Epithelium

“ 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 epithelium of the epidermis is stratified-- that is to say, has many thicknesses of cells; the deeper layers are alive and dividing (stratum mucosum) , while the more superficial are increasingly flattened and drier as the surface is approached (stratum corneum) and are continually being rubbed off and replaced from below.
Section 59. In the branching air-tubes of the lung, the central canal of the spinal cord, and in the ureters of the rabbit, and in most other types, in various organs, we find ciliated epithelium (Figure VII.) . ”

“ It presents two surfaces, which are respectively outer and inner, and is formed of a layer of somewhat thickened germinal epithelium separated by a basement membrane from a central core of stroma. The epithelium is far thicker on the outer surface than on the inner, and the primitive ova are entirely confined to the former. The cells of the germinal epithelium are irregularly scattered around the primitive ova, and have not the definite arrangement usually characteristic of epithelial cells. Each of them has a large nucleus, with a deeply staining small nucleolus, and a very scanty protoplasm. ”

“ According to him, what we call the selective power of the intestinal epithelium is simply the expression of the fact that there exist in those cells substances which have a greater affinity' for certain constituents of the intestinal contents than for others, just as plates of gelatine do not take up the same quantities of different salts and other compounds from solutions containing them. ”

“ At the same time it becomes differentiated into two layers, viz. an outer epithelioid layer which gives rise to part of the peritoneal epithelium, and an inner layer of undifferentiated cells which in time becomes converted into the connective tissue and muscular walls of the mesenteron. The connective tissue layers become first formed, while of the muscular layers the circular is the first to make its appearance. ”

“ 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. ”

“ The granular deeply staining cytoplasm, the clear non-staining nucleus with its bright nucleolus and the nucleolus-like yolk nucleus, all show beyond doubt that these freely floating cells originate in the ovary.
In some of my preparations these cells are found not only floating free, but wandering through the tissues. Fig. 70 shows two such wandering cells fixed just as they were making their way either through the digestive epithelium into the gelatine of the floor of the stomach, or from the gelatine into the epithelium. ”

“ The epithelium is made up of nucleated cells, which are particles of simple living matter, or protoplasm, in the midst of each of which is a rounded body, which is termed the nucleus. It is these cells which are the seat of the manufacturing process which results in the formation of the secretion ”

“ 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.
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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. ”

“ The layer supporting the epithelium is reduced to a basement membrane. The epithelial part of the wall of the stomach is by far the thickest (fig. 20) , and is mainly composed of enormously elongated, fibre-like cells, which in the middle part of the stomach, where they are longest, are nearly half a millimètre in length, and only about .006 mm. in breadth. Their nuclei, as seen in fig. ”

“ 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. ”

“ Eventually the component cells form the limiting epithelium of a series of follicles or vesicles embedded in a matrix of connective tissue and blood-vessels, and the characteristic adult structure is attained. ”

“ These are called villi; they are not unlike the papillæ of the skin (Fig. 15) , if you suppose all the epidermis stripped except the bottom row of cells (d) , and the papilla itself pulled out a good deal. Fig. 18 is a sketch to illustrate the structure of a villus. The epithelium (b) , you see, is made up of a single row of cells. Beneath the epithelium, just as in the papilla of the skin, is a network of blood capillaries, shown, for convenience, in the right-hand villus only. ”

“ It causes the epithelium to contract and to become so disintegrated as to be utterly incapable of performing its functions until such time as the underlying tissue shall have created new cells to take the place of those which have been destroyed. ”

“ If a small portion of the inner coat of an artery be gently scraped with a knife, or if the inside of the cheek be treated in a similar manner, a little white soft substance is brought away on it, called epithelium, a name given to it by Ruysch, from the delicate layer of epidermis investing the female nipple, έπι, upon, θηλή, a nipple. The epithelium of the human body is divided into three kinds by microscopists—the tesselated, pavement, or scaly ”

“ Besides these most individualized cells, so characteristic of every organ that a cytologist can at once distinguish cells that form the epithelium, intestines, brain, muscle, glands, etc., there always remain other far less differentiated or more primitive cells, chiefly leucocytes or white blood corpuscles and the connective tissue cells. ”

“ 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. ”

“ Most commonly the cells are columnar, but other shapes are also found. During life the cilia are always in movement, and set up a current tending to drive fluid or other material on the surface in one direction along the membrane or tube lined by such epithelium. ”

“ The mesenteron now rapidly extends by the invagination of the cells on its dorsal side. These cells grow inwards towards the segmentation cavity as a layer of cells several rows deep. At its inner end, this layer is continuous with the yolk-cells; and is divided into two strata (fig. 71 A) , viz. (1) a stratum of several rows of cells adjoining the epiblast, which becomes the mesoblast (m) , and (2) a stratum of a single row of more columnar cells lining the cavity of the mesenteron, which forms the hypoblast (hy) . ”

“ These two layers are the primary germinal layers. A third layer is subsequently formed between them, by the agency of one or both of them, and is called the mesoblast. From these three simple membranes, which are composed exclusively of cells, are formed all the complex tissues and organs of the adult man. The epiblast develops into the nervous system (brain, spinal cord, and nerves) , and the cuticle, hair, etc. The hypoblast develops into the cellular parts of the digestive canal, the liver, lungs, etc. The mesoblast develops into the muscles, bones, ligaments, blood-vessels, etc. ”

“ Beneath the upper epidermis, the cells of which are fitted closely together without intercellular spaces and are devoid of chlorophyll corpuscles, there are one or two rows of vertical sausage-shaped cells, closely arranged like the wooden railings of a complete palisade—consequently they are termed the palisade cells. The lower moiety of the mesophyll, on the other hand, is composed of irregular cells with large intercellular spaces between them, and this loose, spongy tissue, as it is aptly called, abuts below on the lower epidermis. ”

“ The matter which forms the mass of a cell is called protoplasm, or bioplasm. It resembles somewhat the white of a raw egg, which is almost pure albumen. Cells make up the body, and do its work. Some are employed to construct the skeleton, others are used to form the organs which move the body; liver-cells secrete bile, and the cells in the kidneys separate poisonous matters from the blood in order that they may be expelled from the system. ”

“ They consist mostly of high, columnar epithelial cells surrounding a few [26] central cells and showing here and there a nettle cell just as the reproductive organ does. The epithelial cells differ from those of the ovary in containing one or more large vacuoles, and this vacuolation increases as the embryos, among which the masses float, develop. The idea naturally suggests itself, therefore, that they serve for nourishing and perhaps for protecting the embryos while they are developing in the stomach pockets of the mother individual. ”