epithelium (epitheʻlium)

[NL., orig. used to designate the outer layer of the integument of the lips, which covers the papillæ; The usual meaning of the word, however, is somewhat wider than this, and includes all ti ues similar in structure to the above. It embraces the proper ti ue 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 thelium 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 ti ue 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-ve els or lymphatics, but nerve-fibrils extend into it. The epithelial ti ue, 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, but 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 le elaboration pa 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. The epithelium is the epidermis of the mucous membrane. Wilson, Anat. (1847), p. 540. 2. In ornith., specifically, the dense, tough cuticular lining of the gizzard. It is sometimes even bony, and sometimes deciduous.-3. In bot., 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. - Ciliated epithelium, any variety of true epithelium the cells of which are individually furnished on their free surface with cilia. The cells are usually of columnar form, packed closely side by side, with the cilia on their exposed ends. These cilia are microscopic proce es of the cell, like eyelashes from an eyelid, and keep up a continual lashing or vibratile motion, by which mucus is swept along the pa ages. Ciliated epithelium is found in man in the whole respiratory tract, the middle ear and Eustachian tube, the Fallopian tubes and part of the uterus, in portions of the seminal pa ages, and in the cavities of the brain and spinal cord. - Columnar or cylindrical epithelium, epithelium whose cells are more or le rod-like in shape, set on end, and joined together by their sides into a membrane. These cells are usually flattened or somewhat prismatic by mutual pre ure. Goblet-cells are a modification of ordinary columnar epithelium cells, scattered here and there among the latter.-Germinal epithelium. See the extract. The epithelial investment of the abdominal cavity retains its primitive character along a tract which corre sponds to the rudiment of the primitive kidney longer than it does in other regions; and this epithelial layer may be distinguished as the germinal epithelium. Gegenbauer , Comp. Anat. (trans.), p. 608. Pavement epithelium, epithelium in which the cells are flattened and coherent by their irregular polygonal edges, like the tiles of a mosaic pavement. Also called te ellated, squamous, lamellose, lamellar, and flattened epi thelium. It may be either simple, when it consists of a single layer of cells, as in the epithelium of the pulmonary alveoli, or stratified, when it consists of several layers, as in the epidermis. - Simple epithelium, any epithelium whose cells form a single layer: distinguished from stratified epithelium. - Spheroidal epithelium, glandular epithelium, characteristic of the terminal rece es and crypts of the secreting surfaces of glands, with more or le spherical or polyhedral cells. - Stratified epithelium, any epithelium whose cells are in two or more lay ers or strata, one upon another. Tegumentary epithelium, the epidermis. - Te ellated epithelium. Same as pavement epithelium. - Transitional epithelium, stratified epithelium of three distinguishable layers of cells, such as occurs in the ureters and urinary bladder. -Vascular epithelium, the epithelial or endothelial lining of blood-ve els and lymphatics.