uterus, oviduct, vas deferens, epididymis, a portion of the renal
tubule, &c.
In the instance of some cells there may be but a single process from the exposed surface of the cell, and then the process is usually of large size and length. It is then known as a flagellum. Such cells are common among the surface cells of many of the simple animal organisms.
When the cells of an epithelial surface are arranged several layers deep, we can again distinguish various types:—
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| Fig. 6.—A Stratified Epithelium from a Mucous Membrane. |
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| Fig. 7.—Stratified Epithelium from the Skin. |
| c, Columnar cells resting on the fibrous true skin. p, The so-called prickle cells. g, Stratum granulosum. h, Horny cells. s, Squamous horny cells. |
1. Stratified Epithelium (figs. 6 and 7).—This is found in the epithelium of the skin and of many mucous membranes (mouth, oesophagus, rectum, conjunctiva, vagina, &c.). Here the surface cells are very much flattened (squamous epithelium), those of the middle layer are polyhedral and those of the lowest layer are cubical or columnar. This type of epithelium is found covering surfaces commonly exposed to friction. The surface may be dry as in the skin, or moist, e.g. the mouth. The surface cells are constantly being rubbed off, and are then replaced by new cells growing up from below. Hence the deepest layer, that nearest the blood supply, is a formative layer, and in successive stages from this we can trace the gradual transformation of these protoplasmic cells into scaly cells, which no longer show any sign of being alive. In the moist mucous surfaces the number of cells forming the epithelial layer is usually much smaller than in a dry stratified epithelium.
2. Stratified Ciliated Epithelium.—In this variety the superficial cells are ciliated and columnar, between the bases of these are found fusiform cells and the lowest cells are cubical or pyramidal. This epithelium is found lining parts of the respiratory passages, the vas deferens and the epididymis.
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| Fig. 8.—Transitional Epithelium from the Urinary Bladder, showing the outlines of the cells only. |
3. Transitional Epithelium (fig. 8).—This variety of epithelium is found lining the bladder, and the appearance observed depends upon the contracted or distended state of the bladder from which the preparation was made. If the bladder was contracted the form seen in fig. 8 is obtained. The epithelium is in three or more layers, the superficial one being very characteristic. The cells are cubical and fit over the rounded ends of the cells of the next layer. These are pear-shaped, the points of the pear resting on the basement membrane. Between the bases of these cells lie those of the lowermost layer. These are irregularly columnar. If the bladder is distended before the preparation is made, the cells are then found stretched out transversely. This is especially the case with the surface cells, which may then become very flattened.
Considering epithelium from the point of view of function, it may be classified as protective, absorptive or secretory. It may produce special outgrowths for protective or ornamental purposes, such are hairs, nails, horns, &c., and for such purposes it may manufacture within itself chemical material best suited for that purpose, e.g. keratin; here the whole cell becomes modified. In other instances may be seen in the interior of the cells many chemical substances which indicate the nature of their work, e.g. fat droplets, granules of various kinds, protein, mucin, watery granules, glycogen, &c. In a typical absorbing cell granules of material being absorbed may be seen. A secreting cell of normal type forming specific substances stores these in its interior until wanted, e.g. fat as in sebaceous and mammary glands, ferment precursors (salivary, gastric glands, &c.), and various excretory substances, as in the renal epithelium.
Initially the epithelium cell might have all these functions, but later came specialization and therefore to most cells a specific work. Some of that work does not require the cell to be at the surface, while for other work this is indispensable, and hence when the surface becomes limited those of the former category are removed from the surface to the deeper parts. This is seen typically in secretory and excretory cells, which usually lie below the surface on to which they pour their secretions. If the secretion required at any one point is considerable, then the secreting cells are numerous in proportion and a typical gland is formed. The secretion is then conducted to the surface by a duct, and this duct is also lined with epithelium.
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| Fig. 9.—A Com- pound Tubular Gland. One of the pyloric glands of the stomach of the dog. |
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.
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| 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
