Epithelial tissue, or epithelium, has the following general characteristics:
- Epithelium consists of closely packed, flattened cells that make up the inside or outside lining of body areas. There is little intercellular material.
- The tissue is avascular, meaning without blood vessels. Nutrient and waste exchange occurs through neighboring connective tissues by diffusion.
There are two kinds of epithelial tissues:
- The upper surface of epithelium is free, or exposed to the outside of the body or to an internal body cavity. The basal surface rests on connective tissue. A thin, extracellular layer called the basement membrane forms between the epithelial and connective tissue.
- Covering and lining epithelium covers the outside surfaces of the body and lines internal organs.
- Glandular epithelium secretes hormones or other products.
Epithelium that covers or lines
Epithelial tissues that cover or line surfaces are classified by cell shape and by the number of cell layers. The following terms are used to describe these features.
- Squamous cells are flat. The nucleus, located near the upper surface, gives these cells the appearance of a fried egg.
- Cuboidal cells are cube‐ or hexagon‐shaped with a central, round nucleus. These cells produce secretions (sweat, for example) or absorb substances such as digested food.
- Columnar cells are tall with an oval nucleus near the basement membrane. These thick cells serve to protect underlying tissues or may function to absorb substances. Some have microvilli, minute surface extensions, to increase surface area for absorbing substances, while others may have cilia that help move substances over their surface (such as mucus through the respiratory tract).
- Transitional cells range from flat to tall cells that can extend or compress in response to body movement.
Number of cell layers:
- Simple epithelium describes a single layer of cells.
- Stratified epithelium describes epithelium consisting of multiple layers.
- Pseudostratified epithelium describes a single layer of cells of different sizes, giving the appearance of being multilayered.
Names of epithelial tissues include a description of both their shape and their number of cell layers. The presence of cilia may also be identified in their names. For example, simple squamous describes epithelium consisting of a single layer of flat cells. Pseudostratified columnar ciliated epithelium describes a single layer of tall, ciliated cells of more than one size. Stratified epithelium is named after the shape of the outermost cell layer. Thus, stratified squamous epithelium has outermost layers of squamous cells, even though some inner layers consist of cuboidal or columnar cells. These and other epithelial tissues are illustrated in Figure 1.
Figure 1. Types of epithelial tissues.
Glandular epithelium forms two kinds of glands:
- Endocrine glands secrete hormones directly into the bloodstream. For example, the thyroid gland secretes the hormone thyroxin into the bloodstream, where it is distributed throughout the body, stimulating an increase in the metabolic rate of body cells.
- Exocrine glands secrete their substances into tubes, or ducts, which carry the secretions to the epithelial surface. Examples of secretions include sweat, saliva, milk, stomach acid, and digestive enzymes.
Exocrine glands are classified according to their structure (see Figure 2):
- Unicellular or multicellular describes a single‐celled gland or a gland made of many cells, respectively. A multicellular gland consists of a group of secretory cells and a duct through which the secretions pass as they exit the gland.
- Branched refers to the branching arrangement of secretory cells in the gland.
- Simple or compound refers to whether the duct of the gland (not the secretory portion) does or does not branch, respectively.
- Tubular describes a gland whose secretory cells form a tube, while alveolar (or acinar) describes secretory cells that form a bulblike sac.
Figure 2. Exocrine glands can be classified as simple or compound with either a tubular or alveolar structure.
- In merocrine glands, secretions pass through the cell membranes of the secretory cells (exocytosis). For example, goblet cells of the trachea release mucus via exocytosis.
- In apocrine glands, a portion of the cell containing secretions is released as it separates from the rest of the cell. For example, the apical portion of lactiferous glands release milk in this manner.
- In holocrine glands, entire secretory cells disintegrate and are released along with their contents. For example, sebaceous glands release sebum to lubricate the skin in this manner.
Figure 3. Exocrine glands can be classified according to their function.