Epithelial cell

“ In all the epithelial tissues that we have considered we have one feature in common: they are cells, each equivalent to the amoeba, that have taken on special duties-- in a word, they are specialists. The amoeba is Jack of all trades and a free lance; the protective epidermal cell, the current-making ciliated cell, the bile or urea-making secretory cell, is master of one trade, and a soldier in a vast and wonderfully organized host. We will now consider our second kind of cell in this organization, the cell of which the especial aim is the building round it of a tissue. ”

“ The 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 squamous, 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 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. ”

“ Usually indeed the specialization is only rendered evident by the appearance of the formed secretion, e.g. mucus-secreting epithelial cells (fig. 2, b) , the ovarian ovum and the fat cell (fig. 1, a) . In some cases a distinct fibrillation of the cytoplasm accompanies or precedes the appearance of the cell-secretion (Mathews, pancreas cell of Amphibia) . In many cases the internal secretion is no mere accumulation, e.g. ”

“ Chordata. In the Vertebrata, the primitive generative cells (often known as primitive ova) are early distinguishable, being imbedded amongst the cells of two linear streaks of peritoneal epithelium, placed on the dorsal side of the body cavity, one on each side of the mesentery (figs. 405 C and 410, po) . They appear to be derived from the epithelial cells amongst which they lie; and are characterized by containing a large granular nucleus, surrounded by a considerable body of protoplasm. The peritoneal epithelium in which they are placed is known as the germinal epithelium. ”

“ This epithelium consists really of a modification of the upper layer of the skin, and we see this modification familiarly in the difference between the skin of the face and the layer which is infolded to form the covering of the lips and the lining membrane of the mouth. No tissue is more familiar to the student of physiology than epithelium, composed as it is of epithelial cells or microscopic elements, which in one form or another are found in almost every important tissue of the body. ”

“ There are, however, scattered throughout the skin, and also throughout the lining of the digestive tract, a number of specially modified epithelial cells, which are no doubt delicate organs of sense. They are provided at their free extremity with a long hair, and are prolonged on their inner side into fine processes which penetrate into the deeper part of the epithelial layer of the skin or digestive wall. They eventually join a fine network of protoplasmic fibres which forms a special layer immediately within the epithelium. ”

“ An epithelium of one layer is also found on the body of young stages, but it disappears during growth, and only occasionally do its nuclei persist until adult life. In its place we then find the cuticle, which, moreover, extends into all the body openings more or less deeply.
It is thus a debatable point whether the “investing layer” of flukes is a cuticle—that is, consists of modified epithelial cells—or whether it is a basement membrane, i.e., compressed and modified connective tissue cells; in this latter case the true epidermis and cuticle have been cast off. ”

“ 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. ”

“ Cancer remains as an epithelial tumor, representing the atypical growth of cells resembling epidermis or the epithelium of glands and mucous membranes, extending into parts where epithelium is not found as a normal constituent. A new formation of connective tissue is usually associated with that of the epithelial cells. ”

“ The cells are mainly filled with an immense number of highly refracting spherules, probably secretory globules, but held by Grube, from the fact of their dissolving in ether, to be fat. The epithelial cells are raised into numerous blunt processes projecting into the lumen of the stomach. ”

“ Millions of blood corpuscles are continually dying and being replaced by new ones. On both the internal and external surfaces of the body countless epithelial cells are being incessantly removed, while new ones arise in their place; the activity of many and probably of all glands is accompanied by a change in their cells, for their secretions consist partly of detached and partly of dissolved cells ”

“ 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. ”

“ In simple epithelium, where there is only a single layer of cells, the cells may be pavement or hexagonal, columnar, glandular, or ciliated, according to their different functions. The flat pavement cells occur where a very smooth surface is required, as in the heart, lungs, blood-vessels, serous cavities, etc. None of these surfaces communicate directly with the external surface of the body and the name endothelium is substituted for epithelium. The columnar form of cell in the intestine facilitates the passage of leucocytes between the cells. ”

“ The ectoderm rarely consists of more than one layer of cells: these are divisible by structure and function into nervous, muscular and secretory cells, supported by interstitial cells. The endoderm is generally also an epithelium one cell in thickness, the cells being digestive, secretory and sometimes muscular. Reproductive sexual cells may be found in either of these two layers, according to the class and sub-class in question. ”

“ First of all an account is given of the development of the external epiblast, which is followed by a description of the organs derived from the mesoblast and of the notochord.
External Epiblast.
During stages G to I the epiblast [192] is formed of a single layer of flattened cells; and in this, as in the earlier stages, it deserves to be especially noticed that the epiblast is never more than one cell deep, and is therefore incapable of presenting any differentiation into nervous and epidermic layers. ”

“ The ordinary epidermal cells have thick outer walls and thin inner walls. Beneath many of the epidermal cells are large air-spaces.
Upper Palisade Parenchyma. The palisade parenchyma vary from four to five layers. The inner palisade cells are shorter and broader than the outer layer of cells.
Parenchyma. The parenchyma cells (Fig. 4) are rounded in form and they are arranged in the form of columns which are one cell in thickness above, but two to three cells in thickness near the under epidermis. Between each chain of cells is a larger intercellular space (Fig. 6) . ”

“ Each of the common embryonal cells found in the inflamed tissue usually contains two nuclei, the indication of the active increase by division into two, but when the supply of nutriment is checked the nuclei continue to divide, while the cells remain unchanged, and thus every cell comes to contain several nuclei in addition to fatty granules, and constitute pus-corpuscles. ”

“ Moreover, the cells of the prothallium resemble those of the parenchyma of a leaf, the epidermis with its wavy-margined cells being absent.
When the prothallium has attained its full development, minute scattered protrusions from its cells occur on the margin or under surface, resembling short and blunt hairs; and each of these becomes partitioned off to form a new cell, within which a number of crowded smaller cells are produced. These organs are called antherid ́ia (anther, and εἶδος, resemblance) ”

“ The section from which this particular figure was drawn was in the anterior region, but the corresponding part of a section in either of the other regions would have practically the same appearance.
[183]
The epithelium is of the stratified columnar type. The superficial cells are very tall and narrow, with the nuclei generally at or near the bases, though an occasional nucleus may be seen near the free end of a cell. ”

“ Figure 43 represents the epithelium from the anterior region as seen under high magnification. The outlines of all the cells could not be determined [172] but if each nucleus represents a cell there are twenty-five or thirty layers of cells. The nuclei are arranged in two dense, irregular groups, one along the base of the epithelium, the other about two thirds of the distance from the base to the free border. The basal nuclei are perhaps slightly larger and more rounded than those of the distal group. Between these two groups are numerous more scattered nuclei ”

“ The “dotted” vessel, or duct, is a long, open tube whose sides are marked by pores, or dots. A transverse section of a radish will furnish an example. A fern will furnish an example of “reticulated” cell; almost any common plant contains the “annular” and “spiral” cells.
P. 36.—“Epidermis.” All varieties of cells will be found in the epidermis together with the mouths, or stomata. For examination a layer should be carefully removed with a razor or knife, a little water put on it to prevent too rapid drying, and the whole covered by a thin glass. ”

“ As soon as a spermatozoon enters an ovum, it disappears and is completely absorbed, and, as the ovum in turn is instantly possessed of new powers, the result of this union is a cell which was previously non-existent.
This new cell is not only capable of reproduction by means of segmentation or cell division, but in the course of its sub-division and proliferation, it forms groups of cells which develop into tissues and structures widely different from each other. The entire complex human body, in addition to the placenta, cord, and membranes, arises from the single, extraordinary cell. ”

“ The epithelioid constituent of the cancer, on the contrary, grows often with great luxuriance and with but little tendency to carry out the normal mutual relations of the epithelium and connective tissue of the part from which it proceeds. The epithelioid masses or sprouts are composed of cells whose relation to each other resembles that of normal epithelium in the absence of an intercellular substance, while the shapes of the cells correspond more or less closely with that of the epithelium in the region from which the tumor arises. ”

“ 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. ”

“ In the free state the cytoblasts increase rapidly until they attain a certain size, when they are surmounted by a fine diaphanous bubble; this is the young cell, at first a segment of a sphere, its plane side formed by cytoblasts, and its convex side by young cells (the cell-epidermis) similar to a watch-crystal on a watch. Gradually the bubble expands, becomes more consistent, and the wall is composed of cytoblasts and of a gelatinous substance. ”

“ But if it be a white blood-cell, a lymph-corpuscle, or any of those granular cells which resemble them, it will nearly always take on a different appearance, the cells almost vanishing perhaps, and multiple (2-5) nuclei appearing, as in the pus-cell. Then, if the cell be suspected to be fatty, degenerated, or Gluge's cell, ether may be added, by which the fatty materials will be dissolved and disappear. If no fatty degeneration be present, it is sufficient to add acetic acid."23 Garrigues, on the other hand, contends that the ovarian fluid does not contain a characteristic cell.24 ”

“ In the final stages of the segmentation the cells become distinctly divided into two layers. A layer of small cells is placed at the formative pole, and constitutes the epiblast. The cells composing it are divided, like those of Teleostei, etc., into a superficial epidermic and a deeper nervous layer. The remaining cells constitute the primitive hypoblast (the eventual hypoblast and mesoblast) ; they form a great mass of yolk-cells at the lower pole, and also spread along the roof of the segmentation cavity, on the inner side of the epiblast. ”