The possible causes of senescence and rejuvenescence may be classified as structural and functional, though these two should not be regarded as mutually exclusive. Indeed it is practically certain that both structure and function are involved in these processes as in most other vital phenomena. However, different students of this subject have placed emphasis more or less exclusively upon either the structural or the functional causes of senescence and rejuvenescence.
Under the structural causes may be cited Minot's hypothesis that senescence is caused by an increase in the amount of protoplasm as compared with the nucleus. In 1890 he[1] summarized his views on this subject in the following words:
We have then to state, as the general result of the studies which we har© just made, that the most characteristic peculiarity of advancing age, of increasing development, is the growth of protoplasm; the possession of a large relative quantity of protoplasm is a sign of age. . . . We see that there is a certain antithesis, we might almost say a struggle for supremacy, between the nucleus and protoplasm.
In several subsequent papers and books,[2] Minot has developed this idea at length. In his book on "Age, Growth and Death,"[3] he concludes that
Rejuvenescence depends on the increase of the nuclei, senescence depends on the increase of the protoplasm and on the differentiation of the cells.
R. Hertwig's[4] views are apparently diametrically opposed to those of Minot, He finds that senescence or rather "depression" and "physiological degeneration," in Actinospherium and Infusoria are accompanied by an enormous growth of the nucleus. He regards the immature egg cell with its great nucleus as in a condition of depression similar to that found in the protozoa named. By the processes of maturation and fertilization this nuclear material is greatly reduced and thus the cells are brought back to a normal condition.
As opposed to the hypotheses of Minot and Hertwig, it may be pointed out that the larger part of a resting nucleus is composed of achromatin which has been absorbed from the cell body, and that the size of a nucleus depends chiefly upon the quantity of general protoplasm in a cell and upon the length of the resting period during which the nucleus is absorbing this protoplasm. So far from there being an antithesis between nucleus and general protoplasm, we find that the general protoplasm is common to both; small nuclei occur only in cells
- ↑ Minot, "On Certain Phenomena of Growing Old," Proc. Am. Ass'n Adv. Sci., 29, 1890.
- ↑ Minot, "Ueber Vererbung und Verjüngung," Biol. Centralb., 15, 1895.
- ↑ Minot, "Age, Growth and Death," Putnams, New York, 1908.
- ↑ Hertwig, R., "Ueber die Kernkonjugation der Infusorien," Abh. Bayer. Akad. Wiss., II. Kl., 17, 1889.
