the return of the characteristics of youth to the aging organism; the opposite of the aging process. The first attempt at a scientific approach to the problem of rejuvenescence was made by C. E. Brown-Séquard, who suggested that a decrease in sexual activity is the cause of aging. After experimenting on himself, he proposed the use of extracts from testicles of animals as a means of rejuvenescence (1889). This line of research was pursued by the German scientists W. Garms (1916) and E. Steinach (1920) and the Russian scientist S. A. Voronov (1923). As knowledge about the aging process developed, rejuvenating properties were ascribed to many substances; for example, E. I. Metchnikoff proposed the use of Bulgarian sour clotted milk, and A. A. Bogomolets the use of antireticular cytotoxic serum. Among other alleged rejuvenating substances are Novocain and various hormones, including hormones other than sex hormones. Many of these agents are used in modern geriatrics, although none of them has proved to be the “elixir of youth.”

Rejuvenescence is apparently impossible in a complex multicellular organism, since the progression of ontogenic stages from childhood and youth through maturity and old age is an irreversible process. It is feasible to slow the rate of aging or to correct age-related changes but not to effect rejuvenescence as such.

The concept of rejuvenescence is also applied to individual cells of a multicellular organism and to a population, in which case the term is understood to mean an increase in the number of young individuals in the population. Rejuvenescence is manifested at the cellular level by intensified reproductive activity and by the absence of specific differentiation. Rejuvenescence is a characteristic of, for example, regenerating tissues.