Somatic cell

“ There are distinct and specific lines of cell-lineage in ontogenesis, so the theory states, along which the continuity of germ-protoplasm is kept up, which, in other words, lead from one egg to the other, whilst almost all other lines of cell-lineage end in “somatic” cells, which are doomed to death. What has been stated here is a fact in many cases of descriptive embryology, though it can hardly be said to be more than that. ”

“ Briefly, these views were that in the nucleus of the protozoan cell there are two antagonistic elements—the one somatic, the other reproductive—and that the predominance of either is the characteristic and determinant of differing sex. In a male cell, the somatic or blepharoplastic element is developed at the expense of the female, which atrophies. ”

“ From the intermediate cell-mass a solid knob grows outwards towards the epiblast (woodcut, fig. 4, pd) . This knob consists at first of 20-30 cells, which agree in character with the neighbouring cells of the intermediate cell-mass, and are at this period rounded. It is mainly, if not entirely, derived from the somatic layer of the mesoblast.
From this knob there grows backwards a solid rod of cells which keeps in very close contact with the epiblast, and rapidly diminishes in size towards its posterior extremity. Its hindermost part consists in section of at most one or two cells. ”

“ If we compare the bodies of the higher Metazoa with those of the lower, we see at once that not only has the structure of the body increased in size and complexity as far as the soma is concerned, but we also see that another factor has been introduced, which exercises a most important influence in lengthening the duration of life. This is the replacement of cells by multiplication. Somatic cells have acquired (at any rate in most tissues) the power of multiplying, after the body is completely developed from the reproductive cells. ”

“ But now what about the other term of the antithesis—the alleged inherent mortality of the somatic cells? This we shall, I think, find is no more defensible than the other. Such plausibility as it possesses disappears when, instead of contemplating the vast assemblage of familiar cases which animals present, we contemplate certain less familiar and unfamiliar cases. By these we are shown that the usual ending of multiplication among somatic cells is due not to an intrinsic cause, but to extrinsic causes. ”

“ It has long been known, as we have seen, that unicellular organisms could go on dividing indefinitely and that germ plasm had a potential mundane immortality; but no one had suspected that highly organized and differentiated288 somatic cells, which had lost the power of producing the whole individual and could only produce cells of their own special tissue, had this power. Recent experiments, however, indicate that under certain highly elaborated conditions they, too, can be made to live and even grow indefinitely and that this growth can not only be observed but measured under the microscope. ”

“ The cells which have undergone histological differentiation can increase by fission, and thus supply the place of those which are being continually destroyed in the course of metabolism. The difference between the higher and lower Metazoa in this respect lies in the fact that there is only one generation of somatic cells in the latter, and these are used up in the process of metabolism at almost the same time that the reproductive cells are extruded, while among the former there are successive generations of somatic cells. ”

“ In some species, as early as the first few divisions certain cells are set apart as germ cells, devoted exclusively to the purpose of reproduction. From these ova and spermatozoa arise. While others, far more in number and aggregate bulk and increasingly so as we ascend the scale of life, become more and more specialized for the production of different organs, structures, and tissues. These gradually lose the power to produce entire individuals. It is these that produce, and their descendants that constitute, all the rest of the body, or soma, and so are called the somatic cells. ”

“ Between somatic cells and reproductive cells Weismann places a gulf that cannot be bridged. Only the reproductive cells contain real germplasm, and only these contain the conditions for maintaining the species, as they alone serve for the starting of new generations of development. The somatic cells, on the other hand, are endowed only with fragments of germplasm, and hence they are incapable of preserving the species, and are doomed to death. The reproductive cells, like unicellular organisms, are regarded as immortal, the somatic cells as mortal. ”

“ These cells must represent the whole parent, they must be capable of shedding half their chromatin, and, when they have fused, must be capable of growing into a complete organism like the parent. When these specialized cells have been formed, the others, the somatic cells, will at the same time have been specialized for other functions, and will thus naturally lose the original capacity for interchanging chromatin with other cells, i.e. ”

“ If this became true of the whole mass of somatic cells, we should then meet with natural death for the first time. Whether we ought to regard this limitation of the life of the specific somatic cells as a mere consequence of their differentiation, or at the same time as a consequence of the powers of natural selection especially directed to such an end,—appears doubtful. ”

“ By the time the chick is ready to be hatched there are millions.
Usually, however, a cell is small—just as much protoplasm as its still more minute nucleus can keep going; though here, again, one must be guarded, for there may be several nuclei instead of only one. The protoplasm on the external surface and around the nucleus is specialized into a more or less definite membrane. To this outer envelope are attached fine fibrils, which join up to a small body within the cell, called the centrosome, and by the lengthening and shortening of these its shape can be altered. ”

“ From the adjacent cells distinct outgrowths are being emitted, where the surface tension has been weakened: just as the glass-blower warms and softens a small part of his tube to blow out the softened area into a bubble or diverticulum; and in our Mesocarpus cells (besides a certain amount of potassium rendered visible over the boundary which {290} separates the green protoplasm from the cell-sap) , there is a very large accumulation precisely at the point where the tension of the originally cylindrical cell is weakening to produce the bulge. ”

“ The spindle fibers have bulged out on each side of the old mother-cell nucleus, reaching the cell wall on each side. They have contracted away from the daughter nuclei, and seem at their outer ends to be in the process of gradual transformation into the normal reticulum of the cytoplasm. No visible thickening occurs on the fibers where the new plasma membrane is to be formed, but a space free of granules and stainable matter clearly shows where the new plasma membrane delimiting the two cells will form. This is particularly well shown in a photomicrograph of the cell shown in figure 63. ”

“ So long as that material for the secretion is contained in the cells, the cells appear large, and their protoplasmic bodies do not readily absorb certain of the staining matters, which the microscopist is likely to apply to them. When, however, the accumulated raw material has been changed into the secretion and discharged from the gland, the cell is correspondingly reduced in bulk, and as you see in this figure, it then takes up the stain with considerable avidity, as does also the nucleus which has likewise become reduced in size. ”

“ With regard to the former, we have seen that in the single cells, constituting unicellular organisms, there is always a bounding membrane of denser texture than the rest of the protoplasm. As the cell develops its capabilities, we have a shell or case of non-living matter secreted around it, with apertures for communication with the outside world, and increasingly effective protection is provided as protoplasm, whether in the single cell or the body, leaves the water, and has to face the inclemencies of terrestrial life. ”

“ Uskoff has in particular published experimental work in this province of hæmatology. This has led him to see in the white blood corpuscles the developmental series of one kind of cell, and to distinguish in it, three stages: (1) "young cells," which correspond to our lymphocytes; (2) "ripe cells" (globules mûrs) , large cells with fairly large and irregularly shaped nucleus, which are therefore our large mononuclear and transitional forms; (3) "old cells" (globules vieux) , which represent our polynuclear cells. The eosinophil cells are completely excluded from this classification. ”

“ We have seen how it is always changing, always taking in food, always giving off waste materials. We have seen, too, that it grows and that it does work, and that in a large mass the cells which compose it share the labour instead of each component cell performing all the vital functions. We have now to consider the work which protoplasm does—in a word, the mechanical effect of the chemical actions just described.
The simplest movement of protoplasm is to be seen by the aid of the microscope in certain vegetable cells, where granules seem always streaming about in different directions. ”

“ Until, however, further evidence is forthcoming in support of this syncytial theory of structure, it would be unwise to regard it as established sufficiently to constitute a serviceable working hypothesis; hence, for the time being, we must accept the assertion that the cell represents the ultimate tissue-unit. Our present day definition of a cell is a minute portion of living organized substance or protoplasm. ”

“ Wright's stain the nucleus is less deeply colored than that of the lymphocyte, while the cytoplasm is very pale blue or colorless, and sometimes contains a few reddish granules. The size of the cell, the width of the zone of cytoplasm, and the depth of color of the nucleus are the points to be considered in distinguishing between large mononuclears and lymphocytes. When large forms of the lymphocyte are present, the distinction is often difficult or impossible. It is then advisable to count the two cells together as lymphocytes. ”

“ As a matter of fact, it is not necessary for single cells to have this great length, because it is possible for them to be arranged end to end, making a path of living protoplasm consisting not of one cell but of a chain of them. If we look at a nerve cell under the microscope we see that it is made up of a little central mass of protoplasm to which has been given the name of “cell body.” From this cell body extends a tiny thread of living protoplasm. ”

“ 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. ”

“ The siliceous cell-wall is covered on the outside by a layer of protoplasm called the coleoderm. This layer may be quite thin and evident only when treated with fuchsin or Bismarck brown, or it may be of considerable thickness. The cell contains the cytoplasma, protoplasm, cell-sap, endochrome, pyrenoids, oil globules and nucleus, together with certain other less understood bodies. ”

“ In some cases the protoplasm at the circumference is so modified as to give the appearance of a limiting membrane called the cell wall. In brief, then, a cell is a mass of nucleated protoplasm; the nucleus may have a nucleolus, and the cell may be limited by a cell wall. Every tissue of the human body is formed through the agency of protoplasmic cells, although in most cases the changes they undergo are so great that little evidence remains of their existence. ”

“ This indicates the presence of a substance or substances that take stain more readily than the mass of the protoplasm. This peculiarity led to the naming of the deeply staining portion of the protoplasm chromatin, referring to the ease of staining. The part of the cell which contains chromatin is called the nucleus. In many kinds of cells the nucleus can be made out by an expert observer without resorting to stains, although the details of structure cannot be seen in that way. ”

“ In the different propositions I have quoted, we [Pg 36] have to deal with what is merely a fallacy in rhetorical disguise. For, from the premiss that the chromatin has the power of impressing specific character upon the protoplasm of the cell, it by no means follows that two cells, distinguishable by the nature of their plasma-products, must therefore contain different kinds of protoplasm. There are other possibilities to be reckoned with. ”

“ Instead of the latter, we find in these cells a roundish, nucleus-like form in the protoplasm, which also takes the nuclear stains, and possibly is to be considered an accessory nucleus. We have received the impression that we have here to deal with a vacuole filled with substance secreted by the cell. In a large series of preparations, it is possible to obtain some elucidation of the development and fate of these appearances. They first appear as point-like granules in the protoplasm, bearing no relation to the cell nucleus; they gradually increase, and acquire a considerable circumference. ”