Chapter 5 - Tissue Organization

integrate - CLINICAL VIEW What Are You Planning to Do with Your Baby's Umbilical Cord?

A fetus's blood contains stem cells that are the same as those found in a child's bone marrow, and these cells can be used to treat a variety of life-threatening diseases. Cord blood can be harvested immediately following the birth of a baby, and the blood specimen can be shipped to a cord blood bank for testing, processing, and storage. Some conditions successfully treated to date with cord blood stem cells include lymphoma (cancer of the lymph nodes); leukemia (cancer of the blood); anemia resulting from bone marrow damage, which may happen as a complication of cancer chemotherapy; and even sickle-cell disease. Although this technology is hopeful, it is expensive and the cost is not covered by most insurance plans. Further, each cord blood sample contains relatively few stem cells. Although these stem cells can be used in unrelated recipients, it takes longer for the cells to complete the engraftment process, which leaves the patient vulnerable to infections for a longer amount of time than when using bone marrow-derived stem cells.

Classification of Connective Tissue

All connective tissue is ultimately derived from mesenchyme. Mesenchyme begins to differentiate in the developing fetus as it forms the connective tissues that ultimately are found in the adult body. The connective tissue types present after birth are classified into three broad categories: • connective tissue proper, • supporting connective tissue, and • fluid connective tissue.

integrate - CLINICAL VIEW - Scurvy

Collagen is an important protein that strengthens and supports almost all body tissues, especially connective tissue. Vitamin C (ascorbic acid) is essential for the production and maintenance of healthy collagen fibers. Scurvy, a disease caused by vitamin C deficiency, is marked by weakness, ulceration of gums resulting in tooth loss, hemorrhages, abnormal bone growth, and easily ruptured capillaries. Scurvy was prevalent among nineteenth-century sailors who, on long sea voyages, lacked vitamin C in their food. Sailors eventually learned that eating citrus fruits, such as limes and lemons, on their voyages prevented scurvy (this also explains why sailors received the nickname limeys). Today, collagen production disorders that are caused by nutritional deficiencies are treated by consuming foods high in vitamin C, such as citrus fruits, broccoli, cauliflower, peppers, spinach, and tomatoes, or by taking vitamin C supplements.

Functions of epithelial tissues

Epithelia have several functions, although no one epithelium performs all of them. These functions include the following: • Physical Protection • Selective permeability • Secretions • Sensations

CLINICAL VIEW Gangrene

Gangrene is the necrosis (death) of the soft tissues of a body part due to a diminished or obstructed arterial blood supply to that region. The body parts most commonly affected are the limbs, fingers, or toes. Gangrene is a major complication of diabetics, who often suffer from diminished blood flow to their extremities as a consequence of their disease. Gangrene occurs in several different forms. Intestinal gangrene usually follows an obstruction of the blood supply to the intestines. Without a sufficient blood supply, the tissue will undergo necrosis and gangrene. Untreated intestinal gangrene leads to death. Dry gangrene is where the involved body part is desiccated, sharply demarcated, and shriveled, usually due to constricted blood vessels as a result of exposure to extreme cold. Dry gangrene can be a complication of frostbite or of a variety of cardiovascular diseases that restrict blood flow, primarily to the hands and feet. Wet gangrene is caused by a bacterial infection of tissues that have lost their blood and oxygen supply. The cells in the dying tissue rupture and release fluid. The wet environment allows bacteria to flourish, and they often produce a foul-smelling pus. The most common bacteria associated with wet gangrene include Streptococcus, Staphylococcus, Enterobacter, and Klebsiella. Wet gangrene must be treated quickly with antibiotics and removal of the necrotic tissue. Gas gangrene most often affects muscle tissue, and the bacteria associated with gas gangrene typically are Clostridium. As the bacteria invade the necrotic tissue, a release of gases from the tissue produces gas bubbles. These bubbles make a crackling sound in the tissue, especially if the patient is moved. Symptoms of fever, pain, and edema (localized swelling) occur within 72 hours of the initial trauma to the region. The treatment for gas gangrene is similar to that of wet gangrene.

CLINICAL VIEW Tissue Transplant

Grafting is the process of surgically transplanting healthy tissue to replace diseased, damaged, or defective tissue. Tissue transplant may be characterized in four ways: an autograft, a syngenetic graft, an allograft, or a heterograft. An autograft (aw′t ˉo-graft; autos = self) is a tissue transplant from one site to a different site on the same individual. Autografts are often performed with skin, as healthy skin from one part of the body is grafted to another part of the body where the skin has been damaged by burns or chemicals. Because an autograft is a person's own body tissue, the body will not reject the tissue as foreign. However, autografts may not be feasible in certain situations, such as when the amount of skin damaged is so great that it would not be possible to transplant such a large portion of tissue. A syngenetic (sin-j ˇe-net′ik; syn = together) graft, also called an isograft, is a tissue transplant from one person to a genetically identical person (i.e., identical twins). Very few of us, however, have an identical twin, so this type of graft is not possible for most people. An allograft (al′oˉ-graft; allos = other) is a tissue transplant from one person to another person who is genetically different. Many tissue types have been used as allografts, including skin, muscle, bone, and cartilage. The term allograft also is used for the transplantation of organs or parts of organs, such as heart valves, kidneys, and the liver. The patient and the organ donor must be as genetically similar as possible as the closer the match, the less likely the allograft will be rejected. The recipient of the transplanted organ(s) must take powerful immunosuppressant drugs, which help prevent the body from rejecting the organ. Unfortunately, these same drugs work by suppressing the entire immune system, making the transplant patient more susceptible to illness. A xenograft ( zˉe ′nˉo-graft; xeno = foreign), also called a heterograft ( h˘e′ter-ˉo-graft; heteros = other), is a tissue transplant from an animal into a human being. For example, porcine (pig) and bovine (cattle) tissues have been used as replacements for heart valves, blood vessels, and bone. As with an allograft, xenografts may be prone to rejection and thus may not last as long as a synergetic graft would.

integrate CLINICAL VIEW Marfan Syndrome

Marfan syndrome is a rare genetic disease of connective tissue that causes skeletal, cardiovascular, and visual system abnormalities. It results from an abnormal gene on chromosome 15. Patients with Marfan syndrome tend to have (1) abnormally long fingers, toes, and upper and lower limbs; (2) malformation of the thoracic cage, vertebral column, or both as a result of excessive growth of ribs; and (3) easily dislocated joints, resulting from weak ligaments, tendons, and joint capsules. Cardiovascular system problems involve a weakness in the aorta and abnormal heart valves. Visual system abnormalities develop because the thin fibers that hold the lens of the eye in place are weak, allowing the lens to slip out of place. Patients usually exhibit symptoms of Marfan syndrome by age 10. Marfan syndrome symptoms may range from mild to severe. In severe cases, individuals may die of cardiovascular-related problems before age 50. However, those with milder symptoms and who receive early diagnosis and medical management may have long life spans.

Primary Types of Epithelia

Primary Types of Epithelia: o Simple Squamous Epithelium o Simple Cuboidal Epithelium o Simple Columnar Epithelium o Pseudostratified Columnar Epithelium o Stratified Squamous Epithelium o Stratified Cuboidal Epithelium o Stratified Columnar Epithelium o Transitional Epithelium

Resident cells

Resident cells are stationary cells that are permanently housed within the connective tissue. They help support, maintain, and repair the extracellular matrix. Examples of resident cells include the following: • Fibroblasts (fˉı′broˉ-blast; fibra = fiber, blastos = germ) are relatively flat cells with tapered ends and are the most abundant resident cells in connective tissue proper. They produce the fibers and ground substance components of the extracellular matrix. • Adipocytes (ad′i-pˉo-sˉıt; adip = fat), also called fat cells, appear in small clusters within some types of connective tissue proper. If large clusters of these cells dominate an area, the connective tissue is called adipose connective tissue. • Mesenchymal cells (me-seng′ki-mal) are a type of embryonic stem cell within connective tissue. If the tissue becomes damaged, these cells will divide. One cell that is produced replaces the mesenchymal stem cell, while the other cell becomes a committed cell that moves into the damaged area and differentiates into the type of connective tissue cell that is needed. • Fixed macrophages are relatively large, irregular-shaped cells that are derived from a type of white blood cell called a monocyte (see section 18.3c). They are dispersed throughout the matrix, where they phagocytize (engulf) damaged cells or pathogens. When they encounter foreign materials, the cells also release chemicals that stimulate the immune system and attract numerous wandering cells to the tissue.

functions of connective tissue

The many types of connective tissue collectively perform a wide variety of functions, including: • Physical protection. Bones of the skull and the thoracic cage protect delicate organs such as the brain, heart, and lungs; adipose connective tissue packed both around the kidneys and posterior to the eyes help protect these organs. • Support and structural framework. Bones serve as the framework for the adult body and provide a place for muscle attachment; cartilage keeps air tubes like the trachea and bronchi patent (open); and connective tissue proper forms supportive capsules around organs such as the kidney and spleen. • Binding of structures. Ligaments bind bone to bone, tendons bind muscle to bone, and dense irregular connective tissue anchors the skin to the underlying muscle and bone. • Storage. Adipose connective tissue is the major energy reserve in the body; bone is the primary reservoir for calcium and phosphorus. • Transport. Blood carries nutrients, gases, and wastes between different regions of the body. • Immune protection. Many connective tissues contain leukocytes that protect the body against disease and mount an immune response when necessary. Additionally, the viscous nature of the extracellular matrix restricts the movement and spread of disease-causing organisms.

CAREER PATH - Histologist

The microscopic anatomy of cells and tissues is studied by a histologist. This professional uses various microscopy techniques such as light microscopy (LM) and electron microscopy (EM). In the hospital setting, a histologist may prepare frozen tissue sections taken from patient biopsies for rapid analysis by a pathologist. A thorough understanding of the characteristics of the four basic tissue types is essential for this health-care professional. Having an in-depth knowledge base of normal tissue structure ensures the histologist also can recognize possible tissue abnormalities, and thus possible evidence of disease or infection.

Protein Fibers

The protein fibers in connective tissue usually strengthen and support the tissue. Three basic types of protein fibers may be found in connective tissue: collagen fibers, reticular fibers, and elastic fibers. • Collagen fibers are unbranched, "cablelike" long fibers that are strong, flexible, and resistant to stretching. These fibers are stronger than steel of the same diameter. Collagen forms about 25% of the body's protein, and the fibers appear white in fresh tissue, so they often are called white fibers. In tissue sections stained with hematoxylin and eosin, they appear pink. Collagen fibers are numerous in structures such as tendons and ligaments. • Reticular fibers are similar to collagen fibers but much thinner. They contain the same protein subunits found in collagen, but their subunits are combined in a different way. These fibers form a branching, interwoven framework that is tough but flexible. Reticular fibers are especially abundant in the stroma (connective tissue framework) of organs such as the lymph nodes, spleen, and liver. • Finally, elastic fibers contain the protein elastin. The fibers branch and rejoin, and appear wavy. Elastic fibers stretch and recoil easily. Fresh elastic fibers have a yellowish color and often are called yellow fibers. These fibers are visible only in tissue sections that have been stained with special stains, which make the elastic fibers appear black. Elastic fibers are abundant in the skin, arteries, and lungs, to allow them to return to their normal shape after being stretched.

Tissue Classification

Tissues in the body are classified into four major types: • epithelial tissue, • connective tissue, • muscle tissue, and • nervous tissue. These four tissue types vary in the structure of their cells, the functions of these cells, and the composition of an extracellular matrix. The extracellular matrix is composed of varying amounts of protein fibers, water, and dissolved molecules (e.g., glucose, oxygen). Its consistency ranges from fluid to semisolid to solid.

embryonic connective tissue

Two types of embryonic connective tissue have been identified: o mesenchyme connective tissue and o mucous connective tissue. They have different names because they occupy different locations.

Wandering cells

Wandering cells continuously move throughout the connective tissue proper and are components of the immune system (see chapter 22). They also may help repair damaged extracellular matrix. These cells are primarily types of leukocytes (l ˉu′kˉ o-sˉıt; leukos = white), also known as white blood cells, and protect the body against harmful agents. Examples of wandering cells and their specific functions include the following: • Mast cells are small, mobile cells that usually are found close to blood vessels; they secrete heparin to inhibit blood clotting and histamine to dilate blood vessels and increase blood flow, which is significant in the inflammatory response. • Plasma cells are formed when B-lymphocytes are activated by exposure to foreign materials. Plasma cells produce antibodies, which are proteins that immobilize a foreign material and prevent it from causing further damage. • Free macrophages are mobile, phagocytic cells that wander through the connective tissue. They function like fixed macrophages, yet they are able to move throughout the tissue. • Other leukocytes also migrate through the blood vessel walls into the connective tissue. These include neutrophils, a type of leukocyte that phagocytizes bacteria, and T-lymphocytes, a type of leukocyte which attacks foreign materials.

CLINICAL VIEW Stem Cells

Why all the interest in stem cells? Stem cells are immature, undifferentiated cells. These cells are able to divide into two cells, the first of which is another stem cell, and the other a cell that could differentiate into a specialized, mature cell with a unique function. Stem cells have generated interest in the scientific and medical communities because of their potential for repair or replacement of damaged or dying tissue. What are the two basic characteristics of stem cells? All stem cells exhibit two characteristics: self-renewal and potency. Selfrenewal refers to their ability to divide repeatedly to produce both new cells for maturation and new stem cells. Potency is the potential for differentiation: Different stem cells have varying ability to differentiate into almost any type of cell. Stem cells exhibit the following four levels of potency: totipotency, pluripotency, multipotency, and unipotency: • Totipotent stem cells have a "total potential," meaning that they exhibit the ability to differentiate into any cell type within an organism. A totipotent cell is produced when a secondary oocyte is fertilized by a sperm, giving rise to a zygote. The first few cell divisions of the zygote result in equally totipotent cells. Thus, only embryonic (and not adult) stem cells have the potential to be totipotent. • Pluripotent stem cells are derived from totipotent stem cells. These stem cells are formed from the embryoblast portion (inner cell mass) of the blastocyst. The blastocyst is a ball of cells that develops during the first week of development from the zygote. The embryoblast is the portion of the blastocyst that will eventually become an embryo and then a fetus. Pluripotent stem cells can form cells in any of the tissue layers of the embryo, but they cannot form structures such as the placenta. Again, only embryonic stem cells have the potential to be pluripotent. • Multipotent stem cells are derived from pluripotent stem cells. They have the capability to differentiate into a restricted number of some cell types and not others. For example, stem cells in thebone marrow may be stimulated to mature and differentiate into different types of blood cells, but not into some other types of cells. Some adult stem cells have the potential to be multipotent. • Unipotent stem cells have the ability to differentiate into a single type of cell, yet these cells still retain the ability to renew themselves. Epithelial stem cells (discussed previously) are examples of unipotent stem cells. Many adult stem cells are unipotent. What are the differences between embryonic and adult stem cells? Stem cells may be categorized as either embryonic stem cells or adult stem cells. Embryonic stem cells include those that have begun to divide in the zygote and the cells in the blastocyst. Embryonic stem cells exhibit the greatest degree of potency—and thus, the greatest potential to differentiate into multiple cell types. In contrast, adult stem cells are the immature cells found in postnatal (already born) organisms. Adult stem cells typically are multipotent or unipotent, and thus they exhibit less potency than embryonic stem cells. How are stem cells harvested? Most embryonic stem cells must be harvested from a structure no more differentiated than a blastocyst. Most of these blastocysts were donated by families undergoing in vitro fertilization who had stored more blastocysts than needed for a successful pregnancy. If these blastocysts were not used by the family and not donated for research, they typically would be destroyed. Note that opponents of embryonic stem cell research counter that these blastocysts could be implanted and lead to viable infants who could be adopted, and any medical benefit from embryonic stem cells does not justify using them in research. Opponents also maintain that adult stem cell research should be explored instead. Adult stem cells may be extracted from the bone marrow or tissue of an individual. These adult stem cells have been used to successfully treat certain blood and bone cancers, and research is ongoing about their effectiveness for diseases such as lung inflammation, stroke, and Parkinson disease. The main problem with adult stem cells is their limited potency, which suggests that their use for treatment in diseases is limited. Embryonic stem cells exhibit greater promise for treatment because of their greater potency.

mucous membrane

• A mucous membrane, also called a mucosa, lines passageways and compartments that eventually open to the external environment; these include the digestive, respiratory, urinary, and reproductive tracts. • Mucous membranes perform absorptive, protective, and secretory functions, or a combination of these functions. • A mucous membrane is formed by an epithelium and an underlying connective tissue called the lamina propria. • Often, this membrane is covered with a layer of mucus derived from goblet cells, multicellular glands, or both.

Pseudostratified Columnar Epithelium

• A pseudostratified columnar epithelium (sˉu′d ˉo-strat′i-f ıˉd; pseudes = false, stratum = layer) is so named because upon first glance, it appears to consist of multiple layers of cells. • However, this epithelium is not really stratified because all of its cells are in direct contact with the basement membrane. • Although it may look stratified because the nuclei are scattered at different distances from the basal surface, not all of the cells reach the apical surface in this epithelium. • Its columnar cells always reach the apical surface, and the shorter cells are stem cells that give rise to the columnar cells. • Pseudostratified columnar epithelium consists of two forms: pseudostratified ciliated columnar epithelium, which contains cilia on its apical surface (table 5.2e), and pseudostratified nonciliated columnar epithelium, which lacks cilia (table 5.2f ). • Both types perform protective functions. • The ciliated form houses goblet cells that secrete mucin, which hydrates to become the mucus that traps foreign particles and is moved by the beating cilia. • This type is found in the larger air passageways of the respiratory system (e.g., the nasal cavity, part of the pharynx [throat], larynx [voice box], trachea, and bronchi). • The nonciliated form is rare, lacks goblet cells and cilia, and occurs primarily in part of the male urethra and epididymis.

Pseudostratified Epithelium

• A pseudostratified epithelium appears layered (stratified) because the cells' nuclei are distributed at different levels between the apical and basal surfaces. • Although all of these epithelial cells are attached to the basement membrane, some of them do not reach its apical surface. • For our purposes, we have classified pseudostratified epithelium as a type of simple epithelium, because all of the cells are attached to the basement membrane.

mucous connective tissue

• A second type of embryonic connective tissue is mucous connective tissue, also known as Wharton's jelly (table 5.4b). • The immature protein fibers in this tissue are more numerous than those within mesenchyme. • Mucous connective tissue is located within the umbilical cord only.

serous membrane

• A serous membrane lines body cavities that typically do not open to the external environment, and their locations were first introduced in section 1.4e. • The membrane is composed of a simple squamous epithelium called mesothelium. • Serous membranes produce a thin, watery serous fluid, or transudate, which is derived from blood plasma. • Serous membranes form two associated layers: a parietal layer that lines the inside of the body cavity and a visceral layer that covers the surface of the internal organs. • Between these two layers is a serous cavity, which is a potential space into which the serous fluid is secreted. • The serous fluid reduces the friction between their opposing surfaces. • Examples of serous membranes include part of the pericardium (which is associated with the heart), the pleura (associated with the lungs) and the peritoneum (associated with abdominal organs).

Transitional Epithelium

• A transitional epithelium is limited to the urinary tract (urinary bladder, ureters, and part of the urethra). • It varies in appearance, depending upon whether it is in a relaxed state or a stretched state (table 5.3e). • In a relaxed state, the basal cells appear cuboidal or polyhedral, and the apical cells are large and rounded. • When transitional epithelium stretches, it thins and the apical cells flatten and become almost squamous in shape. • One distinguishing feature of transitional epithelium is the presence of some binucleated (containing two nuclei) cells. • By being able to stretch as the bladder fills, this tissue ensures that urine does not seep into the underlying tissues of these organs.

Simple Columnar Epithelium

• A simple columnar epithelium is composed of a single layer of cells that are taller than they are wide. • The nucleus is oval, oriented lengthwise, and located in the basal region of the cell. • This type of epithelium is ideal for both secretory and absorptive functions. • Simple columnar epithelium has two forms: One type has no cilia, whereas the apical surface of the other type is covered with cilia. • Nonciliated simple columnar epithelium often contains microvilli (see section 4.6c) and a scattering of unicellular glands called goblet cells (table 5.2c). • Individual microvilli cannot be distinguished under the microscope; rather, the microvilli collectively appear as a bright, fuzzy structure known as a brush border. • Goblet cells secrete mucin (mˉu′sin), which is a glycoprotein that when hydrated (mixed with water) forms mucus. • Nonciliated simple columnar epithelium lines most of the digestive tract, from the stomach to the anal canal. • Ciliated simple columnar epithelium has cilia that project from the apical surfaces of the cells (table 5.2d). • Mucus covers these apical surfaces and is moved along by the beating of the cilia. • Goblet cells typically are interspersed throughout this epithelium. • Ciliated columnar epithelium lines the larger bronchioles (air passageways) in the lung. • It also lines the luminal (internal) surface of the uterine tubes, where it helps move an oocyte from the ovary to the uterus.

Simple Cuboidal Epithelium

• A simple cuboidal epithelium contains one layer of uniformly shaped cells that are about as tall as they are wide with a centrally located spherical nucleus. • This epithelium is designed for absorption and secretion. Its cells' uniformity in shape makes them ideal to form the structural components of glands. For example, a simple cuboidal epithelium forms the follicles (spherical structures) of the thyroid gland and covers each ovary. • Simple cuboidal epithelium also composes the walls of small ducts (or tubules), including those of kidney tubules.

Simple Squamous Epithelium

• A simple squamous epithelium consists of a single layer of flattened cells (table 5.2a). • When viewed "en face" (looking onto the surface), the irregularly shaped cells display a spherical to oval nucleus, and the cells are tightly bound together. • Each squamous cell resembles a fried egg, with the slightly bulging nucleus of the cell representing the yolk. • This epithelium is extremely delicate and represents the thinnest possible barrier to allow rapid movement of molecules and ions by membrane transport processes (see section 4.3). • Simple squamous epithelium forms the lining of the air sacs (alveoli) of the lung, where this thin epithelium is well suited for the exchange of oxygen and carbon dioxide between the blood and the inhaled air. • Simple squamous epithelium also is found lining the lumen (inside space) of blood vessel walls, where it allows for rapid exchange of nutrients and waste between the blood and the interstitial fluid surrounding the blood vessels. • Serous membranes, which cover body organs and secrete serous fluid, are also formed by a simple squamous epithelium. • Specific names are used to refer to the simple squamous epithelia in certain locations within the body. • Endothelium (en-doˉ-theˉ′leˉ-u˘m; endon = within) is the name of the simple squamous epithelium that lines both blood vessels and lymph vessels (see sections 20.1a and 21.1), and mesothelium (mez-ˉ o-theˉ′leˉ-u˘m; mesos = middle) is the name given to the simple squamous epithelium that forms the serous membranes of body cavities (see section 1.4e). • Mesothelium gets its name from the embryonic primary germ layer called mesoderm, from which it is derived (see section 5.6a).

Stratified Columnar Epithelium

• A stratified columnar epithelium is relatively rare in the body. • It consists of two or more layers of cells, but only the cells at the apical surface are columnar in shape (table 5.3d). • This type of epithelium protects and secretes. • It is found in the large ducts of salivary glands and in some segments of the male urethra.

Stratified Cuboidal Epithelium

• A stratified cuboidal epithelium contains two or more layers of cells, and the superficial cells tend to be cuboidal in shape (table 5.3c). • Stratified cuboidal epithelium, like simple cuboidal epithelium, forms tubes and coverings. • However, stratified cuboidal epithelium is thicker and functions in protection and secretion. • This tissue forms the walls of the ducts of most exocrine glands (see section 5.1d), such as the ducts of the sweat glands in the skin, the lining of some parts of the male urethra, and the periphery of ovarian follicles.

Stratified Epithelium

• A stratified epithelium contains two or more layers of epithelial cells. • Only the cells in the deepest (basal) layer are in direct contact with the basement membrane. • A stratified epithelium resembles a brick wall, where the bricks in contact with the ground represent the basal layer and the bricks at the top of the wall represent the apical (superficial) layer. • This tissue provides either more structural support or better protection for underlying tissue. • A stratified epithelium is found in areas likely to be subjected to abrasive activities or mechanical stresses, as multiple layers of cells are better able to resist the wear and tear (e.g., the skin, internal lining of the esophagus, and the internal lining of the urinary bladder). • Cells in the basal layer continuously regenerate as the cells in the apical layer are lost due to abrasion or stress.

Stratified Squamous Epithelium

• A stratified squamous epithelium has multiple cell layers, and only the deepest layer of cells is in direct contact with the basement membrane. • The cells in the basal layers have a cuboidal or polyhedral shape, whereas the apical cells display a flattened, squamous shape. • A stratified squamous epithelium is so named because of its multiple cell layers and the shape of its apical cells. • This epithelium is adapted to protect underlying tissues from damage caused by abrasion and friction. • Stem cells in the basal layer continuously divide, to produce a new stem cell and a committed cell that is gradually displaced toward the surface to replace those cells that have been lost. • This type of epithelium exists in two forms: keratinized and nonkeratinized. • In keratinized stratified squamous epithelium, the superficial layers are composed of cells that are dead. • These cells lack nuclei and all organelles, and instead are filled with the protein keratin (ker′˘atin; keras = horn), which is a tough, protective protein that strengthens the tissue (table 5.3a). • New cells produced in the basal region of the epithelium migrate toward the apical surface of the tissue. • During their migration, the cells fill with keratin they produce, which makes them very strong, but as a consequence the cells lose their organelles and nuclei, and die. • Thus, the strength of keratin has a trade-off. The epidermis (outer layer) of the skin consists of keratinized stratified squamous epithelium. • The cells in nonkeratinized stratified squamous epithelium remain alive including those at the tissue's apical surface, and they are kept moist with secretions such as saliva or mucus. • These cells lack keratin. • Because all of the cells are alive, the flattened nuclei characteristic of squamous cells are visible throughout the tissue (table 5.3b). • Nonkeratinized stratified squamous epithelium lines the oral cavity (mouth), part of the pharynx (throat), the esophagus, the vagina, and the anus.

Adipose connective tissue

• Adipose connective tissue (commonly known as fat) is a highly vascularized loose connective tissue composed primarily of adipocytes. • Adipocytes are filled with lipid droplets, with the nucleus pushed to the inside edge of the plasma membrane. • On a histology slide, the lipid is extracted during tissue processing so all that is left is the plasma membrane and nucleus of the adipocyte. • There are two types of adipose connective tissue: white and brown.

Components of connective tissue

• All connective tissues share three basic components: cells, protein fibers, and ground substance (figure 5.8). • Together, the ground substance and the protein fibers it houses form an extracellular matrix. • The specific types of cells may vary between the various classes of connective tissue. • However, diversity in connective tissue is due primarily to the different types and amounts of protein fibers, as well as the varying proportions of the ground substance.

Avascularity (Epithelia)

• All epithelial tissues lack blood vessels. • Nutrients for epithelial cells are obtained either directly across the apical surface or by diffusion across the basal surface from blood vessels within the underlying connective tissue.

Selective permeability (Epithelial)

• All substances that enter or leave the body must pass through an epithelium, and thus epithelial cells act as "gatekeepers." • An epithelium typically exhibits a range of permeability; it may be relatively non-permeable to some substances, while promoting and assisting the passage of other ions and molecules.

changes that occur in tissues with age

• All tissues change as a result of aging. • Proper nutrition, good health, normal circulation, and relatively infrequent wounds promote continued normal tissue functioning past middle age. • Thereafter, the support, maintenance, and replacement of cells and extracellular matrix become less efficient. • Physical damage and physiologic changes can alter the structure and chemical composition of many tissues. • For example, as individuals age, epithelia become thinner and connective tissues lose their pliability and resiliency. • The amount of collagen in the body declines with age, so tissue repair takes longer. • Bones become brittle, and muscle and nervous tissue begin to atrophy. • Poor diet and circulation problems accelerate these tissue declines. • Eventually, cumulative losses from relatively minor damage or injury may contribute to major health problems

Polarity (Epithelia)

• An epithelium has an apical surface (free, or superficial), which is exposed either to the external environment or to some internal body space. • The apical surface may have either microvilli or cilia. o microvilli are small membranous projections on the apical surface of the cell that increase its surface area for secretion and absorption o cilia are numerous, slightly longer, membranous projections that move fluid, mucus, and materials past the cell surface. • The lateral surfaces may contain membrane (intercellular) junctions. • Each epithelium has a basal surface, where the epithelium is attached to the underlying connective tissue.

Epithelial Tissue

• An epithelium, or epithelial tissue, is composed of : o one or more layers of closely packed cells, and o it contains little to no extracellular matrix between these cells. • Epithelial tissue covers the body surfaces, lines the body cavities and organ cavities, and forms most glands.

organ

• An organ is a structure that is composed of two or more tissue types that work together to perform specific, complex functions. • The key to organ structure is that the different tissue types must work in concert. • For example, the stomach contains all four types of tissue (figure 5.11). o It is lined by an epithelium, o has both areolar and dense connective tissue in its walls, o contains three layers of smooth muscle in those walls, and o possesses abundant nervous tissue. o All these tissues work together to perform the functions of the stomach. o Glands associated with epithelial tissue secrete substances for chemical digestion of ingested nutrients. o Connective tissue houses the blood vessels and nerves that supply the stomach as well as provides shape and support. o Smooth muscle contracts and relaxes so that contents within the stomach may be mechanically mixed and broken down. o Nervous tissue is responsible for both regulating contraction of muscle and the stimulation of secretion by glands.

Areolar connective tissue

• Areolar (˘a-rˉe′ˉ o-l˘ ar) connective tissue has a loosely unconfined organization of collagen and some elastic fibers and is highly vascularized (table 5.5a). • This connective tissue type contains all of the fixed and wandering cells of connective tissue proper, although the predominant cell is the fibroblast. • The ground substance is abundant and viscous. • Areolar connective tissue is found nearly everywhere in the body. • It is found in the skin (papillary layer of the dermis) and is a major component of the subcutaneous layer that is deep to the skin. • It binds skin and some epithelia to deeper tissues. • It also surrounds organs, individual nerve and muscle cells, and blood vessels.

metaplasia

• As we place different stresses on our bodies, our tissues may actually transform into another type of tissue. • Sometimes a mature epithelium changes to a different form of mature epithelium, a phenomenon called metaplasia. • Metaplasia may occur as an epithelium adapts to environmental conditions. • For example, smokers typically experience metaplastic changes in the epithelium of the trachea (windpipe). • The smoke and its by-products are the environmental stressors that change the normal pseudostratified ciliated columnar epithelium lining the trachea to a nonkeratinized stratified squamous epithelium. (If a person quits smoking, their metaplastic epithelium will fairly quickly revert back to its pseudostratified ciliated columnar epithelium.) • Another example occurs in some individuals with chronic acid reflux, also known as heartburn. • Here, the nonkeratinized stratified squamous epithelium of the inferior esophagus may transform to a simple columnar epithelium, like that seen in the stomach.

Blood

• Blood is a fluid connective tissue composed of formed elements. • Formed elements include cells, both erythrocytes (red blood cells) and leukocytes (white blood cells), and cellular fragments called platelets (table 5.9). • The liquid ground substance is called plasma, and within it are dissolved proteins. • Blood has numerous functions: o The erythrocytes transport respiratory gases (oxygen and carbon dioxide), and o the leukocytes protect the body from infectious agents. o Platelets and the protein fibers help clot the blood. o Plasma transports nutrients, wastes, and hormones throughout the body.

Connective Tissue Proper

• Connective tissue proper is divided into two broad groups: loose connective tissue and dense connective tissue. This classification is based upon the relative proportions of cells, fibers, and ground substance.

Cuboidal Cells (Epithelia)

• Cuboidal cells are about as tall as they are wide. • The cells do not resemble perfect cubes because their edges are somewhat rounded. • The cell nucleus is spherical and located within the center of the cell.

Bone

• Bone connective tissue is also known as osseous connective tissue and makes up the mass of most of the structures referred to as "bones." • Bone is more solid than cartilage and provides greater support, although it is not as flexible. Section 7.2e provides a detailed description of the histology of bone connective tissue. • The extracellular matrix of bone connective tissue consists of organic components (collagen fibers and glycoproteins) and inorganic components composed of a mixture of calcium salts, primarily calcium phosphate. • The bone cells are called osteocytes (os′tˉe-ˉo-sˉıt) and are housed within spaces in the extracellular matrix called lacunae. • The two forms of bone tissue are compact bone and spongy (cancellous, trabecular) bone. • Compact bone appears completely solid, but is in fact perforated by a number of neurovascular canals (table 5.8). • It has a uniform histologic pattern. • Compact bone is formed from cylindrical structures called osteons (os′tˉe-on), which display concentric rings of bone connective tissue called lamellae. • The lamellae encircle a central canal that houses blood vessels and nerves. • Spongy bone is located within the interior of a bone, and it contains a latticework structure of bone connective tissue that is very strong, yet lightweight (see figure 7.8). • Bone serves a variety of functions. As an organ, bones provide levers for movement, and they support soft tissues as well as protect vital body organs. • The hard extracellular matrix of bone connective tissue stores important minerals, such as calcium and phosphorus. • Finally, some spongy bone houses hemopoietic (hˉe′mˉo-poy-et′ik; hemat = blood) cells, which form a type of reticular connective tissue that makes blood cells (a process called hemopoiesis).

Brown adipose tissue

• Brown adipose tissue is found in newborns and is designed to generate heat. • As we age, we lose most of our brown adipose tissue and instead predominantly have white adipose tissue.

Cardiac muscle tissue

• Cardiac muscle tissue is confined to the thick middle layer of the heart wall, called the myocardium; it is responsible for the contraction of the heart to pump blood. • Cardiac muscle tissue contains visible striations, but unlike skeletal muscle, the cardiac muscle cells are short and often bifurcating (branching) (table 5.10b). • Cardiac muscle cells contain one or two centrally located nuclei. • In addition, the cells are connected by intercalated (in-ter′k˘a-lˉa-ted; intercalates = inserted between) discs, which are intercellular junctions between the cells composed of desmosomes and gap junctions. • Intercalated discs appear as dark, thick lines when viewed in the microscope. Intercalated discs strengthen the connection between cells and promote the rapid conduction of electrical activity through many cells at once, allowing the cells of a heart chamber to contract as a unit. • Cardiac muscle cells are considered involuntary because they cannot be controlled by the somatic (voluntary) nervous system activity to initiate a contraction; instead, specialized cardiac muscle cells (pacemaker cells) in the heart wall initiate the contraction.

Cartilage

• Cartilage has a firm, semisolid extracellular matrix that contains variable amounts of collagen and elastic protein fibers. • Mature cartilage cells are called chondrocytes (kon′drˉ o-sˉıt; chondros = gristle, cartilage). • These cells occupy small spaces called lacunae (l˘ a-kˉu′nˉ e; lacus = a hollow, a lake) within the extracellular matrix. • Most cartilage is surrounded by a dense irregular connective tissue covering called the perichondrium (per-i-kon′dr ˉe-˘ um; peri = around). • The perichondrium has two distinct layers: an outer fibrous layer and an inner cellular layer. • Cartilage is stronger and more resilient than previously discussed connective tissue types, and it provides more flexibility than bone. • It occurs in areas of the body that need support and must withstand deformation, such as the tip of the nose or the auricle (external part) of the ear. • Chondrocytes produce and secrete a chemical that prevents blood vessel growth and formation within the extracellular matrix. • Thus, mature cartilage is avascular, and as a result the chondrocytes must exchange nutrients and waste products by diffusion with blood vessels outside of the cartilage. • Three major types of cartilage are found in the body: o hyaline cartilage, o fibrocartilage, and o elastic cartilage. • They exhibit both differences in density and dispersal of chondrocytes within the extracellular matrix.

Characteristics common to all Epithelia (fig 5.1)

• Cellularity • Polarity • Attachment to a basement membrane • Avascularity • Extensive innervation • High regeneration capacity

Classification of Exocrine Glands by Method of Secretion Glands

• Classification by Method of Secretion Glands may be classified physiologically by their method of secretion. The three basic types of glands in this classification are merocrine glands, apocrine glands, and holocrine glands (figure 5.7). • Merocrine glands package their secretions into secretory vesicles and release the secretions by exocytosis. The glandular cells remain intact and are not damaged in any way by producing the secretion. Examples of merocrine glands include lacrimal (tear) glands; salivary glands; some sweat glands, also known as eccrine glands; the exocrine glands of the pancreas (see section 26.3c); and the gastric glands of the stomach (see section 26.2d). • Apocrine glands produce their secretion in the following way: The apical membrane around a portion of the glandular cell cytoplasm with the secretory product pinches off and becomes the secretion. The glandular cells repair the damage and then continue to produce new secretions in the same manner. Examples include the mammary glands and ceruminous glands of the ear. • Holocrine glands are formed from cells that accumulate a product; the entire cell then disintegrates. Thus, a holocrine secretion is a viscous mixture of both cell fragments and the product the cell produced prior to its disintegration. The ruptured, dead cells are continuously replaced by other epithelial cells undergoing cellular division. The oil-producing glands (sebaceous glands) in the skin are examples of holocrine glands.

Columnar Cells (Epithelia)

• Columnar cells are slender and taller than they are wide. • The cell nucleus is oval and usually oriented lengthwise and in the basal region of the cell.

Connective tissue

• Connective tissue is the most diverse, abundant, and widely distributed of the tissues. • Connective tissue functions to support, protect, and bind organs. • Examples of connective tissue include tendons (structures that attach muscle to bone) and ligaments (structures that attach bone to bone), adipose tissue (fat), cartilage, bone, and blood. • All connective tissues share a common origin; they all originated from an embryonic connective tissue called mesenchyme (discussed in section 5.2c). • In addition, while almost all connective tissue is vascular, the different types of connective tissue exhibit a range of vascularity, from very vascular (in areolar connective tissue) to poorly vascular (in dense regular connective tissue) to avascular (in mature cartilage).

Dense Connective Tissue

• Dense connective tissue is composed primarily of protein fibers and has proportionately less ground substance than loose connective tissue. • It also is known as collagenous tissue because collagen fibers usually are the dominant fiber type. • There are three categories of dense connective tissue: o dense regular connective tissue, o dense irregular connective tissue, and o elastic connective tissue.

Dense irregular connective tissue

• Dense irregular connective tissue contains bundles and clumps of collagen fibers that extend in all directions (table 5.6b). • This tissue provides support and resistance to stress in multiple directions, and has an extensive blood supply. • Dense irregular connective tissue is found in most of the dermis of the skin, the periosteum surrounding bone, and the perichondrium surrounding cartilage. • It also forms capsules around some internal organs, such as the liver, kidneys, and spleen.

Dense regular connective tissue

• Dense regular connective tissue contains limited ground substance yet abundant collagen fibers that are packed tightly and align parallel to one another. • The fibers resemble lasagna noodles stacked one on top of another (table 5.6a). • This tissue type is found in tendons and ligaments, where stress typically is applied in a single direction. • Dense regular connective tissue has few blood vessels, and thus it takes a long time to heal following injury, because a rich blood supply is necessary for quick healing.

Dysplasia

• Dysplasia refers to abnormal tissue development. • For example, cervical dysplasia may develop when a woman is exposed to the human papillomavirus. • Dysplasias have the potential to turn into cancer (and thus are sometimes referred to as precancerous), but they also have the potential to revert back to normal tissue. • Thus, cervical dysplasia has the potential to either turn into cancer or revert back to normal tissue, which is why dysplastic cells are closely monitored by health-care professionals.

Cells of Connective tissue

• Each class of connective tissue contains specific types of cells. For example, dense regular connective tissue, which forms ligaments and tendons, contains fibroblasts, adipose connective tissue (fat) contains adipocytes, and cartilage is composed of chondrocytes. • Unlike cells of epithelial tissue, most connective tissue cells are not in direct contact with each other and usually are widely scattered throughout the tissue. • Connective tissue proper contains two classes of cells: resident cells and wandering cells.

Ectoderm

• Ectoderm is initially located on the dorsal and external surfaces of the embryo. • It is responsible for forming many externally placed structures, such as the epidermis of the skin, hair, nails, and the exocrine glands of the skin. • Thus, some but not all epithelial tissues are derived from ectoderm. • Tooth enamel, the lens of the eye, and the adrenal medulla are derived from ectoderm, as is all nervous tissue such as the brain, spinal cord, and nerves.

Elastic cartilage

• Elastic cartilage is the flexible, springy cartilage. • It is so named because it contains numerous elastic fibers within its extracellular matrix (table 5.7c). • The chondrocytes are closely packed and surrounded by a small amount of extracellular matrix. • The elastic fibers are densely packed together and ensure that this tissue is both resilient and very flexible. • Elastic cartilage is surrounded by a perichondrium. • Note that both elastic cartilage and elastic connective tissue contain abundant amounts of elastic fibers. • However, elastic cartilage has a semisolid ground substance and contains chondrocytes, whereas elastic connective tissue has a fluid ground substance formed by fibroblasts. • Elastic cartilage is found in the external ear and the epiglottis (a structure of the larynx that prevents swallowed materials from entering the trachea). • You can see for yourself how flexible elastic cartilage is by performing this experiment: Fold your external ear over your finger, hold for 10 seconds, and release. • Your ear springs back to its original shape because the elastic cartilage resists the deformational pressure you applied. (This also explains why our ears aren't permanently misshapen if we sleep on them in an unusual way!)

Elastic connective tissue

• Elastic connective tissue is composed of numerous fibroblasts among branching, densely packed elastic fibers (table 5.6c). • The elastic fibers provide the ability for the tissue to stretch and recoil. • This tissue is found in the walls of large arteries, the trachea and vocal cords.

Endocrine Glands

• Endocrine glands lack ducts and secrete their products, called hormones, directly into the blood. • Hormones act as chemical messengers to influence cell activities elsewhere in the body.

Endoderm

• Endoderm becomes the innermost germ layer when the embryo undergoes shape changes. • It forms the epithelial linings of the tympanic cavity (middle ear) and auditory tube, as well as the digestive, respiratory, reproductive, and urinary tracts. • Endoderm also forms organs such as the thyroid gland, parathyroid glands, the thymus and portions of the palatine tonsils, as well as most of the liver, gallbladder, and pancreas.

Classification of Epithelia by Cell Shape

• Epithelia are also classified by the shape of the cell at the apical surface. • In a simple epithelium, all of the cells display the same shape, whereas in a stratified epithelium, a difference in shape can be seen between cells within the basal layer and those within the apical layer. • Figure 5.2b shows the three common cell shapes seen in epithelia: o squamous, o cuboidal, and o columnar. • (Note that the cells in this figure all appear hexagonal when viewed from their apical surface. Thus, these terms describe the cells' shapes when viewed laterally, or from the side.) • Squamous Cells • Cuboidal Cells • Columnar Cells • Transitional Epithelial Cells

Extensive innervation (Epithelia)

• Epithelia are richly innervated (supplied with nerves) to detect changes in the environment at that body or organ region.

body membranes

• Epithelial and connective tissue together form structures called body membranes, which should not be confused with the plasma membranes of cells. Body membranes are formed from an epithelial layer that is bound to an underlying connective tissue. • These membranes line body cavities, cover the viscera, or cover the body's external surface. • There are four types of body membranes: o mucous, o serous, o cutaneous, and o synovial.

Lymph

• Lymph is derived from blood plasma, but it contains no cellular components or fragments (which is why we don't examine it histologically here). • Ultimately, lymph is returned to the bloodstream.

High regeneration capacity (Epithelia)

• Epithelial cells undergo cell division frequently. • This characteristic allows this tissue to regenerate itself at a high rate; a necessary condition for a tissue that is often exposed to the environment and lost by abrasion and damage. • The continual replacement occurs through cell division of the deepest epithelial cells (called stem cells), which are adjacent to the basement membrane.

Cellularity (Epithelia)

• Epithelial tissue is composed almost entirely of tightly packed cells. • There is a minimal amount of extracellular matrix between the cells.

Sensations (Epithelial)

• Epithelial tissues are innervated by sensory nerve endings to detect changes in the external environment at the epithelial surface. • These nerve endings—and those in the underlying connective tissue—continuously relay sensory input to the nervous system concerning touch, pressure, temperature, and pain. • Several organs contain a specialized epithelium, called a neuroepithelium, that houses specific cells responsible for the senses of sight, taste, smell, hearing, and equilibrium.

Physical Protection (Epithelial)

• Epithelial tissues protect both external and internal surfaces from dehydration, abrasion, and destruction by physical, chemical, or biological agents.

Classification of Exocrine Glands by Anatomic Form

• Exocrine glands may be classified anatomically based on the structure and complexity of their ducts: o Simple glands have a single, unbranched duct; o compound glands have branched ducts. • In addition, glands may be classified according to the shape of their secretory portions. o tubular gland = secretory portion & the duct have the same diameter. o acinar gland = secretory portion forms an expanded sac. o tubuloacinar gland = both tubules and acini is present.

Exocrine Glands

• Exocrine glands typically originate from an invagination of epithelium that burrows into the deeper connective tissues. • These glands usually maintain their connection with the epithelial surface by means of a duct, an epithelium-lined tube through which the gland secretions are discharged onto the epithelial surface. • Examples of exocrine glands include sweat glands, mammary glands, and salivary glands. • Exocrine glands may be unicellular (one-celled) or multicellular.

Fibrocartilage

• Fibrocartilage (fˉı′brˉ o-kˉ ar′ti-la˘j; fibro = fiber) is a weight-bearing cartilage. • It has numerous coarse, readily visible protein fibers that are arranged as irregular bundles between large chondrocytes (table 5.7b). • There is only a sparse amount of ground substance. • The densely interwoven collagen fibers contribute to the durability of this cartilage. • There is no perichondrium. • Fibrocartilage acts as a good shock absorber and resists compression. • It is located in the intervertebral discs (circular supportive structures between adjacent vertebrae), pubic symphysis (between the anterior parts of the hip bones), and the menisci of the knee joint.

Glands

• Glands are either individual cells or multicellular organs composed predominantly of epithelial tissue. • They secrete substances either for use elsewhere in the body or for elimination from the body. • Glandular secretions may include mucin, electrolytes, hormones, enzymes, or urea (a nitrogenous waste produced by the body).

Ground Substance

• Ground substance is a noncellular material produced by the connective tissue cells, and it is within this ground substance that the connective tissue cells and protein fibers reside. • The ground substance may be viscous (as in blood), semisolid (as in cartilage), or solid (as in bone). • Together, the ground substance and the protein fibers it houses form an extracellular matrix. • Ground substance contains different large molecules as well as varying amounts of water. Glycosaminoglycans (glˉı′kˉos-am-i-nˉogl ˉı′kan; glykys = sweet, glycan = saccharide), or GAGs, are one type of large molecule in the ground substance. A GAG is a polysaccharide that is composed completely of carbohydrate building blocks, some of which have an attached amine group. GAGs are negatively charged and hydrophilic. The negative charges attract cations, such as sodium (Na+), and as a result water follows the movement of the positive ion. • Thus, GAGs are able to attract and absorb water. Different GAGs attract varying amounts of water, depending on their number of negative charges, so the fluidity of the ground substance varies as a result. Different types of GAGs include chondroitin sulfate, heparan sulfate, and hyaluronic acid. • When a GAG is linked to a protein, it forms an even larger molecule within the ground substance called a proteoglycan. Proteoglycans have over 90% of their structure composed of carbohydrates, in the form of GAGs. The large structure of a proteoglycan is due primarily to the large number of negative charges in its GAGs, which then repel each other and cause the molecule to spread out and occupy more space. As we will see in this and future chapters, GAGs and proteoglycans perform numerous important functions in the body. • The ground substance includes other molecules like adherent glycoproteins (proteins with carbohydrates attached; see section 4.2b), which act like glue to bond connective tissue cells and fibers to the ground substance. Examples of adherent glycoproteins include fibronectin, fibrillin, and laminin.

Hyaline cartilage

• Hyaline (hˉı′˘ a-lin; hyalos = glass) cartilage is the most common type of cartilage. • It is named for its clear, glassy appearance when viewed under the microscope (table 5.7a). • Its chondrocytes are irregularly scattered: The collagen within the extracellular matrix is not readily observed by light microscopy. • Hyaline cartilage is surrounded by a perichondrium. • If this tissue type is stained with hematoxylin and eosin and examined under the microscope, the tissue resembles carbonated grape soda, where the lacunae represent the bubbles in the soda. • Hyaline cartilage is found in many areas of the body, including structures of the respiratory tract (nose, trachea, most of the larynx), costal cartilage (cartilage attached to ribs), and the articular ends of long bones. • It also forms most of the fetal skeleton.

Hyperplasia

• Hyperplasia (plasso = to form) is an increase in the number of cells in a tissue. • Developing a "callus" on the palm of your hand is an example of these skin cells undergoing hyperplasia.

Multicellular Exocrine Glands

• In contrast, multicellular exocrine glands contain numerous cells that work together to produce a secretion (figure 5.5). • The gland often consists of acini, which are the clusters of cells that produce the secretion, and one or more smaller ducts, which merge to form a larger duct that transports the secretion to the epithelial surface. • Multicellular exocrine glands typically are surrounded by a fibrous capsule, and extensions of the capsule called septa partition the gland into lobes.

Loose Connective Tissue

• Loose connective tissue contains relatively fewer cells and protein fibers than dense connective tissue. • The protein fibers are sparse and irregularly arranged (hence, the name "loose connective tissue"), and there is abundant, viscous ground substance. • Loose connective tissues act as the body's "packing material" by supporting and surrounding structures and organs. • There are three types of loose connective tissue: o areolar connective tissue, o adipose connective tissue, and o reticular connective tissue.

Mesenchyme connective tissue

• Mesenchyme (mez′en-kˉım; enkyma = infusion) is the first type of connective tissue to emerge in the developing embryo. • It has star-shaped (stellate) or spindle-shaped mesenchymal cells dispersed within a gel-like ground substance that contains fine, immature protein fibers (table 5.4a). • In fact, ground substance makes up a larger proportion than mesenchymal cells in this type of tissue. • Mesenchyme is the source of all other connective tissues. Adult connective tissues often house numerous mesenchymal (stem) cells that provide support in the repair of the tissue following damage or injury.

Mesoderm

• Mesoderm is the middle primary germ layer. • It forms all muscle tissue and both the epithelial lining of vessels and serous membranes that line the body cavities. • Mesoderm becomes mesenchyme, which then goes on to form all connective tissues in the body. • The dermis of the skin, adrenal cortex, heart, spleen, kidneys and ureters, and internal reproductive structures are mesoderm-derived.

Classification of Exocrine Glands

• Multicellular exocrine glands may be classified either by anatomic form or by method of secretion, which may be thought of as a physiologic classification.

Muscle tissue

• Muscle tissue is composed of specialized cells that can contract when stimulated. • When this tissue contracts, it produces movement, such as the voluntary motion of body parts, contraction of the heart, and propulsion of materials through the digestive and urinary tracts. • The three types of muscle tissue are: o skeletal muscle, o cardiac muscle, and o smooth muscle.

Necrosis

• Necrosis is the term for tissue death. • Necrosis typically occurs due to tissue damage that is not reversible, and an inflammatory response (see section 22.3d) usually occurs in the tissue in response to the damage. • Gangrene is an example of tissue necrosis, and is described in detail (see Clinical View: "Gangrene").

Nervous tissue

• Nervous tissue is located within the brain, spinal cord, and the nerves that traverse through the body. • It consists of cells called neurons (nˉ ur′onz) that receive, transmit, and process nerve impulses. • It also contains a larger number of cells called glial cells (or supporting cells), which do not transmit nerve impulses but instead are responsible for the protection, nourishment, and support of the neurons (table 5.11). • Each neuron has a prominent cell body that houses both the nucleus and other organelles. • Extending from the cell body are branches called nerve cell processes. • The shorter and more numerous processes are dendrites (den′drˉıtes; dendrites = relating to a tree), that receive incoming signals and transmit the information to the cell body. • The single long process extending from the cell body is the axon (ak′son; axon = axis), which carries outgoing signals to other cells. Because of the extensive lengths of some axons, neurons are usually the longest cells in the body; some are longer than 1 meter.

Reticular connective tissue

• Reticular connective tissue houses abundant leukocytes and some fibroblasts within a meshwork of reticular fibers (table 5.5c). This tissue forms the stroma (structural framework) of many lymphatic organs, such as the spleen, lymph nodes, and red bone marrow.

Atrophy

• Shrinkage of tissue by a decrease in either cell size or cell number is called atrophy. • Atrophy may result from normal aging (senile atrophy) or from failure to use an organ or tissue (disuse atrophy). • If an individual becomes bedridden or must wear a cast for a broken bone, the affected muscles exhibit disuse atrophy as the skeletal muscle fibers become smaller. • If the atrophy is not due to long-term problems, then typically physical therapy and a reuse of the tissues can minimize or reverse the atrophic changes.

Classification of Epithelia by Number of Cell Layers

• Simple Epithelium • Stratified Epithelium • Pseudostratified Epithelium

Simple Epithelium

• Simple epithelium is one cell layer thick, and all of the epithelial cells are in direct contact with the basement membrane. • A simple epithelium is found in areas where stress is minimal and filtration, absorption, or secretion is the primary function. • Examples of locations include the lining of the air sacs in the lung, the intestines, and blood vessels.

Skeletal muscle tissue

• Skeletal muscle tissue, also known as striated or voluntary muscle tissue, is primarily responsible for movement of the skeleton (although this tissue also moves some nonskeletal structures, such as the skin of the face). • It is composed of long cylindrical cells called skeletal muscle fibers. • These fibers are arranged in parallel bundles that typically run the length of the entire muscle. • Such long fibers need more than one nucleus to control and carry out all cellular functions; thus, each skeletal muscle fiber is multinucleated (see figure 10.2), with nuclei located at the periphery of the fiber (table 5.10a). • Under the light microscope, skeletal muscle fibers exhibit alternating light and dark bands, called striations, that reflect the overlapping pattern of parallel thick and thin contractile protein filaments. • Additionally, skeletal muscle is considered voluntary because it usually does not contract unless stimulated by the somatic (voluntary) nervous system (see section 12.1).

Smooth muscle tissue

• Smooth muscle tissue, also called visceral or involuntary muscle tissue, is so named because it lacks the striations seen in other muscle tissue, and so this tissue appears smooth (table 5.10c). • Smooth muscle cells are fusiform (spindle-shaped), which means they are thick in the middle and tapered at their ends. • These cells are relatively short and contain one centrally located oval nucleus. • Smooth muscle tissue is also called visceral muscle tissue because it is found in the walls of most viscera, such as the intestines, stomach, airways, urinary bladder, uterus, and blood vessels. • The contraction of smooth muscle helps propel material movement through these organs or controls the size of the lumen. • This tissue is considered involuntary because we do not have voluntary control over the muscle.

Secretions (Epithelial)

• Some epithelial cells are specialized to produce and release secretions. • Individual gland cells may be scattered among other cell types in an epithelium or arranged in small, organized clusters within a gland (see section 5.1d).

synovial membrane

• Some joints in the body are lined by a synovial membrane that is composed of a superficial layer of squamous epithelial cells that lack a basement membrane and rests on an areolar connective tissue layer. • The epithelial cells secrete a synovial fluid that reduces friction among the moving bone parts and distributes nutrients to the cartilage on the articular surfaces of bone.

Squamous Cells (Epithelia)

• Squamous cells are flat, wide, and somewhat irregular in shape. • The cells are arranged like floor tiles, and the nucleus is somewhat flattened

Attachment to a basement membrane (Epithelia)

• The epithelial layer is bound at its basal surface to a thin basement membrane. • It may be seen as a single noncellular (or molecular) layer using the light microscope—however, in reality it consists of three molecular layers that can be viewed using an electron microscope: o the lamina lucida, o the lamina densa, and o the reticula lamina. • These molecular layers are formed by secretions of both the epithelium and the underlying connective tissue, and are composed of collagen, glycoproteins (e.g., laminin, fibronectin), and proteoglycans. • The two laminae closest to the epithelium (lamina lucida and lamina densa) contain collagen fibers as well as specific proteins and carbohydrates, some of which are secreted by the epithelial cells. • Cells in the underlying connective tissue secrete the reticular lamina, which contains protein fibers and carbohydrates. • Together, these basement membrane components strengthen the attachment and form a selective molecular barrier between the epithelium and the underlying connective tissue.

cutaneous membrane

• The largest body membrane is the cutaneous membrane, also known as the skin, which covers the external surface of the body. • The cutaneous membrane is composed of a keratinized stratified squamous epithelium (called the epidermis) and an underlying layer of connective tissue (called the dermis). • Its many functions include protecting internal organs and preventing water loss.

Fluid Connective Tissue

• There are two types of fluid connective tissue: o blood and o lymph.

Supporting Connective Tissue

• There are two types of supporting connective tissue: o cartilage and o bone. • Both form a strong, durable framework that protects and supports the soft body tissue. • The extracellular matrix contains many protein fibers and a ground substance that ranges from semisolid (cartilage) to solid (bone).

Transitional Epithelial Cells (Epithelia)

• These cells can readily change their shape from polyhedral to more flattened, depending upon the degree to which the epithelium is stretched. • The shape change occurs when the epithelium cycles between distended and relaxed states, such as in the lining of the bladder, which fills with urine and is later emptied.

Hypertrophy

• Tissues may change in size, form, or number of cells in response to a stimulus. • Hypertrophy refers to an increase in the size of the existing cells in a tissue, although the number of cells remains constant. • For example, skeletal muscle cells may hypertrophy when a person undergoes a long-term rigorous exercise regimen.

the stages of tissue development in the embryo

• To understand how all tissues form, some basic information about the human embryo is required. • When a secondary oocyte (egg) is fertilized by a sperm, it forms a diploid cell called a zygote. • The zygote undergoes multiple cell divisions; eventually, a multicellular structure called a blastocyst is formed. • The cells that form the embryo are collectively known as the embryoblast. • Embryoblast cells differentiate in the second and third weeks of development. • By the third week of development, three primary germ layers are formed. • All body tissues develop from these layers (figure 5.13). • The three primary germ layers are called o ectoderm, o mesoderm, and o endoderm. • When these three primary germ layers have formed, the growing structure may now be referred to as an embryo.

Unicellular Exocrine Glands

• Unicellular exocrine glands typically do not contain a duct, and they are located close to the surface of the epithelium in which they reside. • The most common type of unicellular exocrine gland is the goblet cell, which is commonly found in both simple columnar epithelium and pseudostratified ciliated columnar epithelium.

neoplasia

• When tissue growth proceeds out of control, a tumor composed of abnormal tissue develops, and the condition is called neoplasia. • Neoplasms (tumors) may be benign or malignant. A benign neoplasm typically is localized in its growth and does not spread, whereas a malignant neoplasm is characterized by invading local tissues and potentially metastasizing, or spreading, to other tissues of the body. • A malignant neoplasm is commonly known as cancer. It is believed that most cancers are a result of DNA damage, either through environmental factors or genetics or a combination thereof. • The growth and proliferation of malignant cells can interfere with the normal functioning of other tissues and organs, leading to the morbidity and mortality of the individual.

White adipose tissue

• White adipose tissue stores energy, acts as an insulator, and serves both as packing around structures as well as a cushion against shocks. • It is located throughout the body in places such as the subcutaneous layer deep to the skin and surrounding various organs. • Typically, the number of adipocytes remains relatively stable in an individual, and weight gain or loss is due to the adipocytes enlarging or shrinking in size, respectively.