EPITHELIUM, in anatomy. Every surface of the body which may come into contact with foreign substances is covered with a protecting layer of cells closely bound to one another to form continuous sheets. By the formation of outgrowths or in growths from these surfaces, structures, consisting largely or en tirely of cells directly derived from the surface epithelium, may be formed. In this way originate the central nervous system, the sensitive surfaces of the special sense organs, the glands, and the hairs, nails, etc. The epithelial cells possess typical microscopical characters which enable them to be readily distinguished from all others. Thus the cell outline is clearly marked, the nucleus large and spherical or ellipsoidal. The protoplasm of the cell is usually large in amount and often contains large numbers of granules.
I. Columnar Epithelium (figs. I and 2) .—This variety covers the intestinal tract from the end of the oesophagus to the com mencement of the rectum. It also lines the ducts of many glands. In a highly typical form it covers the villi of the small intestine (fig. I). The close apposition of these cells to form a closed membrane is well seen when a surface covered by them is examined from above (fig. 3).
2. Cubical Epithelium.—This differs from the former in that the cells are less in height. It is found in many glands and ducts (e.g. the kidney), in the middle ear, choroid plexuses of the brain, etc.
3. Squamous or Flattened Epithelium (fig. 4).—In this variety the cell is flat tened, very thin and irregular in outline. It occurs as the covering epithelium of the alveoli of the lung, of the kidney glomer ules and capsule, etc. The surface epithelial cells of a stratified epithelium are also of this type.
4. Ciliated Epithelium (fig. 5) .—The surface cells of many epithelial membranes carry fine protoplasmic processes or cilia. Most commonly the cells are columnar, but other shapes are also found. During life the cilia are always in movement, and set up a current tending to drive fluid or other material on the surface in one direction along the membrane or tube lined by such epithe lium. Ciliated epithelium lines the trachea, bronchi, parts of the nasal cavities and the uterus, oviduct, vas deferens, epididymis, etc. If there be but a single process from the exposed surface of the cell it is usually large and long, and is known as a flagellum. Such cells are common on the surface of many simple animal organisms.
When the cells of an epithelial surface are arranged several layers deep, we can again distinguish various types: 5. Stratified Epithelium (figs. 6 and 7) . —This is found in the epithelium of the skin and of many mucous membranes (mouth, oesophagus, rectum, conjunctiva, vagina, etc.). Here the surface cells are very much flattened, those of the middle layer are polyhedral and those of the lowest layer are cubical or columnar. This type of epithelium covers surfaces exposed to friction. The surface may be dry, e.g., skin, or moist, e.g., mouth. The surface cells are constantly being rubbed off, and are replaced by new cells growing up from below. Hence the deepest layer is formative and in successive stages from this we can trace the gradual transfor mation of these protoplasmic cells into scaly cells, which no longer show any sign of being alive. In the moist mucous sur faces the number of cells forming the epithelial layer is usually much smaller than in a dry stratified epithelium.
6. Stratified Ciliated Epithelium.—In this variety the superficial cells are ciliated and columnar; between the bases of these are found f usif orm cells and the lowest cells are cubical or pyramidal. This epi thelium lines parts of the respiratory passages, the vas deferens and the epididymis.
7. Transitional Epithelium (fig. 8).—This variety of epithelium lines the urinary bladder, and its appearance depends upon the contracted or distended state of the bladder from which the preparation was made. If the bladder was contracted the form seen in fig. 8 is obtained. The epithelium is in three or more layers, the superficial one being very characteristic. The cells are cubical and fit over the rounded ends of the cells of the next layer. These are pear-shaped, the points of the pear resting on the basement membrane. Between the bases of these cells lie those of the lower most layer. These are irregularly columnar. If the bladder is dis tended before the preparation is made, the cells are stretched transversely. The surface cells then become very flattened.
Considering epithelium from the point of view of function, it may be protective, absorptive or secretory. It may produce spe cial outgrowths (hairs, nails, horns, etc.) , and for such pur poses it may manufacture within itself chemical material best suited for that purpose, e.g. ker atin ; here the whole cell be comes modified. In other in stances it contains fat droplets, granules of various kinds, pro tein, mucin, watery granules or glycogen, etc. In a typical ab sorbing cell granules of material being absorbed may be seen. A secreting cell of normal type forming specific substances stores these in its interior until wanted, e.g., fat as in sebaceous and mammary glands, fer ment precursors (salivary, gastric glands, etc.), and various excretory substances, as in the renal epithelium (see GLANDS). EPITOME, an abridgment, abstract or summary giving the salient points of a book, law case, etc., a short and concise ac count of any particular subject or event.
By transference epitome is also used to express the representation of a larger thing, concrete or abstract, reproduced in miniature. Thus St. Mark's was called by Ruskin the "Epitome of Venice," as it embraces examples of architecture from the loth to the I 9th centuries.