The origin of myoblasts and the problem of dedifferentiation

Abstract

Skeletal muscle of dystrophic mice was injured, thymidine- 3H was administered and the mice were left for a period of 10 to 27 days to allow the regeneration of muscle fibers which would then contain many radioactive nuclei. When the muscle was reinjured, it was shown by radioautography that the newly regenerating fibers also contained many radioactive nuclei. These experimental results were consistent with the hypothesis that new muscle fibers are formed by cells originating from the injured fibers, but did not exclude the possibility of participation by connective tissue cells. Connective tissue overlying skeletal muscle was injured in normal and dystrophic mice. Thymidine- 3H was injected to label connective tissue cells proliferating in response to injury and then the underlying muscle was injured. Many strongly radioactive leucocytes and connective tissue cells were seen in radioautographs four days after muscle injury but the basophilic myotubes did not contain any nuclei which gave strong reactions. However, very weak reactions were commonly seen over myotube nuclei and were attributed to reutilization of thymidine- 3H released through degeneration of leucocytes. This was tested by repeating the experiment with the modification that a two week interval separated the injury to connective tissue from the injury to muscle. Radioautographs of such doubly injured areas revealed a lack of strongly radioactive leucocytes and the complete absence of radioautographic reactions over nuclei in basophilic myotubes. With this demonstration of non-participation by connective tissue cells in muscle regeneration, and in conjunction with a previous experiment which ruled out leucocyte participation, it has been shown rather conclusively that the origin of myoblasts during regeneration is from cells which form within injured muscle fibers. Results of the current and previous experiments on muscle regeneration were reviewed in respect to dedifferentiation and redifferentiation and were considered to support the hypothesis that cells in the adult are irreversibly differentiated in respect to being members of a particular cell population even though the morphology of a cell might vary greatly due to various environmental influences.