Cell growth

“ This growth is effected by the multiplication of the constituent cells. The cells themselves have a very limited power of growth. But, especially in the early stages of [28] the life of the organism, when well supplied with nutriment, the cells multiply rapidly, by a process of fission, or the division of each cell into two daughter cells. The first part of the cell to divide is the nucleus, the protoplasmic network of which shows, during the process, curious and interesting arrangements and groupings of the fibres. ”

“ The cell is the unit of growth. It is so with all forms of life—plant or animal, insect or bacterium. In the beginning the start is with a single cell, an egg, if you please. After fertilization has taken place, this single cell enlarges or grows. Many changes now occur, all rather rapidly, until the cell walls become too small, when it breaks apart and forms two cells just like the first used to be. This is known as cell division. As growth increases, the number of cells increases also—until in the end there are millions. ”

“ Gradually, through the formation of new cells and by the growth of these cells after they have been formed, the body attains its full size. When growth is complete, cell reproduction is supposed to cease except where the tissues are injured, as in the breaking of a bone, or where cells, like those at the surface of the skin, are subject to wear. Then new material continues to be added to the protoplasm throughout life, but in amount only sufficient to replace that lost from the protoplasm as waste. ”

“ The ova of animals, the spores of plants, the isolated cells of lower organisms in general, all show the power of separate assimilation and development. "We must therefore, in general, ascribe to the cell an individual life, that is to say, the combination of the molecules in the single cell does suffice to produce the force whereby the cell is enabled to draw to itself new molecules. The ground of nutrition and growth lies not in the organism as a whole, but in the separate elementary parts, the cells. ”

“ The division of the cell-protoplasm is, indeed, a most essential part of the life-process, and to it growth and differentiation of tissue are principally due. The cell, when furnished with nutriment, manifests individual growth for a short period. ”

“ I should propose to include under this term every cell, cytode, or group of cells which, while not possessing the structure of the mature individual of the species, possesses the power of developing into it under certain circumstances. The emphasis is now laid upon the expression development, which is something opposed to simple growth, without change of form. A cell which becomes a complete individual by growth alone is not a germ but an individual, although a very small one. ”

“ In other words, as these growing regions consist of cells, the growth of the entire organ or plant will depend upon the behaviour of the cells or protoplasts of which the merismatic tissues are composed.
The growth of such a cell will be found to depend mainly upon five conditions: (1) There must be a supply of nutritive or plastic materials, at the expense of which the increase of its living substance can take place and which supply the needed potential energy. ”

“ It is however of great importance to observe that the living cell is one of those cases where the phenomena of surface tension are by no means limited to the outer surface; for within the heterogeneous substance of the cell, between the protoplasm and its nuclear and other contents, and in the alveolar network of the cytoplasm itself (so far as that “alveolar structure” is actually present in life) , we have a multitude of interior surfaces ”

“ Figure 6 Stages of the mitotic cycle in a hypothetical cell with four chromosomes.
Mitosis
One of the most remarkable characteristics of cells is their ability to grow and divide. New cells come from preexisting cells. When a cell reaches a certain stage in its life, it divides into two parts. These parts, after another period of growth, can in turn divide. In this way plants and animals grow to their normal size and injured tissues are repaired. Cell division occurs when some of the contents of the cell have been doubled by replication, or copying (to be discussed later) . ”

“ As in the general growth and the vegetative reproduction of plants cell-division is the chief method of cell formation, so in the reproduction of plants by special cells the great feature is the part played by cells which are produced not by the ordinary method of cell division, but by one or the other processes of cell formation, namely, free-cell formation or rejuvenescence. ”

“ Hofmeister and Sachs have taught us that in the plant the growth of the mass, the growth of the organ, is the primary fact, that “cell formation is a phenomenon very general in organic life, but still only of secondary significance.” “Comparative embryology” says Whitman, “reminds us at every turn that the organism dominates cell-formation, using for the same purpose one, several, or many cells, massing its material and directing its movements and shaping its organs, as if cells did not exist261.” ”

“ As the cell increases in size as a result of growth, it elongates in one direction, and finally a new cell wall is formed, dividing the so-called mother-cell into two, equal-sized daughter-cells. This process of cell division, known as fission, is continued until growth ceases and is especially characteristic of bacteria. ”

“ After birth decline in the growth power continues, but gradually the decline becomes slower and slower, so that though growth is slight in rate, the growth power is long continued. A study of the condition of cells while this decline of the growth power is going on reveals to us that while the growth power is rapid the nucleus of the cell is active and well developed, and that the protoplasm of the cell is but slightly developed. ”

“ Cell-reproduction is, however, in the metazoa of two kinds. There is the direct reproduction of differentiated cells, by which muscle-cells, nerve-cells, or others reproduce their kind in the growth of tissues or organs; and there is the developmental reproduction, by which the germinal cells under appropriate conditions reproduce an organism similar to the parent. ”

“ Among the lower animals, such as the hydroids and sponges, the process is still more simple, and cells may become converted into a bud at almost any point upon the body of the parent. That the process of reproduction by budding is not in any way absolutely distinguished from the process of ordinary growth by cell-multiplication, is shown by the fact that an accident may determine which of these processes is to result from the activity of a given cell. ”

“ On the contrary, it is supposed that both formative and formed material are one in kind—that [183] all the cellular tissues of a multicellular organism, like the single cell of a unicellular organism, are per se endowed with the vital property of self-multiplication; and that whether this property finds its expression in normal growth, in abnormal increments of growth (such as tumours) , in processes of repair, in the various forms of a-sexual reproduction, or in the more specialized form of sexual fertilization, there is everywhere an exhibition of one and the same capacity. ”

“ And yet, in spite of this fact, there are few fields of scientific work possessing more general interest than that of cytology. At the present day the greatest problems of biology are centered in the cell. Assimilation, growth, metabolism, reproduction, differentiation, inheritance and variation—these are at bottom cellular phenomena, the result of the structure and functions of cells. ”

“ It is a fact well known to physiologists that every part of the living body is made up of cellular elements which have the power to reproduce themselves in the individual, thus repairing the damage resulting from waste and injury. Each cell produces cells like itself. It is further known that there are found in the body numerous central points of growth. In every group of cells is found a central cell from which the others originated, and which determines the form of their growth. Every minute structure possesses such a center. ”

“ In a similar way we must be careful, in our wonder at the marvellous rapidity of cell-division and growth of bacteria, that we do not exaggerate the significance of the phenomenon. It takes any ordinary rodlet 30-40 minutes to double its length and divide into two equal daughter cells when growth is at its best ”

“ Facts which hear on the First Step, viz., Derivation of NonSexual Modes from Ordinary Processes of Growth.—The transition between the lowest form of non-sexual reproduction, viz., fission, and ordinary growth, is so obvious that it is hardly necessary to insist on it. A single cell divides itself into two; each half grows, and again divides itself into two, and so on. Now, if the cells cohere, we call it growth; if they separate, we call it reproduction. Again: a mass of cells grows by continued cell-multiplication, as above. ”

“ The fact that the germ-cell develops into a very complex structure, is no absolute proof that the cell itself is structurally a very complicated mechanism: nor yet, though this is somewhat less obvious, is it sufficient to prove that the forces at work, or latent, within it are especially numerous and complex. ”

“ When these conditions are present, the course of the growth of a cell appears to be the following: The young cell, immediately it is cut off from its fellow, absorbs water, in consequence of the presence in it of osmotically active substances. ”

“ Outwards through the protoplasm, and stretching far beyond the spherical surface of the cell, there run stiff linear threads of modified or differentiated protoplasm, replaced or reinforced in some cases by delicate siliceous needles. In either case we know little or nothing about the forces which lead to their production, and we do not hide our ignorance when we ascribe their development to a “radial polarisation” of the cell. ”

“ 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. ”

“ It is daily becoming more apparent that epigenesis with the three layers of the germ furnishes no explanation of developmental phenomena." [11]
In accordance with the general principle of a division of labor, certain cells become early set apart to particular functions, and in accordance with the varying demands of these functions the developing cells may become greatly changed in form and in vital characteristics. That is, one cell specializes, let us say, in secretion, another in contractility, another in receiving and carrying stimuli, etc. ”

“ Speculation aside, however, we may say that the two functions of the chromatin do not exclude one another, but that the general activity which originates from it may be but a lower phase of that special activity caused by fertilization. [2]
§ 74g. Here we come unawares to the remaining topic embraced under the title Cell-Life and Cell-Multiplication. We pass naturally from asexual multiplication of cells to sexual multiplication—from cell-reproduction to cell-generation. The phenomena are so numerous and so varied that a large part of them must be passed over. ”

“ 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. ”

“ This tends to increase the amount of the protoplasm, thereby causing the cells to increase in size. A general increase in the size of the cells has the effect of increasing the size of the entire body, and this is one way by which they cause it to grow. There is, however, a fixed limit, varying with different cells, to the size which they attain, and this is quite low. (The largest cells are scarcely visible to the naked eye.) Any marked increase in the size of the body must, therefore, be brought about by other means. Such a means is found in the formation of new cells, or cell reproduction. ”

“ In the second, life reaches its greatest development. The body of man, which represents the highest type of life, is recognized as a group of cells. In this group each cell is usually separate and distinct from the others, but is attached to them, and is held in place by the intercellular material.
Protoplasm, the Cell Substance.—The cell is properly regarded as an organized bit of a peculiar material, called protoplasm. This is a semi-liquid and somewhat granular substance which resembles in appearance the white of a raw egg. ”

“ In conclusion, we find that the life of a cell is dependent upon the continued interaction of nucleus and protoplasm; that as the protoplasm is transformed into products of differentiation this interaction of nucleus and protoplasm is reduced and constructive metabolism is diminished; that when the quantity of protoplasm present has been reduced beyond a certain point, either by its transformation into metaplasm, or by other means, constructive processes fail to compensate for destructive ones, and the cell grows old and finally dies. ”