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.