Centrosome

“ Again, Platner (’89, 1) probably followed the centrosome and attraction sphere, as well as the nebenkern, in the second part of the paper on Helix and Paludina. In the pancreas the body is a result of secretion, and is not a nebenkern. In Pygæra Platner’s large mitosome is the real nebenkern, as shown by its fibrous structure, its surrounding clear space and destiny. His small mitosome is what I have described as the acrosome, and he is mistaken as to its final use. The body he called centrosome in the nucleus is the persisting accessory chromosome. ”

“ The process by which one cell (a mother cell) divides into two cells (daughter cells) is called mitosis, and is inaugurated by the centrosome (Fig. 8, A, at) . The centrosome divides into two centrosomes, which at first remain close together (Fig. 8, B, sp) , and then gradually separate from one another. Each centrosome becomes the center of a system of fine achromatin fibers arranged round it in a radiating manner and forming what is called the attraction sphere ”

“ In 1897 he called the collection of granules around the centrosome, the centrodeutoplasm. He considers them to be identical with St. George’s cytomicrosomes and the archoplasm (or attraction sphere) of other writers. In order to harmonize results, he suggests that, since the centrosome sometimes wanders around the nucleus, the centrodeutoplasm (or sphere) may unite with the spindle remains in some cases to form one body, as shown by the descriptions of Henking, Henneguy, Meves, and others. He later (’97, 2) describes the so-called “sphere,” and distinguishes between it and the true nebenkern. ”

“ It is not yet certain, however, whether the centrosome is a constant part of the cell. In some cells it cannot yet be found, and there are some reasons for believing that it may be formed out of other parts of the cell. The nucleus is always a direct descendant from the nucleus of pre-existing cells, so that there is an absolute continuity of descent between the nucleii of the cells of an individual and those of its antecedents back for numberless generations. ”

“ They are always surrounded by ra r s of kinoplasm, which become very abundant during spindle formation and division and are not surrounded by any differentiated sphere of any sort, as so commonly occurs in the centrosomes of animals. In spindle formation the centrosome appears to be of prime importance, a bundle of fibers starting from each centrosome and gradually extending into the nucleus until the spindle is completely formed. Swingle's studies were made with growing vegetative tips, which would indicate that the centrosome here is probably a permanent organ in all stages of growth. ”

“ Concerning the protoplasm contained in each cell, named by some cytoplasm, it remains to say that it always includes a small body called the centrosome, which appears to have a directive function. Usually the centrosome lies outside the nucleus, but is alleged to be sometimes within it. ”

“ If it is a centrosome, as claimed by some writers, does it function as a centrosome in the segmentation of the eggi It seems to have been established that in many animals the spermatozoid brings in the centrosome which forms the amphiaster for the first division, but this has not yet been proved in the case of any plant. That the blepharoplast or the cilia-forming organ enters the egg and remains in the cytoplasm in Zamla and Cycax is certain and not open to any question. ”

“ The cell-organ by which division is carried out is the centrosome. But in the course of the two maturation divisions of the germ cell, that cell loses its centrosome, which seems to be absorbed into the protoplasmic substance of the cell when once its task is accomplished. No fusion of any number of such cells can therefore lead to any further change or growth, for growth is based on cell division, and the centrosome is the organ of division. The sperm cell, on the other hand, does not lose its centrosome ”

“ It would seem, therefore, as if there were reason to regard the centrosome, like the nucleus, as a permanent constituent of a cell. This view, however, is not universally entertained. If not always capable of demonstration in the resting stage of a cell, it is doubtless to be regarded as potentially present, and ready to assume, along with the radiations, a characteristic appearance when the process of nuclear division is about to begin. ”

“ Centrosomes of almost exactly the form of those of Stypocaulon have been described by Strasburger (111) in Fucuh. A veiy distinct deeply staining centrosome is described, which he believes to be a permanent organ of the cell, reproducing by division at the end of each nuclear division, thus forming two which control the next division. The observations of Farmer and Williams (37) are also of interest in this connection. They describe very marked centrospheres at the poles of the spindle of Fucux, in which an irregular number of granules, possibly representing centrosomes, can beobserved. ”

“ What is the nature of the coercion exercised by the centrosome—a body hardly distinguishable in size from the microsomes or granules of protoplasm around—is not known. It can scarcely be a repelling force ”

“ In the simplest unicellular animals the fluid protoplasm is contained in a membrane, or denser bounding layer, to which are attached fine filaments springing from a minute body known as the centrosome. These centrosomes—for there are sometimes several in a cell—seem to control the mechanical department, just as the nucleus does the chemical. ”

“ Trypanosoma) .28 In almost all cases the centrosome of the resting cell, when this can be detected, lies in the cytoplasm, and is often already divided in preparation for the next mitotic division (e.g. spermatogenic cells of the salamander; Meves) . In some cases, however, it resides in, or arises from, the nucleus (Brauer; spermatogenesis of Ascaris, var. univalens) . This indifferent nuclear or cytoplasmic position for the centrosome is paralleled by the attraction sphere or homologue of the centrosome in many Protozoa. ”

“ The polar and spindle-like radiations visible during the process of karyokinesis, which have already been referred to, and the presence of the centrosome, possibly even during the resting stage of the cell, furnished additional illustrations of differentiation within the cell plasm. In many cells there appears also to be a difference in the character of the cell plasm which immediately surrounds the nucleus and that which lies at and near the periphery of the cell. ”

“ No final conclusion can be reached, furthermore, until our knowledge of the typical centrosome has been extended and systematized so that it is possible to state what constitutes a centrosome. If the writer by his efforts has in any degree aided in paving the way to an earlier understanding of the matter, he is satisfied. ”

“ They form the center of a series of radiations, and do not become visible until the radiations are fairly well developed. The center of the system of radiations was not always occupied by a single granule or centrosome; often several distinct granules were visible, forming a microcentrum in the lleidenhain sense. The centrospheres disappear at the close of the division, and before each division are apparently originated de novo in the cytoplasm of the cell in close relation to the nuclei. ”

“ In some cells, probably in all, the centrosome presides over division. Cells, however, do not always divide in the same way. Some simply lengthen, the nucleus also lengthening inside, become constricted in the middle like a dumb-bell, and separate. (See Diagram 15.)
Diagram 16.—Cell Division.
Others manage differently. In them the nucleus simply bursts, and turns its essential elements, a number—always a constant number—of coarse threads, adrift. Meanwhile, two centrosomes have moved to opposite ends of the cell, and there anchored themselves by fibrils ”

“ This centrosome, when the time comes for the cell-division, itself divides in two, and the two starlike centres so formed (which are to become centrosomes of the two new cells) , slowly move to opposite ends or poles of the original cell—all the time, as they do so, throwing out raylike threads or fibrils which connect them somehow with the chromosomes and which seem to regulate the movements of the latter, till, as described, 14 the latter form themselves in a line across the centre of the cell, transversely to the line joining the poles. ”

“ Those who maintain their autogenous character lay great stress upon the polarity of cells, and the relation of the centrosome to the nucleus, indicating a change in the polar axis in cells about to be cancer-genetic, as does Ewing in his recent classical study of pre-cancerous lesions. But whatever changes are observed microscopically we must recognize that the cells themselves must be influenced ultimately by that mysterious force which we call life, which ends with its extinction 35from the body as a whole, and which is ultimately related to nerve action. ”

“ In reports upon Pygæra and Sphinx he changed the name nebenkern to mitosome. The centrosome lies in front of the nucleus and forms the point of the head. This he calls the nebenkern. From the spindle remains arise two bodies. A large, fibrous one from the equatorial end has a clear space around it and the axial filament passes through it. It soon elongates and forms the tail. This is the large mitosome. The other is much smaller and arises from the polar end of the spindle fibers. It takes its place in the angle between the large mitosome and the nucleus. ”

“ According to Boveri, the ovum has all the organs and qualities necessary for the development of the foetus, except that its centrosome, which starts segmentation, is in a state of inactivity, while the spermatozoön possesses the active centrosome but lacks the protoplasma or the material by means of which this organ could begin its activity.
AA Among animals, says Walker, there are species that never marry and those that do. ”

“ These spaces collect and unite into larger vacuoles, which surround the darker center. Finally there is only a dark ball with a surrounding clear space shut in by a ring. The centrosome with attached axial filament fastens itself to the nucleus; then both begin to grow longer. The axial filament passes over the surface of the ring surrounding the ball—the “mitochondrion körper” or nebenkern. This body elongates a great deal then the darker mass puts forth numerous threads which surround the axial filament. ”

“ In addition to the chromatin there may be present in the nucleus one or more, usually spherical, and as yet somewhat enigmatical bodies, the “nucleoli.” In addition to the nucleus and cytoplasm, a third body, the “centrosome,” has often been considered as a constant cell-structure. It is a minute granule, usually lying in the cytoplasm not far from the nucleus, and plays an important part in cell-division and fertilization (see below) .
Cell-differentiation.—Both among unicellular and multicellular individuals the cell assumes the most varied forms and performs the most diverse functions. ”

“ The centrosome now divides, the membranes of both nuclei disappear, and a spindle is formed. The twenty-four chromosomes arrange themselves at the centre of this spindle and split longitudinally, so that forty-eight chromosomes are formed. Twenty-four of these, twelve male and twelve female, as it is supposed, travel to each pole of the spindle and assist in giving rise to the two nuclei. At the next nuclear division twenty-four chromosomes appear in each nucleus, each of which divides longitudinally ”

“ The reorganizing daughter nuclei, in the only section secured in this stage, are too densely stained to show their structure well; they appear simply to be densely granular. The spindle fibers show very plainly, but do not as yet show any thickenings in the center preparatory to the formation of a cell membrane. A most careful search has failed to reveal any suggestions of a centrosphere or centrosome at the apex of the spindle where one might be expected to occur. ”

“ The former, for instance, contains a rounded nucleolus or “germinal spot,” certain conspicuous granules or strands of the peculiar substance called chromatin, and a coarse meshwork of a protoplasmic material known as “linin” or achromatin; the outer protoplasm, or cytoplasm, is generally believed to consist throughout of a sponge-work, or rather alveolar meshwork, of more and less fluid substances; and lastly, there are generally to be detected one or more very minute bodies, usually in the cytoplasm, sometimes within the nucleus, known as the centrosome or centrosomes. ”

“ As observed by Morgan, these eggs often become filled with large numbers of asters, each of which contain a centrosome. One of the most interesting results of Wilson's work is the discovery that these asters may multiply by division and form centers of cytoplasmic division, even when they have no connection with nuclear material. ”

“ Hirase, in his study of Ginkgo and the attractive spheres formed in the spermatogenous cells, also concludes that they are to be considered as centrosomes, though, as shown by his figure 18 (61, pi. 8) , they remain distinct from the spindle, the radiations around the sphere not connecting with the radiations around the pole of the spindle in the center of which a centrosome should be located, if present. Hirase says, furthermore:
The attractive spheres which we have just described are different from those made known by many scientists heretofore. ”