EPITHELIUM, 6p-T-theli-um, one of the simplest forms of tissue derived chiefly but not alone from the outer embryonic layers and characterized by its non-vascularity. It con sists of flattened or columnar cells united into continuous membranes by an intercellular ce ment substance. Epithelium serves for the most part to protect exposed surfaces of the body, and performs the functions of absorp tion, secretion and excretion. The epithelial tissues are developed from all three layers of the developing embryo. They themselves secrete their own cement substance. This takes on the form of thin plates between the cells, gluing them together. Occasionally the epithelial cells develop short lateral projections (prickles), forming with similar structures of neighboring cells intercellular bridges, between which are intercellular spaces filled with lymph for the nourishment of the cells. Inasmuch as practically all epithelial cells have one ex posed surface and one surface lying in contact with tissues underneath, the upper and lower surfaces show certain variations of structure.
Occasionally the free surface develops fine hairs or cilia. These are found in various localities. The outer surface of the cell, be ing exposed, develops more truly animal func tions, the inner more vegetative. Blood ves sels and lymph vessels do not penetrate, as a rule, into epithelial tissues, but they are richly supplied with nerve-end organs.
According to their shape and relation, the epithelial cells are divided into these varieties: (1) Simple epithelial cells, with or without cilia, comprising (a) squamous epithelium; (b) cubical epithelium; (c) columnar epithelium; (d) pseudostratified columnar epithelium. (2) Stratified epithelium, comprising (a) stratified columnar epithelium, with superficial flattened cells without cilia; (b) transitional epithelium; (c) stratified columnar epithelium. (3) Glan dular epithelium. (4) Neuro-epithelium.
1. Sample Ep:thel:um.— This is that type in which the cells lie in a single continuous layer. This form is found lining almost the entire alimentary tract, the smaller air-passages, the majority of the gland-ducts, the ovarian ducts, the uterus, the central canal, the spinal cord and the ventricles of the brain. In (a) simple squamous epithelium the cells are flattened, forming a mosaic with the nuclei lying in the middle of the cell. It is found in the alveoli of the lungs. In (b) simple cubical epithelium the appearance is that of short polygonal prisms. It occurs in the smaller bronchioles of the lung, in certain portions of the urinifer otis tubules, the liver, pancreas, salivary and mucous glands. In (c) simple columnar epi thelium the cells are pyramidal or prismatic This type is found in the intestinal tract from the cardiac end of the stomach to the anus, and in certain portions of the kidney. Ciliated
columnar is found in the ovarian duct and uterus, in the central canal of the spinal cord and in some of the smaller bronchi. (d).
2. Stratified Epithelium.—When the cells of simple epithelium increase to such an extent that layer upon layer is developed, the epithe lium no longer remains simple, but becomes stratified. The lower layers are richly plied with blood vessels, and multiply very rapidly, pushing out the upper layers that are constantly dying and being cast off. The various forms are: (a) Stratified columnar epithelium, with superficial flattened cells, forming the outer covering of the body, the epidermis and its continuations inside of the body, as, for instance, the walls of the oesopha gus, the epithelium of the conjunctiva, the ex ternal auditory canal, the sheath of the hair follicles, the walls of the rectum, the anus and the vagina. The deeper cells are usually cu bic-cylindrical, and are followed, as a rule, by one or more layers of slightly flattened cells, until finally the outmost layers become very much flattened and horny, or they may be developed into distinct horn-like substances such as that found in the nails. (b) tional epithelium, a type of stratified epithe lium found in the kidney, uterus, urethra and bladder. It is somewhat similar to the stratified columnar epithelium, but does not show the characteristic deep papilla in the basal mem brane, so characteristic of the former. (c) Stratified columnar epithelium, consisting of a superficial layer of columnar cells and deeper layers of irregular, triangular, cubical or spindle-shaped cells. This type is found in the larger duct-glands, in the mucous mem brane of the nose, portions of the male urethra and in parts of the larynx. Many of this type of cells have cilia, particularly those found in the back of the nose, larynx, respiratory tract, larger bronchi, Eustachian tube, epididymis and portions of the vas deferens.
3. Glandular Ep:therum.—This is a type of epithelium occasionally found scattered among other epithelial cells, and which shows the char acteristic of gland-structures. See GLANDS.
4. Neuro-epithelium.— Neuro-epithelial cells are highly specialized cells in which special nerve-end organs are to be found. In one sense nearly all epithelium is neuro-epithelium since many nervous receptors are located in flat, stratified epithelium and also both receptors and effectors are found in the epithelium of mucous membranes. Consult Bailey, 'Histol ogy.'