Epithelial tissue covers the body’s outer surface and lines the internal closed cavities (including blood vessels) and body tubes that communicate with the exterior (gastrointestinal, respiratory, and genitourinary tracts). Epithelium also forms the secretory portion of glands and their ducts.
Origin and Characteristics
Epithelial tissue is derived from all three embryonic layers. Most epithelia of the skin, mouth, nose, and anus are derived from the ectoderm. Linings of the respiratory tract, gastrointestinal tract, and glands of the digestive system are of endodermal origin. The endothelial lining of blood vessels originates from the mesoderm. Many types of epithelial tissue retain the ability to differentiate and undergo rapid proliferation for replacing injured cells.
The cells that make up epithelium have three general characteristics:
• They are characterized by three distinct surfaces: a free surface or apical surface, a lateral surface, and a basal surface.
• They are closely apposed and joined by cell-to-cell adhesion molecules (CAMs), which form specialized cell junctions.
• Their basal surface is attached to an underlying basement membrane (Fig. 4.18).
The characteristics and geometric arrangement of the cells in the epithelium determine their function. The free or apical surface is always directed toward the exterior surface or lumen of an enclosed cavity or tube, the lateral surface communicates with adjacent cells and is characterized by specialized attachment areas, and the basal surface rests on the basement membrane anchoring the cell to the surrounding connective tissue.
Epithelial tissue is avascular (i.e., without blood vessels) and must therefore receive oxygen and nutrients from the capillaries of the connective tissue on which the epithelial tissue rests (see Fig. 4.18). To survive, epithelial tissue must be kept moist. Even the seemingly dry skin epithelium is kept moist by a nonvitalized, waterproof layer of superficial skin cells called keratin, which prevents evaporation of moisture from the deeper living cells.
Basement Membrane. Underneath all types of epithelial tissue is an extracellular matrix, called the basement membrane. A basement membrane consists of the basal lamina and an underlying reticular layer. The terms basal lamina and basement membrane are often used interchangeably. Epithelial cells have strong intracellular protein filaments (i.e., cytoskeleton) that are important in transmitting mechanical stresses from one cell to another.
Cell Junctions and Cell-to-Cell Adhesions. Cells of epithelial tissue are tightly bound together by specialized junctions. These specialized junctions enable the cells to form barriers to the movement of water, solutes, and cells from one body compartment to the next. Three basic types of intercellular junctions are observed in epithelial tissues: continuous tight junctions, adhering junctions, and gap junctions (Fig. 4.19).
Continuous tight or occluding junctions (i.e., zonula occludens), which are found only in epithelial tissue, seal the surface membranes of adjacent cells together. This type of intercellular junction prevents materials such as macromolecules in the intestinal contents from entering the intercellular space.
Adhering junctions represent sites of strong adhesion between cells. The primary role of adhering junctions may be that of preventing cell separation. Adhering junctions are not restricted to epithelial tissue; they provide adherence between adjacent cardiac muscle cells as well. Adhering junctions are found as continuous, beltlike adhesive junctions (i.e., zonula adherens) or scattered, spotlike adhesive junctions, called desmosomes (i.e., macula adherens). A special feature of the adhesion belt junction is that it provides an anchoring site to the cell membrane for microfilaments. In epithelial desmosomes, bundles of keratin-containing intermediate filaments (i.e., tonofilaments) are anchored to the junction on the cytoplasmic area of the cell membrane. A primary disease of desmosomes is pemphigus, which is caused by a buildup of antibodies to desmosome proteins.
Affected people have skin and mucous membrane blistering. Hemidesmosomes, which resemble a half-desmosome, are another type of junction. They are found at the base of epithelial cells and help attach the epithelial cell to the underlying connective tissue.
Gap or nexus junctions involve the close adherence of adjoining cell membranes with the formation of channels that link the cytoplasm of the two cells. Gap junctions are not unique to epithelial tissue; they play an essential role in many types of cell-to-cell communication. Because they are low-resistance channels, gap junctions are important in cell-to-cell conduction of electrical signals (e.g., between cells in sheets of smooth muscle or between adjacent cardiac muscle cells, where they function as electrical synapses). These multiple communication channels also enable ions and small molecules to pass directly from one cell to another.
Types of Epithelium
Epithelial tissues are classified according to the shape of the cells and the number of layers that are present: simple, stratified, and pseudostratified. The terms squamous (thin and flat), cuboidal (cube shaped), and columnar (resembling a column) refer to the cells’ shape (Fig. 4.20).
Simple Epithelium. Simple epithelium contains a single layer of cells, all of which rest on the basement membrane. Simple squamous epithelium is adapted for filtration; it is found lining the blood vessels, lymph nodes, and alveoli of the lungs. The single layer of squamous epithelium lining the heart and blood vessels is known as the endothelium. A similar type of layer, called the mesothelium, forms the serous membranes that line the pleural, pericardial, and peritoneal cavities and cover the organs of these cavities. A simple cuboidal epithelium is found on the surface of the ovary and in the thyroid. Simple columnar epithelium lines the intestine. One form of a simple columnar epithelium has hairlike projections called cilia, often with specialized mucus-secreting cells called goblet cells. This form of simple columnar epithelium lines the airways of the respiratory tract.
Stratified and Pseudostratified Epithelia. Stratified epithelium contains more than one layer of cells, with only the deepest layer resting on the basement membrane. It is designed to protect the body surface. Stratified squamous keratinized epithelium makes up the epidermis of the skin. Keratin is a tough, fibrous protein found as filaments in the outer cells of skin. A stratified squamous keratinized epithelium is made up of many epithelium is found on moist surfaces such as the mouth and tongue. Stratified cuboidal and columnar epithelia are found in the ducts of salivary glands and the larger ducts of the mammary glands. In smokers, the normal columnar ciliated epithelial cells of the trachea and bronchi are often replaced with stratified squamous epithelium cells that are better able to withstand the irritating effects of cigarette smoke.
Pseudostratified epithelium is a type of epithelium in which all of the cells are in contact with the underlying inter-cellular matrix, but some do not extend to the surface. A pseudostratified ciliated columnar epithelium with goblet cells forms the lining of most of the upper respiratory tract. All of the tall cells reaching the surface of this type of epithelium are either ciliated cells or mucus-producing goblet cells. The basal cells that do not reach the surface serve as stem cells for ciliated and goblet cells.6 Transitional epithelium is a stratified epithelium characterized by cells that can change shape and become thinner when the tissue is stretched. Such tissue can be stretched without pulling the superficial cells apart. Transitional epithelium is well adapted for the lining of organs that are constantly changing their volume, such as the urinary bladder.
Glandular Epithelium. Glandular epithelial tissue is formed by cells specialized to produce a fluid secretion. This process is usually accompanied by the intracellular synthesis of macromolecules. The chemical nature of these macromolecules is variable. The macromolecules typically are stored in the cells in small, membrane-bound vesicles called secretory granules. For example, glandular epithelia can synthesize, store, and secrete proteins (e.g., insulin), lipids (e.g., adrenocortical hormones, secretions of the sebaceous glands), and complexes of carbohydrates and proteins (e.g., saliva). Less common are secretions that require minimal synthetic activity, such as those produced by the sweat glands.
All glandular cells arise from surface epithelia by means of cell proliferation and invasion of the underlying connective tissue, and all release their contents or secretions into the extracellular compartment. Exocrine glands, such as the sweat glands and lactating mammary glands, retain their connection with the surface epithelium from which they originated. This connection takes the form of epithelium-lined tubular ducts through which the secretions pass to reach the surface. Exocrine glands are often classified according to the way secretory products are released by their cells. In holocrine-type cells (e.g., sebaceous glands), the glandular cell ruptures, releasing its entire content into the duct system. New generations of cells are replaced by mitosis of basal cells. Merocrine- or eccrine-type glands (e.g., salivary glands, exocrine glands of the pancreas) release their glandular products by exocytosis. In apocrine secretions (e.g., mammary glands, certain sweat glands), the apical portion of the cell, along with small portions of the cytoplasm, is pinched off the glandular cell. Endocrine glands are epithelial structures that have had their connection with the surface obliterated during development. These glands are ductless and produce secretions (i.e., hormones) that move directly into the bloodstream.