4.3 Epithelial Tissue

Cell connections

- Cells have structures that hold them to one another or to the basement membrane. These structures do three things: (1) mechanically bind the cells together. (2) help form a permeability barrier. (3) provide a mechanism for intercellular communication. - Epithelial cells secrete glycoproteins that attach the cells to the basement membrane and to one another. This relatively weak binding between cells is reinforced by desmosomes, disk-shaped structures with especially adhesive glycoproteins that bind cells to one another and intermediate filaments that extend into the cytoplasm of the cells. Many desmosomes are found in epithelial tissues that are subjected to stress, such as the stratified squamous epithelium of the skin. Hemidesmosomes, similar to one-half of a desmosome, attach epithelial cells to the basement membrane. - Tight junctions hold cells together and form a permeability barrier. Tight junctions are formed by plasma membranes of adjacent cells that join one another in a jigsaw fashion to make a tight seal. Near the free surface of simple epithelial cells, the tight junctions form a ring that completely surrounds each cell and binds adjacent cells together to prevent the passage of materials between cells. For example, in the stomach and the urinary bladder, chemicals cannot pass between cells. Thus, water and other sub- stances must pass through the epithelial cells, which can actively regulate what is absorbed or secreted. Tight junctions are found in areas where a layer of simple epithelium forms a permeability barrier. For example, water can diffuse through epithelial cells, and active transport, symport, and facilitated diffusion move most nutrients through the epithelial cells of the small intestine. - An adhesion belt of glycoproteins is found just below the tight junction. It is located between the plasma membranes of adjacent cells and acts as a weak glue that holds cells together. These connections are not as strong as those of desmosomes. - A gap junction is a small, specialized contact region between cells containing protein channels that aid intercellular communication by allowing ions and small molecules to pass from one cell to another. In epithelium, the function of gap junctions is not entirely clear; gap junctions between ciliated epithelial cells may coordinate the movements of cilia. In cardiac and smooth muscle tissues, gap junctions are important in coordinating important functions. Because ions can pass through the gap junctions from one cell to the next, electrical signals can pass from cell to cell to coordinate the contraction of cardiac and smooth muscle cells. Thus, electrical signals that originate in one cell of the heart can spread from cell to cell and cause the entire heart to contract. The gap junctions between cardiac muscle cells are found in specialized cell-to-cell connections called intercalated disks (see chapter 20). In addition to containing gap junctions, the intercalated disks have desmosomes that help hold adjacent cells in close contact.

Glands

- Glands are secretory organs. Many glands are composed primarily of epithelium, with a supporting network of connective tissue. These glands develop from an infolding or outfolding of epithelium in the embryo. If the gland maintains an open contact with the epithelium from which it developed, a duct is present. Glands with ducts are called exocrine glands, and their ducts are lined with epithelium. Alternatively, some glands become separated from the epithelium of their origin and have no ducts; these are called endocrine glands. Endocrine glands have extensive blood vessels. The cellular products of endocrine glands, which are called hormones, are secreted into the bloodstream and carried throughout the body. - Most exocrine glands are composed of many cells and are called multicellular glands, but some exocrine glands are composed of a single cell and are called unicellular glands. Goblet cells are unicellular glands that secrete mucus. - Multicellular glands can be classified according to the structure of their ducts and secretory regions. Glands that have a single duct are called simple, and glands with ducts that branch are called compound. Glands with secretory regions shaped as tubules (small tubes) are called tubular, whereas those shaped in saclike structures are called acinar or alveolar. Tubular glands can be straight or coiled. Glands with a combination of the two are called tubuloacinar or tubuloalveolar. If multiple acinar or tubular secretory regions (not ducts) are branched off a single duct, the gland is called branched. - Exocrine glands can also be classified according to how products leave the cell. The most common type of secretion is merocrine secretion. In merocrine secretion, products are released, but no actual cellular material is lost. Secretions are either actively transported or packaged in vesicles and then released by the process of exocytosis at the free surface of the cell. Merocrine secretion is used by water-producing sweat glands and the exocrine portion of the pancreas. In apocrine secretion, the secretory products are released as fragments of the gland cells. Secretory products are retained within the cell, and portions of the cell are pinched off to become part of the secretion. The milk-producing mammary glands release milk by a combination of apocrine and mostly merocrine secretion. Holocrine secretion involves the shedding of entire cells. Products accumulate in the cytoplasm of each epithelial cell, the cell ruptures and dies, and the entire cell becomes part of the secretion. Holocrine secretion is used by the sebaceous (oil) glands of the skin.

Epithelial tissue

- Or epithelium, covers and protects surfaces, both outside and inside the body. - The characteristics common to most types of this tissue: * Mostly composed of cells. This tissue consists almost entirely of cells, with very little extracellular matrix between them. * Covers body surfaces. This tissue covers body surfaces and forms glands that are derived developmentally from body surfaces. The body surfaces include the exterior surface, the lining of the digestive and respiratory tracts, the heart and blood vessels, and the linings of many body cavities. * Distinct cell surfaces. Most epithelial tissues have cells with one free, or (apical), surface not attached to other cells; a lateral surface attached to other epithelial cells; and a basal surface attached to a basement membrane. The free surface often lines the lumen of ducts, vessels, and cavities. The basement membrane is a specialized type of extracellular material secreted by epithelial and connective tissue cells. Like the adhesive on scotch tape, the basement membrane helps attach the epithelial cells to the underlying tissues, and it plays an important role in supporting and guiding cell migration during tissue repair. The basement membrane is typically porous, which allows substances to move to and from the epithelial tissue above it. A few epithelial tissues, such as those in the lymphatic capillaries and liver sinusoids, do not have basement membranes, and some epithelial tissues, such as those in some endocrine glands, do not have a free surface or a basal surface with a basement membrane. * Cell and matrix connections. Specialized cell contacts bind adjacent epithelial cells together and to the extracellular matrix of the basement membrane. * Nonvascular. Blood vessels in the underlying connective tissue do not penetrate the basement membrane to reach the epithelium; thus, all gases and nutrients carried in the blood must reach the epithelium by diffusing from blood vessels across the basement membrane. In epithelial tissues, with many layers of cells, diffusion must also occur across cells, and the most metabolically active cells are close to the basement membrane. * Capable of regeneration. Epithelial cells retain the ability to undergo mitosis and therefore are able to replace damaged cells with new epithelial cells. Undifferentiated cells (stem cells) continuously divide and produce new cells. In some types of epithelial tissues, such as those in the skin and the digestive tract, new cells continuously replace the cells that die.

Functions of the epithelial tissues

- Protecting underlying structures. For example, the outer layer of the skin and the epithelium of the oral cavity protect the underlying structures from abrasion. - Acting as a barrier. Epithelium prevents many substances from moving through it. For example, the skin acts as a barrier to water and reduces water loss from the body. The skin also prevents many toxic molecules and microorganisms from entering the body. - Permitting the passage of substances. Epithelium allows many substances to move through it. For example, oxygen and carbon dioxide are exchanged between the air and blood by diffusion through epithelium in the lungs. Epithelium acts as a filter in the kidney, allowing many substances to pass from the blood into the urine but retaining other substances,, such as blood cells and proteins in the blood. - Secreting substances. Mucous glands, sweat glands, and the enzyme secreting portions of the pancreas are all composed of epithelial cells that secrete their products onto surfaces or into ducts that carry them to other areas of the body. - Absorbing substances. The plasma membranes of certain epithelial tissues contain carrier proteins, which regulate the absorption of materials.

Free surfaces

- The free surfaces of epithelial can be smooth or folded; they may have microvilli or cilia. Smooth surfaces reduce friction. For example, the lining of the blood vessels is a simple squamous epithelium that reduces friction as blood flows through the vessels. - Microvilli are nonmotile and contain microfilaments. They greatly increase free surface area and occur in cells that absorb or secrete, such as serous membranes and the lining of the small intestine. Stereocilia are elongated microvilli found in sensory structures, such as the inner ear, and they play a role in sound detection. They are also found in some places where absorption is important, such as the epithelium of the epididymis. Motile cilia, which contain microtubules, move materials across the free surface of the cell. Three types of ciliated epithelium line the respiratory tract, where cilia move mucus that contains foreign particles out of the respiratory passages. Cilia are also found on the apical surface of the simple columnar epithelial cells of the uterus and uterine tubes, where the cilia help move mucus and oocytes. - Transitional epithelium has a rather unusual plasma membrane specialization: rigid sections of membrane separated by very flexible regions in which the plasma membrane is folded. When transitional epithelium is stretched, the folded regions of the plasma membrane can unfold. Transitional epithelium is specialized to expand in tissues such as the urinary bladder.

Classification of epithelial tissues

Epithelial tissues are classified primarily according to the number of cell layers and the shape of the superficial cells. There are three major types of epithelium based on the number of cell layers in each: 1. Simple epithelium- consists of a single layer of cells, with each cell extending from the basement membrane to the free surface. 2. Stratified epithelium- consists of more than one layer of cells, but only the basal layer attaches the deepest layer to the basement membrane. 3. Pseudostratified columnar epithelium- a special type of simple epithelium. Pseudo means "false," so this type of epithelium appears to be stratified but it's not. It consists of one layer of cells, with all the cells attached to the basement membrane. There appears to be two or more layers of cells because some of the cells are tall and extend to the free surface, whereas others are shorter and do not extend to the free surface. There are three types of epithelium based on idealized shapes of the epithelial cells: 1. Squamous- cells that are flat or scalelike. 2. Cuboidal- cells are cube shaped and are about as wide as they are tall. 3. Columnar- cells tend to be taller than they are wide. In most cases, epithelium is given two names, such as simple squamous, stratified squamous, simple columnar, or pseudostratified columnar. The first name indicates the number of layers , and the second indicates the shape of the cells at the free surface. Simple squamous epithelium consists of one layer of flat, or scalelike, cells that rest on a basement membrane. Stratified squamous epithelium consists of several layers of cells. Near the basement membrane, the cells are more cubed shaped, but at the free surface the cells are flat or scalelike. Pseudostratified columnar epithelial are columnar in shape (taller than they are wide) and, although they appear to consist of more than one layer, all the cells rest on the basement membrane. Stratified squamous epithelium can be classified further as either nonkeratinized or keratinized, according to the condition of the outermost layer of cells. Nonkaratinized (moist) stratified squamous epithelium, found in areas such as the mouth, esophagus, rectum, and vagina, consists of living cells in the deepest and outermost layers. A layer of fluid covers the outermost layers of cells, which makes them moist. In contrast, keratinized stratified squamous epithelium, found in the skin, consists of living cells in the deepest layers, and the outermost layers are composed of dead cells containing the protein keratin. The dead, keratinized cells give the tissue a dry, durable, moisture-resistant character. In addition to the skin, keratinized stratified squamous epithelium is also found in the gums and hard plate of the mouth. A unique type of stratified epithelium called transitional epithelium lines the urinary bladder, ureters, pelvis of the kidney, and superior part of the urethra. These are structures where considerable expansion can occur. The shape of the cells and the number of cell layers vary, depending on the degree to which transitional epithelium is stretched. The surface cells and the underlying cells are roughly cuboidal or columnar when the epithelium is not stretched, and they become more flattened or squamouslike as the epithelium is stretched. Also, as the epithelium is stretched, the epithelial cells can shift on one another, so that the number of layers decreases from five or six to two to three.

Functional Characteristics

Epithelial tissues have many functions, including forming a barrier between a free surface and the underlying tissues and secreting, transporting, and absorbing selected molecules. The structure and the organization of cells within each epithelial type reflect these functions. - Cell Layers and cell shapes: + Simple epithelium, with its single layer of cells, covers surfaces. In the lungs it facilitates the diffusion of gases; in the kidneys it filters blood; in glands it secretes cellular products; and in the intestines it absorbs nutrients. + Stratified epithelium is found in areas where protection is a major function because it is able to hinder the selective movement of materials through the epithelium. The multiple layers of cells in stratified epithelium are well adapted for a protective role. As the outer cells are damaged, they are replaced by cells from deep layers; thus, a continuous barrier of epithelial cells is maintained in the tissue. + Stratified squamous epithelium is found in areas of the body where abrasion can occur, such as the skin, mouth, throat, esophagus, anus, and vagina. + Differing functions are also reflected in cell shape. Cells that filter substances and allow diffusion are normally flat and thin. For example simple squamous epithelium forms blood and lymphatic capillaries, alveoli (air sacs) of the lungs, and parts of the kidney tubules. Cells that secrete or absorb are usually cuboidal or columnar. They have greater cytoplasmic volume relative to surface area than seen with squamous cells. This greater volume results from the presence of organelles responsible for the tissues' functions. For example, pseudostratified columnar epithelium, which secretes large amounts of mucus, lines the respiratory tract and contains large goblet cells, which are specialized columnar epithelial cells. The goblet cells contain abundant organelles, such as ribosomes, endoplasmic reticulum, Golgi apparatuses, and secretory vesicles, that are responsible for synthesizing and secreting mucus.