Biological regeneration

“ There must, then, be contained in the cells of the bud the germinal rudiments of the whole animal; this conclusion is more necessary as the individuals, produced from the buds, in due course bear sexual products.
Although in many higher animals and plants one sees that cells with the capacity for reproduction are limited to special areas, still, the capacity for regeneration often is very great. In a wonderful fashion animals will reproduce lost parts, sometimes of most complicated structure ”

“ To recall only one point, we have not said that regeneration, merely because it is a kind of restitution of the disturbed whole, compels us to admit that biological events happen in a specific and elemental manner, but, indeed, regeneration does prove vitalism, because it is founded upon the existence of certain complex-equipotential systems, the analysis of the genesis of which leads to the understanding of life-autonomy. ”

“ In these normal cases the regeneration depends on the existence of special formative layers or groups of cells, and must be regarded in each case as a special adaptation, with individual limitations and peculiarities, rather than as a mere exhibition of the fundamental power of growth and reproduction displayed by living substance. Many tissues, even in the highest animals, are capable of replacing an abnormal loss of substance. ”

“ The word "regeneration" has therefore in biology "come to mean, in general usage, not only the replacement of a lost part, but also the development of a new whole organism, or even a part of an organism, from a piece of an adult, or of an embryo, or of an egg." ”

“ With the growth of the internal organs there probably goes on a series of gradual regenerative processes, and Korschelt and Heider state that we may venture to assume that each changed cell or group of cells which have become exhausted by the exercise of the functions of life are reabsorbed and become restored through the vital powers of the tissues, so that as the result there goes on a constant, gradual regeneration of the organs. ”

“ We cannot therefore avoid referring the processes of regeneration to particular 'regeneration-determinants,' which are contained in the germ-plasm and are handed on in ontogeny with the other determinants from cell-division to cell-division, till ultimately they reach the cells which are to respond, or may have to respond, to the stimulus of injury by some expression of their regenerative capacity. ”

“ It is much less common in the tissues of higher plants, in which the adult cells have usually lost the power of reproduction, and in which the regeneration of lost parts is replaced by a very extended capacity for budding. ”

“ But if in each organism the physiological units which result from the compounding of highly compound molecules have a more or less distinctive character, the germ-cell is not so very indefinite after all.
Many of the facts of regeneration are very striking. A crab may regrow its complex claw, a starfish arm may regrow an entire body. ”

“ If we took a narrower point of view, and if we only considered regeneration proper from the wound itself, we might probably at first be inclined to advocate the doctrine that the removing of some obstacles might in fact be the stimulus to the process of restoration; but, even then, why is it that just what is wanted grows out? Why is there not only growth, but specific growth, growth followed by specification? ”

“ My own view is that the regenerative capacity is not something primary, but rather an adaptation to the organism's susceptibility to injury, that is, a power which occurs in organisms in varying degrees, proportionate to the degree and frequency of their liability to injury. Regeneration prevents the injured animal from perishing, [Pg 8] or from living on in a mutilated state, and in this lies an advantage for the maintenance of the species, which is the greater the more frequently injuries occur in the species, and the more they menace its life directly or indirectly. ”

“ Thus there is a normal physiological regeneration, which is, so to speak, the prolongation of the ontogenesis—i.e., of the work of formation of the individual. We have examples in the reconstitution of the skin of mammals—in the throwing off of the epidermic products constantly used up in their superficial and distal parts and regenerated in their deeply-seated parts ”

“ It appears that, in the same fashion as more simply organized animals display a capacity for reproduction of lost parts greater than that of higher animals, so embryos and embryonic structures generally have a higher power of renewal than that displayed by the corresponding adult organs or organisms. ”

“ It is a great merit of the modern tendency in evolution theory that it has demonstrated the importance of such internal influences. Although we are still far from being able to define the manner in which these influences operate, we may say so much, that it depends essentially on the nature and extent of the loss which parts are reproduced by the regenerating cells, and, also, on the position and direction of the injured surface from which the regeneration starts. ”

“ And in truth, if the body were really able to replace, after artificial injury, parts which are never liable to injury in natural conditions, and to do so in a most beautiful and appropriate manner, then there would be nothing for it but at least to regard the faculty of regeneration as a primary power of living creatures, and to think of the organism as like a crystal, which invariably completes itself if it be damaged in any part. ”

“ If we consider the introduction of natural death in connection with the foregoing statements, we may imagine the process as taking place in such a way that,—with the differentiation of Heteroplastids from Homoplastids, and the appearance of division of labour among the homogeneous cell-colonies,—natural selection not only operated upon the physiological peculiarities of feeding, moving, feeling, or reproduction, but also upon the duration of the life of single cells. ”

“ The phenomenon of regeneration, or the restoration of lost or amputated parts, is a particular case of growth which deserves separate consideration. As we are all aware, this property is manifested in a high degree among invertebrates and many cold-blooded vertebrates, diminishing as we ascend the scale, until at length, in the warm-blooded animals, it lessens down to no more than that vis medicatrix which heals a wound. ”

“ From the above theory it follows that an explanation can be given of the healing of wounds (as in ourselves) , of the regeneration of lost parts (as the limb of a newt) , or even of the reproduction of an entire organism from a mere fragment of somatic-tissue [35] (as in the cases already alluded to at the commencement of this chapter—viz. the leaf of Begonia, portions of sea-anemones, jelly-fish, &c.) ”

“ The very diverse potency of the faculty of regeneration in animals belonging to the same small group, and nearly, if not quite, at the same level of organization, seems to show clearly that we have here to do with adaptation to different conditions of life, although we cannot demonstrate this in detail. ”

“ To say nothing of phagocytes, which push independence to the point of attacking the organism that nourishes them, or of germinal cells, which have their own life alongside the somatic cells—the facts of regeneration are enough: here an element or a group of elements suddenly reveals that, however limited its normal space and function, it can transcend them occasionally ”

“ Here, again, we do not know the processes which cause the arrangement of the cells and their often manifold locally-conditioned differentiations, in short, we do not know the essential nature of regeneration. But, in the meantime, we can endeavour to find out which cell-groups regeneration is bound up with in particular cases, so as to know where the vital particles, the 'determinants,' which condition regeneration, are placed by nature. ”

“ But beyond this there is normally no regeneration. The soldier who has lost an arm in battle cannot return home and in quiet seclusion reproduce a new limb. That which seems to be among lower animals a well-established law of organic growth does not here obtain. This is probably due to the fact that the higher histological differentiation of the tissues in the more highly developed forms of life is a bar to regeneration. In their devotion to special and minute details of physiological work, the cells have, so to speak, forgotten their more generalized reproductive faculties. ”

“ Cells are being born and cells are dying every moment of our existence. Cells reproduce themselves by enlarging and subdividing, the original cell swelling until it finally forms two parts with a small connecting "waist"; then the connection parts and there are two independent cells instead of one. The new cell in turn divides itself up, and so on.
Cells enable the body to carry on its work of continual regeneration. Every part of the human body is undergoing a constant change and tissues are being continually renewed. ”

“ If, as we have seen, “formative material” and “germ-plasm” agree in being particulate; in constituting the material basis of heredity; in being mainly lodged in highly specialized, or germinal, cells; in being nevertheless also distributed throughout the general cellular tissues, where they are alike concerned in all processes of regeneration, repair, and a-sexual reproduction; in having an enormously complex structure, so that every constituent part of the future organism is already represented in them by corresponding particles ”

“ Development has to go back of the point of origin of these several variations in order to include them all. In other words, regeneration has to start with relatively undifferentiated material. This is excellently illustrated by many of the lower, particularly the unicellular, animals, in which reproduction is not yet sexual, but by the simple method of division. A cell comes to rest, divides into two, and each half then leads an independent existence. ”

“ I have already pointed out the importance of the rôle played by liberating stimuli in regeneration, and not only of extra-organismal stimuli, such as gravity, but above all of intra-organismal stimuli that is, the influences exerted in a mysterious manner by other parts of the animal on the parts which are in process of regeneration. ”

“ It will make our study of regeneration clearer, more significant, I think, if we pause for a moment to consider certain fluctuations in the natural development of the organism. We see, for instance, in the brain that early the cells begin to assume the character of nerve cells and that thereafter their multiplication ceases. But, curiously, there will be a spot in the spinal cord, for example, where the change of the cells into nerve cells has not taken place, and from that growth will go on. Cells will migrate from that spot and reach their ultimate destination. ”

“ There seems to be some evidence that we may stunt the growth of a plant or animal by insufficient or unsuitable food, and that all the cells of the body may thereby be reduced in size, the sexual cells among the rest, and that these reduced cells give rise to small progeny in the next generation. ”

“ Physical parallels to the phenomena of regeneration are not easy to find and we still cannot penetrate beyond the empirical facts. Przibram has laid stress on the general resemblance between the new growth of an amputated part in an animal and the way in which a broken crystal repairs itself when placed in the mother-solution. ”

“ The object of the embryo is obviously to protect and nourish the germ-cells which have been placed within it, so that they may be available in due time for the production of further germ-cells, and so for the continuity of the race. Hence it is the all-pervading truth of natural selection that the interest and survival of the individual is almost of no account; that of the species or the race being the paramount consideration. ”

“ This depends on the one hand upon the mode of reproduction by fission which obtains among the Monoplastides (unicellular organisms) , and on the other upon the necessity, induced by sexual reproduction, for the maintenance of a unicellular stage in the development of the Polyplastides (multicellular organisms) .
V. Death itself, and the longer or shorter duration of life, both depend entirely on adaptation. Death is not an essential attribute of living matter; it is neither necessarily associated with reproduction, nor a necessary consequence of it. ”

“ In the regeneration of Planarians we must think of the regeneration-idioplasm as containing the full complex of the collective determinants of the three germinal layers, and possibly we must add to this cells with the complete germ-plasm for giving rise to the reproductive cells. But when the amputated tail of the newt is regenerated, or its leg, or the arm of a starfish, or the bill of a bird, we have no ground for assuming that the cells, from which regeneration starts, contain the whole germ-plasm, since the determinants of the replaceable parts suffice to explain the facts. ”

“ But it is unnecessary to depend upon this assumption alone, for when a useless organ degenerates, there are also other factors which demand consideration, namely, the higher development of other organs which compensate for the loss of the degenerating structure, or the increase in size of adjacent parts. If these newer developments are of advantage to the species, they finally come to take the place of the organ which natural selection has failed to preserve at its point of highest perfection. ”