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The phenomena of regeneration are of interest in connection with this aspect of the biological basis for rational therapeutics. The nature and conditions of the regeneration of lost and injured parts have been investigated by a number of biologists, as these phenomena are displayed both by simple organisms and by those of more complex structure, including mankind. The investigations into the reactions of the simpler animals have added most to our knowledge of the processes of regeneration. This is due in part to the greater ease of investigating their physiology, in part to their greater capacity for regeneration, and in part to the greater simplicity of their life processes under both normal and abnormal conditions.
The effects of variations of temperature upon the regeneration of lost and injured parts has been studied by many persons. Even the most superficial resume of these experiments would require too much space in such a volume as this. Without any significant exception it is found that the limits of temperature at which normal growth may take place, represents also the limits of temperature for regeneration. The temperature which is the optimum for any animal during the time of its most active growth is the optimum for the most rapid and perfect regeneration of its lost and injured parts.
The regeneration which occurs in a worm, Planarian lugubris, has been investigated by Morgan with significant results. The effects of starvation upon its regeneration were first studied. This animal is very well adapted for this work, since it endures starvation remarkably well. The planarian is able to live until it has been starved to one-thirteenth its normal size. If a planarian is cut into two equal pieces by a sagittal lengthwise incision, both pieces regenerate their lost parts, and two complete and normal worms are formed. In Morgan’s experiments, one such half was kept well supplied with food, and the other was kept without food. The well fed half regenerated the lost parts rapidly, and the resulting individual was full grown and normal. In the starved animal, regeneration proceeded very slowly, and the resulting animal was very small, much less in size than the half immediately after the operation. It appears that this worm is able to reform its lost parts from the tissues which remain normal. It is shown also that the presence of a normal supply of food is essential to the most rapid and perfect regeneration. Nevertheless, there are animals in whom the beginning of regeneration occurs more quickly during starvation than during full feeding. This is probably due to the fact that the material from which the new organs are to be formed is derived from the pre-existing protoplasm in the case of the starving animal. This material is more nearly of the chemical form of the new molecules than are the food stuffs which are probably used, in part at least, for the rebuilding of the lost organs by the well fed animal.
The effect of light upon regeneration has not been well studied. In the case of a hydroid, eudendrium racemosum, Loeb found that hydranths were regenerated in light but not in darkness, in blue light, but not in red. This hydroid is one of the few animals known in which light has an influence upon the direction of growth. It is, so far, the only one known in which the regeneration of lost parts is influenced by light. That is, in this animal, the same conditions of light which are best adapted to normal growth are also best adapted to the regeneration of lost parts.
A supply of oxygen is essential to normal growth and function, and is also essential to regeneration. If the stem of a tubularia, for example, is suspended so that it rests just above the surface of the sand, where the oxygen is somewhat deficient, the process of regeneration is either hindered or lacking altogether. If the stem is placed in a tube which it fits rather closely, regeneration usually does not occur at all. These points have been demonstrated by several investigators independently.
Among animals who rejoice in the possession of a nervous system, regeneration often depends to a certain extent upon the nature of the nerve impulses to the injured locality. Several biologists have found that when the eye only is cut from the eye stalk of certain crustaceae, (Palaemon and Sicyonia and one or two others), the eye is regenerated, but when the eye stalk is entirely removed an antenna grows in the place of the eye. This is held to be due to the fact that when the eye alone is removed the optic ganglion is left intact and the eye regenerated, but the removal of the entire eye stalk carries with it the ganglion, and the simpler structure, the antenna, is developed. Or, the regenerative attributes of the cranial structures are ordinarily effective in producing antennae, but the influence of the nerve impulses changes the character of these regenerative efforts in such a way that the eye is produced instead. In any event, the normal regeneration of the eye depends upon the maintenance of the nerve impulses normally sent to the eye. Even among human beings, the regeneration of a nerve trunk is facilitated by the maintenance of normal nutritive conditions in the area of the normal distribution of the nerve trunk affected. The whole process of regeneration, recovery and hypertrophy are as much of a mystery as are all other physiological processes. It is not at present possible to offer any adequate theory for the explanation of the effects of nerve impulses upon regeneration. In our own bodies, physiological regeneration occurs only to a slight extent. The most conspicuous example of physiological regeneration is displayed in the continual renewal of the continually wasted epithelial cells.
Pathological or accidental regeneration occurs after injury to the skin, and to certain other structures. Only small areas of skin can be regenerated, but large areas may be healed by the multiplication of the cells of the connective tissues. In order to facilitate the renewal of the injured skin, the environment which facilitates the normal growth of skin is most effective. Normal skin cells which are undergoing the processes of reproduction are protected from bacteria and all irritating substances by the old, dead cells of the upper layers. In order to render growth of the new cells most rapid and normal these conditions must be secured. The injured part must be protected from bacteria and irritants, as are the cells growing under normal conditions, and the heat, blood supply, etc., must be kept as nearly as possible the same as are present under normal conditions. The healing of all injured tissues, of broken bones, sprained joints, bruised or cut, torn or burned tissues anywhere in the body is facilitated by those conditions which are normal to those tissues during their period of growth.
The number of neurons is fixed at a very early period of embryonic life. There is, so far as our present knowledge goes, no possibility of regeneration of nerve cells as such. There is a possibility of the development of embryonic cells, however, and thus the recovery from injury to the nervous system may be symptomatic, though it can never be absolute. The number of embryonic cells potentially capable of development is almost inexhaustible, under our present conditions of life. The processes of differentiation which rendered the neurons so irritable and conductible render them also incapable of reproduction. It did not render them altogether incapable of regenerating their own lost parts, however. The axons which are supplied with both the neurilemma and the white substance of Schwann may be regenerated after section, if conditions are favorable, and they may then perform their functions in a fairly normal manner. After a nerve trunk is injured, the regeneration of its fibers may be rendered more certain, more perfect, and more speedy by securing the following conditions: The ends of the nerve must be sutured. If the injury is an old one, the ends must be freshened; if the nerve has been crushed, the crushed part must be removed. Transplantation may be employed if necessary. The blood supply to the nerve both above and below the injury must be kept free, both upon the arterial and the venous side of the circulation. The blood itself must be kept good, by good food, good air, and good elimination. The condition of the structures normally supplied by the injured nerve must be kept normal. This is very essential in the case of the muscles. They must be stimulated to a normal amount of exercise, in order that they may not atrophy. Not only is this done for the sake of having them normal when the connections are made, but their activity seems to exert a favorable influence upon the growth of the developing nerve fibers. It is evident that the factors which exert the most favorable influence upon the regeneration of the nerve fibers are just those which exert the most favorable influence upon normal growth and function.
Cells within the body may be injured by various abnormal conditions in their environment. If bacteria are present, the cells which are in the most normal conditions of metabolism are those which are most efficient in destroying the invaders. If there has been injury to any part of the body, the environment which is normal to that part of the body during its period of growth is the environment which best facilitates recovery. There is no such thing as a “healing application” anywhere in all nature, except the things always present in the environment of the normal cells of the body. The normal environment of the normal cell during its period of normal growth is the best environment for the abnormal cell during its period of repair.
Nerve Surgery, in The American Text Book of Surgery. The Healing of Wounds, in any good text book of pathology. |