(ep‵i-thē'lē-ŭm , ep‵i-thē''lē-ă )

Plural: epithelia

[epi- + Gr. thēlē, nipple, teat + -ium (2)]

The layer of avascular cells forming the epidermis of the skin and the surface layer of mucous and serous membranes. The cells rest on a basement membrane and lie close to one another with little intercellular material between them. The epithelium may be simple (consisting of a single layer) or stratified (consisting of several layers). Cells making up the epithelium may be flat (squamous), cuboidal, or cylindrical (columnar). Modified forms of epithelium include ciliated, pseudostratified, gland ular, and neuroepithelium. The epithelium may include goblet cells, which secrete mucus. Stratified squamous epithelium may be keratinized for a protective function or abnormally keratinized in response to physical forces or disease. Squamous epithelium is classified as endothelium, which lines the blood vessels and the heart, and mesothelium, which lines the serous cavities. Epithelium serves the general functions of protection, absorption, and secretion, and specialized functions such as movement of substances through ducts, production of germ cells, and reception of stimuli. Its ability to regenerate is excellent: it may replace itself as often as every 24 hr.

SEE: illus.; skin.

epithelial,

(ep‵i-thē''lē-ăl )

ciliated e.Epithelium with hairlike processes on the surface that wave actively only in one direction. This type is present in the respiratory tract and fallopian tubes.

columnar e.Epithelium composed of cylindrical cells.

cuboidal e.Epithelium consisting of cuboid or prismatic cells with a height approx. equal to their width.

germinal e.1Epithelium that covers the surface of the genital ridge of the urogenital folds of an embryo. It gives rise to the seminiferous tubules of the testes and the surface layer of the ovary. It was once thought to produce the germ cells (spermatozoa and ova).2The epithelium that covers the surface of a mature mammalian ovary.

gland ular e.Epithelium consisting of secretory cells.

junctional e.A band of nonkeratinized stratified squamous epithelium that attaches both to the gingiva (on one side) and to the crown of the tooth (on the other). SYN: epithelial attachment; gingival cuff.

laminated e.Stratified epithelium.

mesenchymal e.Squamous epithelium that lines the subarachnoid and subdural cavities, the chambers of the eye, and the perilymphatic spaces of the ear.

olfactory e.A yellowish-brown thickened patch of epithelium that is found along the upper back walls of the nasal cavity and contains olfactory receptors.

pavement e.Epithelium consisting of flat, platelike cells in a single layer.

pigmented e.Epithelium containing pigment granules.

pseudostratified e.Epithelium whose cell bases rest on the basement membrane. The distal ends of some cells do not reach the surface. Their nuclei lie at different levels, giving the appearance of stratification.

reduced enamel e.Combined epithelial layers of the enamel organ, which form a protective layer over the enamel crown as it erupts and then become the primary epithelial attachment surrounding the tooth.

squamous e.The flat form of epithelial cells.

stratified e.Epithelium with the cells in layers. Mitosis takes place in the deepest layer. SYN: laminated epithelium.

sulcular e.The nonkeratinized epithelium that lines the gingival sulcus.

transitional e.A form of stratified epithelium whose cuboidal cells adjust to mechanical changes such as stretching and recoiling. This type of tissue is found only in the urinary system (renal pelvis, ureter, bladder, and a part of the urethra).