DNA synthesis and nuclear reproduction during embryonic development and regeneration of muscle tissue.

DESPITE a large number of investigations devoted to the development and regeneration of skeletal musculature, the problem of the mode of reproduction of the muscle nuclei remains unsolved (Boyd, 1960; Murray, 1960; Holtzer, 1961; and many others). The majority of investigators believe that the symplast nuclei reproduce by amitosis (Bucher, 1959). Only a few investigations have shown the presence of mitoses in developing muscle fibres. Purely morphological investigations of the development and regeneration of muscle tissue seem to be unable to solve this problem. The nuclei of muscle fibres developing in vitro have recently been shown to synthesize DNA, moreover, the experiments showed the synthesis of DNA by the nuclei to be antagonistic to that of specialized muscle proteins (Stockdale & Holtzer, 1961). Bintliff & Walker (1960) showed that a considerable percentage of de-differentiated nuclei synthesized DNA upon regeneration of the skeletal musculature. These observations make it possible to assume that nuclei in developing and regenerating muscle fibres reproduce in the same manner as those in any other tissue. The present investigation, carried out in two experimental series, concerns the relationships between reproduction by mitosis or amitosis and DNA synthesis in muscle nuclei, and their relation to muscle differentiation. The first series was devoted to a study of nuclear reproduction during embryonic development, while the second dealt with nuclear reproduction during the regeneration of striated muscles. To detect DNA synthesis, tritiated thymidine was applied as a specific indicator.