DNA Synthesis and Nuclear Reproduction during Embryonic Development and Regeneration of Muscle Tissue

Abstract

ABSTRACT Nuclear reproduction in myosymplasts and myotubes proceeds mitotically. Nuclei synthesizing DNA are found during the development of muscles. The percentage of 3H-thymidine labelled nuclei diminishes with the development of the embryo as mitotic activity decreases. A mitotic cycle in the nuclei of muscle fibres of a 17-day-old rat embryo takes about 20 hr. and the duration of mitosis itself is about 2·5 hr. A comparison of the number of nuclei participating in a mitotic cycle with the mitotic index allows us to suggest that as differentiation proceeds some nuclei provide the synthesis of specialized proteins while others undergo reproduction. During regeneration of muscle fibres, 2 days after operation, a considerable percentage of 3H-thymidine labelled nuclei is found at the ends of cut muscle fibres and, correspondingly, the mitotic index is high. The number of labelled nuclei, as well as that of mitoses, decreases with distance from the wound. Subsequently labelled nuclei move to muscle buds where they only very rarely undergo division. All the facts obtained show that the reproduction of muscle nuclei during embryonic development and during regeneration of muscle fibres in adult animals proceeds in the same manner as in any other cellular tissues where cell reproduction by mitosis is not in doubt.