See also:surface of the
See also:body which may come into Epithet contact with
See also:foreign substances is covered with a turn . protecting layer of cells closely bound to one another to
See also:form continuous sheets . These are
See also:epithelial cells (from BilXii, a nipple) . By the formation of outgrowths or in- growths from these surfaces further structures, consisting largely or entirely of cells directly, derived from the surface epithelium, may be formed . In this way originate the central
See also:system, the sensitive surfaces of the
See also:special sense
See also:organs, the glands, and the hairs, nails, &c . The epithelial cells possess typical microscopical characters which enable them to be readily distinguished from all others . Thus the
See also:cell outline is clearly marked, the nucleus large and spherical or ellipsoidal . The
See also:protoplasm of the cell is usually large in amount and often contains large numbers of granules . The individual cells forming an epithelial membrane are classified 'according to their shape . Thus we find flattened, or squantous, cubical, columnar; irregular, ciliated or flagellated cells . Many of the membranes formed by varieties. these cells are only one cell thick, as for instance is the case for the major
See also:part of the alimentary canal .
In other instances the epithelial membrane may consist of a number of layers of cells, as in the case of the epidermis of the skin . Considering in the firstplace those membranes of which the cells are in a single layer we may distinguish the following: 1 . Columnar Epithelium (
See also:figs . 1 and 2).—This variety covers the
See also:main part of the intestinal
See also:tract, i.e. from the end of the oesophagus to the commencement of the rectum . It is also found lining the ducts of many glands . In a highly typical form it is found covering the villi of the small in- testine (fig . 1) . The
See also:external layer of the cell is commonly modified to form a thin membrane showing a number of very
See also:fine radially 'arranged lines, which are probably the expression of very minute tubular perforations through the membrane . The close apposition of these cells to form a closed membrane is well seen when a surface covered by them is examined from above (fig . 3) . The surfaces of the cells are then seen to form a
See also:mosaic, each cell
See also:area having a polyhedral shape . 2 .
Cubical Epithelium.—This differs from the former in that the cells are less in height . It is found in many glands and ducts (e.g. the
See also:kidney), in the
See also:middle ear, choroid plexuses of the
See also:brain, &c . 3 . Squamous or Flattened Epithelium (fig . 4).—In this variety the cell is flattened, very thin and irregular in outline . It occurs as the covering epithelium of the alveoli of the
See also:lung, of the kidney glomerules and capsule, &c . The surface epithelial cells of a stratified epithelium are also of this type (fig . 4) . Closely resembling these cells are those known as endothelial (see later) . 4 . Ciliated Epithelium (fig . 5).-The surface cells of many epithelial membranes are often provided with a number of very fine proto-FIG .
4 . Squamous plasmic processes FIG . 5.–Isolated Epithelial Cells from the orcilia . Most corn- ciliated Epithelial Mucous Membrane of the or cilia . the cells Cells from the Mouth. monly Trachea. are columnar, but other shapes are also found . During
See also:life the cilia are always in
See also:movement, and set up a current tending to drive fluid or other material on the surface in one direction along the membrane or
See also:tube lined by such epithelium . It is found lining the trachea, bronchi, parts of the nasal cavities and the 706 uterus, oviduct, tubule, &c . In the instance of some cells there may be but a single
See also:process from the exposed surface of the cell, and then the process is usually of large
See also:size and length . It is then known as a flagellum . Such cells are
See also:common among the surface cells of many of the
See also:simple animal organisms . When the cells of an epithelial surface are arranged" several layers deep, we can again distinguish various types:—I . 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
See also: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
See also: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
See also:respiratory passages, the vas deferens and the epididymis . 3 . Transitional Epithelium (fig .
8).—This variety of epithelium is found lining thebladder, 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 Considering epithelium from the point of view of
See also: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 thecells many chemical substances which indicate the nature of their
See also:work, e.g,
See also: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 . 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 Gland& may start as a solid
See also:column of cells which subsequently becomes tubulated . As growth proceeds, the column of cells may
See also: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 sub-division . As a
See also:rule the branches do not unite with one another, but in one instance, the
See also: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
See also: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 . I(a) . 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 . II) . These are termed alveoli . The typical secretory cells of the glands are found lining the vas deferens, epididymis, a portion of the renal ium 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 .
EPITHALAMIUM (Gr. hirl, at or upon, and BhXayos, a ...
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