respiratory, digestive, and reproductive systems and along some excretory ducts (Figure 5-9).

Epithelial tissue is composed of sheets of cells that cover and line other tissues.

the bladder, mouth, blood vessels, thorax, and all of the body cavities and ducts in the body. Although well grounded to underlying structures,

The surface of a cell covered with microvilli is called

the brush borderThe brush border greatly increases the surface area of the cell, thereby increasing the absorptive ability of the cell. For this reason, microvilli usually occur on cells that are involved in absorption or secretion, such as the epithelia in the intestinal and urinary tracts. Remarkably, a cell with microvilli has about 20 times the surface area of a cell without them. Cilia are also found on the free surface of cells, usually in the respiratory and urogenital tracts. In the trachea, for example, the cilia help to propel mucus and debris up and away from the lungs toward the mouth. In the opening of the oviduct, called the infundibulum, cilia encourage newly expelled ova into the oviduct. Ciliary movement occurs in coordinated "beats," which enable the efficient transport of material. This coordinated action is brought about by an electrical potential that moves through junctional complexes connecting adjacent cells. The movement crosses the entire epithelial surface as a perfectly synchronized wave. Epithelial cells of the skin become filled with a protective, waterproof substance called keratin. The accumulation of keratin occurs as the cell matures and moves from the basal layer to the superficial layer of the integument. These cells are called keratinized epithelium and are discussed in greater detail in

The basement membrane

the foundation of the epithelial cell. It is a nonliving meshwork of fibers that cements the epithelial cell to the underlying connective tissue. Its strength and elasticity help prevent the cell from being torn off by intraluminal pressures, such as stretching or erosion caused by the rubbing of luminal material. The basement membrane (also called basal lamina) is manufactured and laid down by epithelial cells in varying degrees of thickness. The basement membrane in skin, for example, is thin, but in the trachea it is much thicker. Oxygen and nutrient molecules are supplied to the epithelial cells by diffusing through the basement membrane from capillaries in the underlying connective tissue. Similarly, nutrient substances that are absorbed and waste that is excreted by the epithelium diffuse across the basement membrane into the blood supply of the connective tissue. In this way, the basement membrane acts as a partial barrier between the epithelial cell and the underlying connective tissue. Cancerous epithelia do not respect this boundary and aggressively invade the connective tissue layer underneath.

muscle tissus organs

the heart is powerful muscle tissus that moves blood throughhout the boody

connexons

which extend from the cytoplasm of one cell to the cytoplasm of the other. These transmembrane proteins allow the exchange and passage of ions and nutrients, such as nucleotides, sugars, and amino acids, from one cell to the other. Gap junctions are most commonly found in intestinal epithelial cells, the heart, and smooth muscle tissue. Although the exact function of gap junctions in epithelial cells is not yet fully understood, their role in cardiac and smooth muscle cells centers around their ability to transport electrical signals quickly from one cell to another. In this way, they coordinate the contraction of cardiac and smooth muscle.