Glandular Tissues.—Every gland is formed by an ingrowth from an epithelial surface. This ingrowth may from the beginning possess a tubular structure, but in other instances may start as a solid column of cells which subsequently Glands. becomes tubulated. As growth proceeds, the column of cells may divide or give off offshoots, in which case a compound gland is formed. In many glands the number of branches is limited, in others (salivary, pancreas) a very large structure is finally formed by repeated growth and subdivision. As a rule the branches do not unite with one another, but in one instance, the liver, this does occur when a reticulated compound gland is produced. In compound glands the more typical or secretory epithelium is found forming the terminal portion of each branch, and the uniting portions form ducts and are lined with a less modified type of epithelial cell.
Glands are classified according to their shape. If the gland retains its shape as a tube throughout it is termed a tubular gland, simple tubular if there is no division (large intestine), compound tubular (fig. 9) if branching occurs (pyloric glands of stomach). In the simple tubular glands the gland may be coiled without losing its tubular form, e.g. in sweat glands. In the second main variety of gland the secretory portion is enlarged and the lumen variously increased in size. These are termed alveolar or saccular glands. They are again subdivided into simple or compound alveolar glands, as in the case of the tubular glands (fig. 10). A further complication in the case of the alveolar glands may occur in the form of still smaller saccular diverticuli growing out from the main sacculi (fig. 11). These are termed alveoli.
| Fig. 10.—A Tubulo-alveolar Gland. One of the mucous salivary glands of the dog. On the left the alveoli are unfolded to show their general arrangement. d, Small duct of gland subdividing into branches; e, f and g, terminal tubular alveoli of gland. | Fig. 11.—A Compound Alveolar Gland. One of the terminal lobules of the pancreas, showing the spherical form of the alveoli. |
The typical secretory cells of the glands are found lining the terminal portions of the ramifications and extend upwards to varying degrees. Thus in a typical acinous gland the cells are restricted to the final alveoli. The remaining tubes are to be considered mainly as ducts. In tubulo-alveolar glands the secreting epithelium lines the alveus as well as the terminal tubule.
The gland cells are all placed upon a basement membrane. In many instances this membrane is formed of very thin flattened cells, in other instances it is apparently a homogeneous membrane, and according to some observers is simply a modified part of the basal surface of the cell, while according to others it is a definite structure distinct from the epithelium.
In the secretory portion of the gland and in the smaller ducts the epithelial layer is one cell thick only. In the larger ducts there are two layers of cells, but even here the surface cell usually extends by a thinned-out stalk down to the basement membrane.
The detailed characters of the epithelium of the different glands of the body are given in separate articles (see [Alimentary Canal], &c.). It will be sufficient here to give the more general characters possessed by these cells. They are cubical or conical cells with distinct oval nuclei and granular protoplasm. Within the protoplasm is accumulated a large number of spherical granules arranged in diverse manners in different cells. The granules vary much in size in different glands, and in chemical composition, but in all cases represent a store of material ready to be discharged from the cell as its secretion. Hence the general appearance of the cell is found to vary according to the previous degree of activity of the cell. If it has been at rest for some time the cell contains very many granules which swell it out and increase its size. The nucleus is then largely hidden by the granules. In the opposite condition, i.e. when the cell has been actively secreting, the protoplasm is much clearer, the nucleus obvious and the cell shrunken in size, all these changes being due to the extrusion of the granules.
Endothelium and Mesothelium.—Lining the blood vessels, lymph vessels and lymph spaces are found flattened cells apposed to one another by their edges to form an extremely thin membrane. These cells are developed from the Endothelium and mesothelium. middle embryonic layer and are termed endothelium. A very similar type of cells is also found, formed into a very thin continuous sheet, lining the body-cavity, i.e. pleural pericardial, and peritoneal cavities. These cells develop from that portion of the mesoderm known as the mesothelium, and are therefore frequently termed mesothelial, though by many they are also included as endothelial cells.
| Fig. 12.—Mesothelial Cells forming the Peritoneal Serous Membrane. Three stomata are seen surrounded by cubical cells. One of these is closed. The light band marks the position of a lymphatic. (After Klein.) |
A mesothelial cell is very flattened, thus resembling a squamous epithelial cell. It possesses a protoplasm with faint granules and an oval or round nucleus (fig. 12). The outline of the cell is irregularly polyhedral, and the borders may be finely serrated. The cells are united to one another by an intercellular cement substance which, however, is very scanty in amount, but can be made apparent by staining with silver nitrate when the appearance reproduced in the figure is seen. By being thus united together, the cells form a continuous layer. This layer is pierced by a number of small openings, known as stomata, which bring the cavity into direct communication with lymph spaces or vessels lying beneath the membrane. The stomata are surrounded by a special layer of cubical and granular cells. Through these stomata fluids and other materials present in the body-cavity can be removed into the lymph spaces.