The cells are nearly always large and rich in granular cytoplasm. They contain distinct round or oval, vesicular nuclei, of which there is usually only one in each cell.
The intercellular substance is very small in amount and is clear and homogeneous. The arrangement of the cells and their size and shape all vary greatly, giving rise to a number of varieties of epithelium, which are classified according to the shape and arrangement of the cells. In pavement-epithelium the cells are thin and arranged in a single layer, not unlike endothelium. In cubical epithelium the cells are thicker and also usually arranged in but a single layer. In columnar epithelium the cells are prismatic in form and rest with their bases upon the surface of the tissues beneath.
They are usually separated at their bases by pyramidal cells, so that the layer of epithelium cannot be said to consist strictly of but one layer of cells, and in some situations there are several distinct layers. In stratified epithelium the cells are superimposed upon each other to form a layer of cells, the thickness of which is several times the diameter of a single cell. The cells of the variety of epithelium called ciliated epithelium differ from those of the other varieties in possessing delicate, hair-like processes which project from the free surface of the tissue.
Epithelium resembles endothelium in being composed almost exclusively of cells separated by a minimal amount of intercellular substance. Like endothelium, it is nearly always found covering other tissues and having one free surface. The two tissues differ greatly in the character of their cells, with one notable exception.
This exception is found in the epithelial lining of the pulmonary alveoli, where the pavement-epithelium contains cells that closely resemble those of endothelium. These cells are, however, directly exposed to the inspired air, while endothelium is only found in situations where it is protected from all contact with the exterior.
The cells of this variety of epithelium are approximately of the same diameter in all directions. They may be almost strictly cubical or spherical, but are usually polyhedral as the result of mutual compression, their contiguous surfaces being flattened. They are usually disposed in a single layer upon a surface furnished by the underlying tissues, as, for example, in tubular or racemose glands, but they may be aggregated to form a solid mass of cells filling a sac, as in the sebaceous glands of the skin, or in strands or columns, variously disposed, as in the liver and suprarenal bodies.
It is this form of epithelium that is chiefly concerned in performing the functions of secretion, and, for this reason, it is frequently designated as "glandular epithelium".
The appearance of the individual cells varies considerably according to the functions that they perform and the stage of functional activity which obtained at the time cellular changes were arrested when the particular specimen was prepared for study. It will suffice for present purposes of description to call attention to the fact that the cytoplasm is usually highly granular, partly because of its own structure, partly because many of the substances elaborated and stored within the cells as the result of their functions appear in the form of granules (metaplasm). The nature of these granules varies. They may be albuminoid, zymogenic granules, or minute drops of fatty substances, which may coalesce to form distinct oily globules, or they may consist of carbohydrates, e. g., glycogen. The granular condition of the cytoplasm may be so marked as to render the detection of the nucleus difficult in unstained specimens.
In this form of epithelium the presence of two nuclei in a single cell is more frequent than in the other varieties.
This variety of epithelium consists of thin cells arranged edge to edge to form a single layer. With the exception of certain regions on the surfaces of the pulmonary alveoli, the cells are more cytoplasmic and granular than are those of endothelium which this tissue in other respects closely resembles.
During foetal life the smaller air-passages and alveoli of the lung are lined by a pavement-epithelium, the cells of which are nearly as thick as those of some varieties of cubical epithelium. When, however, the lung is expanded by the respiratory acts following birth, many of the cells lining the alveoli become greatly extended and flattened until their bodies are thin and membranous and their nuclei inconspicuous or even destroyed. These greatly flattened epithelial cells are found covering those portions of the alveolar walls in which the capillary bloodvessels are situated and permit a ready interchange of gases between the air in the alveolar cavities and the blood circulating in their walls. Many of the epithelial cells covering the tissues in the meshes between the capillaries retain the cytoplasmic and granular character possessed before birth and appear capable of multiplying and, perhaps, replacing such of the thinner cells as may be thrown off or destroyed.
It will be evident, from the foregoing descriptions, that there is no sharp structural line separating cubical from pavement-epithelium. Functionally, pavement-epithelium is a much less active tissue than the cubical variety.
The cells of this form of epithelium are of a general columnar or prismatic shape and possess a single nucleus and a cytoplasm that is usually distinctly granular. They are arranged with their long axes parallel to each other, so that their free ends form the surface of the epithelium, while their deeper ends either rest upon the tissues beneath the epithelium or upon other epithelial cells of different shape which form one or more layers between the columnar cells and the underlying tissues. When they rest directly upon the tissues beneath there are usually other epithelial cells of a pyramidal or oval shape which may be regarded as immature cells ready to take the place of such fully developed cells as may become detached or destroyed. The presence of these cells occasions a narrowing of the deep ends of the columnar cells, so that they are not strictly prismatic in form. In cross-section, or when viewed in a direction parallel to their long axes, the cells have a polygonal form due to the lateral pressure they exert upon each other.
The nuclei of the columnar cells are oval, situated nearer the base of the cell than its superficial end with their long axes parallel to those of the cells themselves, and are vesicular in structure with a distinctly reticular arrangement of the chromatin filaments.
Columnar epithelium is found chiefly upon the free surfaces of mucous membranes, but also occurs in some of the secreting glands.
The minute structure of the cells varies somewhat in different situations, but the consideration of these minutiae must be deferred until a description of the structure of the different organs is undertaken in a subsequent chapter.
Ciliated epithelium is merely a variety of either columnar or cubical epithelium in which the free ends of the cells are beset with delicate hair-like processes, which execute lashing movements in some one direction.
It is found lining the trachea and bronchi, the cilia here serving to propel toward the larynx such particles of dust as are brought into the respiratory passages by the currents of air during respiration.
Ciliated epithelium also occurs on the lining membranes of the nose and the adjoining bony cavities, the mucous membrane of the uterus and the Fallopian tubes, the vasa efferentia of the testis and a part of the epididymus, the ventricles of the brain (except the fifth), the central canal of the spinal cord, and the ducts of some glands.
The possession of cilia, which are very motile organs, presents a marked departure in specialization from the usual metabolic functions of epithelium. Ciliated epithelium rarely exercises a secretory function, its stock of energy being utilized to produce motion instead of chemical change. But there are secreting varieties of epithelium possessing a "cuticle" which appears to be morphologically analogous to the cilia, but in which the fibrils are less highly developed, probably not motile, and, therefore, functionally not the equivalents of cilia. This cuticle is highly developed in the cells covering the mucous membrane of the intestine.
In the varieties of epithelium hitherto considered the cells are, in the main, disposed upon some surface in a single layer, some, at least, of the cells usually extending from the bottom of the layer to its surface.
Stratified epithelium is distinguished from these by being of greater depth and consisting of several layers of cells. The epithelium lining the cheek or the oesophagus may be taken as a typical example of this variety.
The most deeply situated cells are small and nearly filled by the round or oval nucleus. They undergo frequent division, and as they multiply some of them are crowded toward the surface. For a time these increase in size through a growth of their cytoplasm.
But as they are pushed nearer to the surface and farther from the sources of nutrition in the vascular tissues underlying the epithelium, they become flattened and their bodies lose their cytoplasmic character, being converted into a dry, horny substance, keratin.
Upon the free surface they are reduced to thin scales, closely adhering to each other and their subjacent neighbors, but entirely devoid of both cytoplasm and nucleus.
Stratified epithelium is found upon surfaces exposed to friction, which it serves to protect against mechanical injury, and, in some cases, against desiccation. It forms the epidermis of the skin, and lines the mouth, oesophagus, rectum, and vagina. In these situations the scaly or squamous cells of the surface are constantly being removed by the attrition to which they are exposed, but are as constantly replaced by fresh cells from the deeper layers of the epithelium. Pressure and moderate friction stimulate the multiplication of the cells in the deepest layers of the tissue, so that parts, e. g. of the skin, which are especially subjected to such influences, acquire a thicker epidermis (callus).
Where the stratified epithelium consists of many layers of cells, as is the case, for instance, upon the skin, there is a provision for the nourishment of the growing cells which are somewhat removed from the vascularized subjacent tissues. The cells of the deeper layers are somewhat separated from each other, leaving a space between them through which nutrient fluids can circulate. Across this space numerous minute projections or "prickles," springing from neighboring cells, join each other, forming connecting bridges between the cells. When isolated, such cells appear covered with these small spicules (" prickle-cells "), and their presence probably increases the tenacity with which the cell-remains adhere to each other when they become hardened and toughened on the surface of the epithelial layer.
These delicate bridges connecting neighboring cells are not peculiar to stratified epithelium, though they are more conspicuous in that tissue than elsewhere. They have been observed between the cells of the columnar epithelium of the intestinal mucous membrane, and also between the cells of other elementary tissues ; e. g., smooth muscular tissue.
This variety resembles stratified epithelium in forming layers several cells in thickness, but differs in the character of its superficial cells. These do not undergo the horny change peculiar to stratified epithelium, but continue to increase in size, forming a covering of very large cells lying upon those beneath. Under these largest superficial cells are pyriform cells lying with their larger, rounded ends next to the topmost layer, while their deeper and more attenuated ends lie between the oval or round cells that form the one or two deepest layers of the epithelium and rest upon the underlying tissues.
Transitional epithelium is found lining the renal pelves, ureters, and bladder. Its structure permits of a considerable stretching of the tissues beneath without rupture of the epithelial layer over them, the cells of which become flattened to cover the increased surface, to return to their first condition when the viscus which they line is emptied. This is notably the case in the bladder, the epithelial lining of which may be taken as a type of this variety of tissue.
The functional activities of epithelium are in marked contrast to the comparatively inert character of endothelium. The cytoplasmic nature of the epithelial cell, when contrasted with the poverty in cytoplasm of the cell in endothelium, would lead us to expect this difference in the cellular activities of the two tissues. At the beginning of this chapter a sketch of the manifold functions of epithelium was given. It is a fair general statement of its usefulness to say that epithelium is chiefly concerned in bringing about chemical changes in substances brought to it. Sometimes these substances are elaborated into fresh cell-constituents, and the activity of the tissue is displayed chiefly in an active multiplication and growth of its cells. This is especially true in the stratified variety, where protection is provided by a constantly renewed supply of cells. In other cases the substances received by the cells are elaborated into definite compounds destined to form the essential constituents of a secretion. This secretory function of epithelium is an extremely important one, and for its performance that tissue is usually arranged in a special structure or organ, called a gland. A brief statement of the general characters and classification of these organs may here appropriately find a place.
The simplest type of secreting structure consists of a surface covered with a layer of epithelium, the cells of which are endowed with the power of elaborating a secretion and discharging it upon their free surfaces. The tissues supporting the epithelium belong to the connective tissues, and are fibrous in character and well provided with bloodvessels, lymphatics, and nerves. These bring to the epithelium the substances necessary for its nourishment and work, and place its activities under the control of the nervous system. Between the epithelium and the fibrous tissue supporting it there is frequently a thin membranous layer of tissue that often appears quite homogeneous, evidently belongs to the connective tissues, and has received the name of "basement-membrane." This appears to offer a smooth surface for the attachment of the epithelial cells, which receive their nourishing fluids through it.
The epithelial surfaces of many of the mucous membranes are examples of the foregoing simple secreting structure. The secretory function is here of use as an adjunct to the protective function assigned to the epithelial covering, and the quantity of secretion i& but slight under normal conditions. Where the volume of secretion required is considerable some provision for an increase in the extent of secreting surface is necessary. This may be accomplished by an invagination of that surface, which then forms the lining of one or more tubes or sacs, into which the secretion furnished by the epithelial cells is discharged. Such an arrangement of the tissues constitutes a gland, and it is evident that these may be arranged into groups or classes according to whether the secreting surface forms a single tube or sac, or several such tubes or sacs, uniting to form a single gland. Thus, there may be simple or compound tubular glands, or simple or compound saccular glands. Whether the deeper portions of the gland have a tubular or saccular structure, the secretion of the gland is discharged upon some free surface through a tubular outlet, called the duct. This is frequently lined with a non-secreting layer of epithelial cells differing in character from the actively secreting epithelium in the deeper portions of the glandular passages.
It is rarely possible to trace the connection between the ducts and other portions of a gland in sections, for the axes of these different parts seldom lie in one plane. As a result of this circumstance, sections of glands usually present a collection of round or oval sections of tubes or sacs, which are lined with a single layer of epithelial cells, surrounding a lumen. The cells in the deeper portions are usually granular and cubical; those lining the ducts are generally more columnar in shape and less granular in character.
The deeper portions are called the alveoli or acini of the gland, to distinguish them from the ducts, and the character of the epithelium they contain differs according to the function of the gland. Sometimes the cells are so large that they nearly fill the acini, leaving a scarcely perceptible lumen. In other glands the cells are less voluminous and the lumen of each acinus is distinct. It occasionally happens, e.g., in the submaxillary glands, that the acini contain two sorts of cells which secrete different materials. Both kinds of cell may be present in the same acinus, or each kind may be confined to different acini. In studying sections of glands it must be borne in mind that the tangential section of an acinus would appear as a group of cells surrounded by fibrous tissue, with no trace of a lumen among the epithelial cells.
Glands develop from surfaces which are covered by epithelium.
The cells of this epithelium multiply and penetrate into the underlying tissues, forming little solid tongues or columns of cells.
If the gland is destined to be of the simple tubular variety, this column of cells then becomes hollowed to form the lumen, the cells being arranged in a single layer lining the tubule. If the gland is to be compound, the solid column of cells branches within the tissues, and then the lumina of the different portions are formed, the epithelium in the different parts becoming differentiated as specialization of function develops.
The foregoing general description of the structure of secreting glands applies to those glands which have a purely secretory function, discharging the products of their activities upon some free surface, such as the skin or a mucous membrane. There are other glandular organs which perform more complicated functions and the structure of which deviates from that of the simpler glands.
Examples of these are furnished by the liver and kidney, the structures of which must be deferred to a subsequent chapter. Other exceptions are exemplified in the thyroid body and other "ductless" glands, which discharge no secretion into a viscus or upon a free surface, but which have an alveolar structure similar to an ordinary secreting gland. These alveoli do not communicate with ducts, which are wanting; but whatever products they may contribute to the whole organism are apparently discharged into the circulating fluids of the body by a process of absorption similar to that through which the glandular epithelium obtains its materials from those fluids, or by a direct discharge into the lymphatics. This process is indicated by the term "internal secretion," and is probably of commoner occurrence than is usually supposed. In fact, it but represents a special interpretation of the phenomena of interchange of material that is constantly going on between all the cells of the body and its circulating fluids.
Epithelium is developed from the epiderm or hypoderm; never from the mesoderm. In this respect, as well as in its functional role, it differs from endothelium.
From “Normal Histology” (1905) by Edward K. DUNHAM (1860-1922), Pn.B., M.D., Professor of general Pathology, bacteriology and hygiene, in the University and Bellevue Hospital Medical College, New York.