epithelium

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Membranous tissue composed of one or more layers of cells separated by very little intercellular substance and forming the covering of most internal and external surfaces of the body and its organs.

Century Dictionary and Cyclopedia

1. n. In anatomy, the superficial layer of cells of mucous membranes, covering the connective-tissue layer, corresponding to the epidermis of the outer skin and continuous with it at the mouth and other natural openings. The usual meaning of the word, however, is somewhat wider than this, and includes all tissues similar in structure to the above. It embraces the proper tissue of secreting glands, whether derived from the hypoblast, as in the case of the gastric and intestinal glands, the liver and the pancreas, or from the epiblast, as in the case of the sudoriparous, sebaceous, and mammary glands, or from the mesoblast, as in the case of the kidneys, ovaries, and testes; it is applied, moreover, to the ependyma of the cerebrospinal ventricular cavities and to the epidermis itself. With what seems a distinct widening of its meaning, the term is not infrequently employed to designate the endothelium of blood- and lymph-channels and of serous membranes. The epithelium is thus the covering of all free surfaces, mucous, external, and even serous, and forms the glands and other organs derived from these coverings. Epithelial tissue consists of cells, usually compactly set; the nuclei are usually distinct, with an intranuclear network and nucleoli. The intercellular substance is scanty, often inappreciable, and is called cement. It contains no blood-vessels or lymphatics, but nerve-fibrils extend into it. The epithelial tissue, forming the outermost covering of free surfaces, is favorably situated for performing protective and secreting functions. The protective function is not only exhibited by the general layer of easily replaced cells coating the mucous membrane and outer skin, hut in the latter case by a peculiar tendency to form keratin, and this results in a quite impervious outer horny layer, which guards against minor violence, the absorption of deleterious substances, and the invasion of pathogenic bacteria, as well as in the development of such especial means of protection as scales and feathers, hair and nails. This chemical feature of that epithelium which is especially devoted to protection, the production of keratin, can be matched by no single peculiarity on the part of the secretory epithelium; for that must respond equally whether it is called upon to eliminate waste products, or to elaborate digestive ferments, or to manufacture milk. It is probable that some of the cells lining the digestive tract have an active absorptive function with reference to the products of digestion, and that they select and take up certain substances from the intestine, and after more or less elaboration pass them on to the blood- or lymph-channels. This forms a kind of inverted secretion. The epithelial cells of secreting glands are, in part at least, under the direct control of the nervous system. Whether epithelial cells having a purely protective function are, as regards their nutrition, under similar control is still a question. See cuts under Malpighian and villus.
2. n. In ornithology, specifically, the dense, tough cuticular lining of the gizzard. It is sometimes even bony, and sometimes deciduous.
3. n. In botany, a delicate layer of cells lining the internal cavities of certain organs, as the young ovary, etc.: also applied to the thin epidermis of petals.

Wiktionary

1. n. anatomy A membranous tissue composed of one or more layers of cells which forms the covering of most internal and external surfaces of the body and its organs: internally including the lining of vessels and other small cavities, and externally being the skin.

GNU Webster's 1913

1. n. (Anat.) The superficial layer of cells lining the alimentary canal and all its appendages, all glands and their ducts, blood vessels and lymphatics, serous cavities, etc. It often includes the epidermis (i. e., keratin-producing epithelial cells), and it is sometimes restricted to the alimentary canal, the glands and their appendages, -- the term endothelium being applied to the lining membrane of the blood vessels, lymphatics, and serous cavities.

WordNet 3.0

1. n. membranous tissue covering internal organs and other internal surfaces of the body