EPITHELIAL, ENDOTHELIAL and GLANDULAR TISSUES, 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 Epithelium. to form continuous sheets. These are epithelial cells (from θηλή, a nipple). By the formation of outgrowths or ingrowths from these surfaces further structures, consisting largely or entirely 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, &c. 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.
The individual cells forming an epithelial membrane are classified according to their shape. Thus we find flattened, or squamous, cubical, columnar, irregular, ciliated or flagellated cells. Many of the membranes formed by Varieties. these cells are only one cell thick, as for instance is the case for the major part of the alimentary canal. In other instances the epithelial membrane may consist of a number of layers of cells, as in the case of the epidermis of the skin. Considering in the first place those membranes of which the cells are in a single layer we may distinguish the following:—
1. Columnar Epithelium (figs. 1 and 2).—This variety covers the main part of the intestinal tract, i.e. from the end of the oesophagus to the commencement of the rectum. It is also found lining the ducts of many glands. In a highly typical form it is found covering the villi of the small intestine (fig. 1). The external layer of the cell is commonly modified to form a thin membrane showing a number of very fine radially arranged lines, which are probably the expression of very minute tubular perforations through the membrane.
| Fig. 1.—Isolated Epithelial Cells from the Small Intestine of the Frog. | Fig. 2.—Columnar Epithelial Cells resting upon a Basement Membrane. | Fig. 3.—Mosaic appearance of a Columnar Epithelial Surface as seen from above. |
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). The surfaces of the cells are then seen to form a mosaic, each cell area having a polyhedral shape.
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, &c.
| Fig. 4.—Squamous Epithelial Cells from the Mucous Membrane of the Mouth. |
3. Squamous or Flattened Epithelium (fig. 4).—In this variety the cell is flattened, very thin and irregular in outline. It occurs as the covering epithelium of the alveoli of the lung, of the kidney glomerules and capsule, &c. The surface epithelial cells of a stratified epithelium are also of this type (fig. 4). Closely resembling these cells are those known as endothelial (see later).
| Fig. 5.—Isolated ciliated Epithelial Cells from the Trachea. |
4. Ciliated Epithelium (fig. 5).—The surface cells of many epithelial membranes are often provided with a number of very 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 epithelium. It is found lining the trachea, bronchi, parts of the nasal cavities and the uterus, oviduct, vas deferens, epididymis, a portion of the renal tubule, &c.