REJUVENATION. In all multicellular animals the process of regeneration, growth and rejuvenation is constant as is shown typically by the hair and nails. It is now possible to cultivate tissues in suitable media (see TISSUE CULTURE) and to keep them alive long after the death of the parent organism. This proves that death is not the inevitable end of cellular vitality, but is in every case the result of unfavourable conditions to which the cells are subjected at a given moment.
The endocrine glands produce substances (hormones), which they pour into the circulatory system and thereby influence the metabolic processes, the growth and morphology of the cells. One of these glands should have as its special function the secretion of a substance which gives tone and stimulus to cellular vitality during a certain period of life and ceases to do this on the approach of old age. This cannot be the special function of the thyroid, parathyroid, pituitary or suprarenal glands, since they continue to act during old age. The only gland which constitutes an exception to this rule is the genital gland. It plays a double role. It secretes spermatozoa externally, and it secretes internally hormones which it passes into the blood stream, actively at puberty and during maturity, but less and less thereafter, so that the diminution and disappearance of its activity correspond with old age.
Examination of male vertebrate animals, including man, after removal of the genital glands shows distinctly the nature of the influence of the internal secretion of these glands on the whole organism as affecting, not only the secondary male sexual char acters, but also the growth and development of the body as a whole, the brain and skin cells, the bones and tissues. The physical and intellectual qualities of animals and of man are as intimately conditioned by the hormone secreted by the testicles as are the secondary sexual characters.
There can thus be no doubt as to the nature of the relation between the general reduction of the forces of the organism and the disappearance of the internal secretion of the testicles. No organ can keep its vital energy and yield a full return if the cells are not stimulated and vivified by the testicular hormone.
It acts more or less directly on other endocrine glands, since castration is followed by hypertrophy of the anterior lobe of the pituitary gland and by regression of the thyroid body and epiphysis. If the genital glands remained active in old age, were they not the only glands which cease to secrete hormones, old age would certainly be delayed.
Eunuchs in Cairo who had been castrated at an early age and had theref ore never been exposed to the activity of the testicular hormones, were never known to have been more than 6o at death. All had the appearance of old men, with desiccated skins, hag gard eyes, bent, and looking like centenarians.
It is obvious that the deprivation of the internal secretion of the testicles accelerates the advance of old age and shortens life. The only remedy is to graft a young testicle, whether that of a young human being or of an ape, by which the tone-giving sub stance is provided, so as to increase the vitality of all the cells which are weakened but are not yet atrophied and therefore still able to renew themselves, and thus effectively to rejuvenate the whole organism. So long as an organism, however old, continues to exist, its cells continue to be renewed and rejuvenated. Un fortunately, in old age this process of rejuvenation is slowed down, a certain number of the functional cells regress and are replaced by conjunctive tissue. The cells which escape this are renewed more slowly, but continue to be renewed to the extreme limit of their vitality.
With a rich addition of the testicular hormone, the cells ac quire new energy, grow more rapidly, proliferate more intensely and rejuvenate the whole system. At the end of several years the beneficent action of the grafted gland is exhausted because the grafts in turn are subject to positive regression. The organism is again deprived of the stimulating hormones and the symptoms of old age reappear. In most cases testicular grafting is adequate, but in some cases thyroid grafts have to be added. See S. Voron off, Rejuvenation by Grafting (1925) . (S. V.)