EPITHELIAL TISSUE

There are two major categories of epithelia: membranous and glandular. Membranous epithelia are located throughout the body and form such structures as the outer layer of the skin; the inner lining of body cavities, tubes, and ducts; and the covering of visceral organs. Glandular epithelia are specialized tissues that form the secretory portion of glands.

Characteristics of Membranous Epithelia

Membranous epithelia always have one free surface exposed to a body cavity, a lumen (hollow portion of a body tube), or to the skin surface. Some membranous epithelia are derived from derm, such as the outer layer of the skin; some from mesoderm, such as the inside lining of blood vessels; and others from endoderm, such as the inside lining of the digestive tract (gastrointestinal, or GI, tract). Membranous epithelia may be one or several cell layers thick. The upper surface may be exposed to gases, as in the case of epithelium in the integumentary and respiratory systems; to liquids, as in the circulatory and urinary systems; or to semisolids, as in the GI tract. The deep surface of most membranous epithelia is bound to underlying supportive tissue by a basement membrane, that consists of glycoprotein from the epithelial cells and a meshwork of collagenous and reticular fibers from the underlying connective tissue. With few exceptions, membranous epithelia are avascular (without blood vessels) and must be nourished by diffusion from underlying connective tissues. Cells that make up membranous epithelia are tightly packed together, with little intercellular matrix between them. Some of the functions of membranous epithelia are quite specific, but certain generalities can be made. Epithelia that cover or line surfaces provide protection from pathogens, physical injury, toxins, and desiccation. Epithelia lining the GI tract function in absorption. The epithelium of the kidneys provides filtration, whereas that within the pulmonary alveoli (air sacs) of the lungs allows for diffusion. Highly specialized neuroepithelium in the taste buds and in the nasal region has a chemoreceptor function.

FIGURE 1-1 (a) Simple squamous epithelium lines the lumina of vessels, where it permits diffusion. (b) A photomicrograph of this tissue and (c) a labeled diagram. Simple squamous epithelia that line the lumina of vessels are referred to as endothelia, and that which cover visceral organs are referred to as mesothelia.

Many membranous epithelia are exposed to friction or harmful substances from the outside environment. For this reason, epithelial tissues have remarkable regenerative abilities. The mitotic replacement of the outer layer of skin and the lining of the GI tract, for example, is a continuous process. Membranous epithelia are histologically classified by the number of layers of cells and the shape of the cells along the exposed surface. Epithelial tissues that are composed of a single layer of cells are called simple; those that are layered are said to be stratified. Squamous cells are flattened; cuboidal cells are cubeshaped; and columnar cells are taller than they are wide.

Simple Epithelia

Simple epithelial tissue is a single cell layer thick and is located where diffusion, absorption, filtration, and secretion are principal functions. The cells of simple epithelial tissue range from thin, flattened cells to tall, columnar cells. Some of these cells have cilia that create currents for the movement of materials across cell surfaces. Others have microvilli that increase the surface area for absorption.

Simple Squamous Epithelium

Simple squamous epithelium is composed of flattened, irregularly shaped cells that are tightly bound together in a mosaiclike pattern (fig. 1-1). Each cell contains an oval or spherical central nucleus. This epithelium is adapted for diffusion and filtration. It occurs in the pulmonary alveoli within the lungs (where gaseous exchange occurs), in portions of the kidney (where blood is filtered), on the inside walls of blood vessels, in the lining of body cavities, and in the covering of the viscera. The simple squamous epithelium lining the inner walls of blood and lymphatic vessels is termed endothelium (fig. 1-1b). That which covers visceral organs and lines body cavities is called mesothelium.

Simple Cuboidal Epithelium

Simple cuboidal epithelium is composed of a single layer of tightly fitted cube-shaped cells (fig. 1-2). This type of epithelium is found lining small ducts and tubules that have excretory, secretory, or absorptive functions. It occurs on the surface of the ovaries, forms a portion of the tubules within the kidney, and lines the ducts of the salivary glands and pancreas.

Simple Columnar Epithelium

Simple columnar epithelium is composed of tall, columnar cells (fig. 1-3). The height of the cells varies, depending on the site and function of the tissue. Each cell contains a single nucleus which is usually located near the basement membrane.

Lumen of renal tubule

Basement membrane

Nucleus

Kidney

Ureter

(a)

(b)

(c)

FIGURE 1-2 (a) Simple cuboidal epithelium lines the lumina of ducts; for example, in the kidneys, where it permits movement of fluids and ions. (b) A photomicrograph of this tissue and (c) a labeled diagram.

FIGURE 1-3 (a) Simple columnar epithelium lines the lumen of the digestive tract, where it permits secretion and absorption. (b) A photomicrograph of this tissue and (c) a labeled diagram.

Specialized unicellular glands called goblet cells are scattered through this tissue at most locations. Goblet cells secrete a lubricative and protective mucus along the free surfaces of the cells. Simple columnar epithelium is found lining the inside walls of the stomach and intestine. In the digestive system, it forms a highly absorptive surface and also secretes certain digestive chemicals. Within the stomach, simple columnar epithelium has a tremendous mitotic rate. It replaces itself every 2 to 3 days.

Simple Ciliated Columnar Epithelium

Simple ciliated columnar epithelium is characterized by the presence of cilia along its free surface (fig. 1-4). By contrast, the simple columnar type is unciliated. Cilia produce wavelike movements that transport materials through tubes or passageways. This type of epithelium occurs in the female uterine tubes to move the ovum (egg cell) toward the uterus.

Pseudostratified Ciliated Columnar Epithelium

As the name implies, this type of epithelium has a layered appearance (strata = layers). Actually, it is not multilayered (pseudo = false), because each cell is in contact with the basement membrane. Not all cells are exposed to the surface, however (fig. 1-5). The tissue appears to be stratified because the nuclei of the cells are located at different levels. Numerous goblet cells and a ciliated exposed surface are characteristic of this epithelium. It is found lining the inside walls of the trachea and the bronchial tubes; hence, it is frequently called respiratory epithelium. Its function is to remove foreign dust and bacteria entrapped in mucus from the lower respiratory system.

Stratified Epithelia

Stratified epithelia have two or more layers of cells. In contrast to the single-layered simple epithelia, they are poorly suited for absorption and secretion. Stratified epithelia have a primarily protective function that is enhanced by rapid cell divisions. They are classified according to the shape of the surface layer of cells, because the layer in contact with the basement membrane is always cuboidal or columnar in shape.

Stratified Squamous Epithelium

Stratified squamous epithelium is composed of a variable number of cell layers that are flattest at the surface (fig. 1-6). Mitosis occurs only at the deepest layers. The mitotic rate approximates the rate at which cells are sloughed off at the surface. As the newly produced cells grow in size, they are pushed toward the surface, where they replace the cells that are sloughed off. Movement of the epithelial cells away from the supportive basement membrane is accompanied by the production of keratin, progressive dehydration, and flattening. There are two types of stratified squamous epithelial tissues: keratinized and nonkeratinized.

1. Keratinized stratified squamous epithelium contains keratin, a protein that strengthens the tissue. Keratin makes the epidermis (outer layer) of the skin somewhat waterproof and protects it from bacterial invasion. The outer layers of the skin are dead, but glandular secretions keep them soft.
2. Nonkeratinized stratified squamous epithelium lines the oral cavity and pharynx, nasal cavity, vagina, and anal canal. This type of epithelium, called mucosa,
is well adapted to withstand moderate abrasion but not fluid loss. The cells on the exposed surface are alive and are always moistened.

FIGURE 1-4 (a) Simple ciliated columnar epithelium lines the lumen of the uterine tube, where currents generated by the cilia propel the ovum (egg cell) toward the uterus. (b) a photomicrograph of this tissue and (c) a labeled diagram.

FIGURE 1-5 (a) Pseudostratified ciliated columnar epithelium lines the lumen of the respiratory tract, where it traps foreign material and moves
it away from the pulmonary alveoli of the lungs. (b) a photomicrograph of this tissue and (c) a labeled diagram.

FIGURE 1-6 Stratified squamous epithelium forms the outer layer of skin and the lining of body openings. In the moistened areas, such as in the
vagina (a), it is nonkeratinized, whereas in the epidermis of the skin it is keratinized. (b) a photomicrograph of this tissue and (c) a labeled diagram.

Stratified Cuboidal Epithelium

Stratified cuboidal epithelium usually consists of only two or three layers of cuboidal cells. This type of epithelium is confined to the linings of the large ducts of sweat glands, salivary glands, and the pancreas, where its stratification probably provides a more robust lining than would simple epithelium.

Transitional Epithelium

Transitional epithelium is similar to nonkeratinized stratified squamous epithelium except that the surface cells of the former are large and round rather than flat, and some may have two nuclei. Transitional epithelium is found only in the urinary system, particularly lining the cavity of the urinary bladder and lining the lumina of the ureters. This tissue is specialized to permit distension (stretching) of the urinary bladder as it fills with urine. The inner, exposed cells actually transform from being rounded when the urinary bladder is empty to being somewhat flattened as it distends with urine.