Endothelial cell activation during angiogenesis in freely transplanted skeletal muscles in mice and its relationship to the onset of myogenesis.

The activation time of endothelial cells and myogenic cells (presumptive satellite cells) in small skeletal muscle transplants was examined, as well as in the muscle bed underlying the transplants, using autoradiography and light microscopy. At varying intervals, from 12 to 336 hours (14 days) after transplantation, the transplants and some of the underlying muscle were removed from two or four mice (at each interval), after the injection of each animal with tritiated thymidine 1 hour prior to transplant removal. Thus premitotic cells synthesising DNA were labelled. Only one labelled endothelial cell nucleus was seen in all 20 transplants examined during the first 60 hours. Contrastingly, labelled endothelial nuclei were plentiful in muscles underlying the transplants from 36 hours post-transplantation. There was no evidence of a functional vascular supply in the transplants until 72 hours when one of the eight transplants examined at this time showed very slight peripheral revascularisation, with these peripheral vessels containing labelled endothelial cell nuclei. Ninety six hours after transplantation half the transplants sampled showed peripheral revascularisation, with these vessels containing labelled nuclei, and by 120 hours all transplants showed functional blood vessels and contained labelled endothelial cell nuclei. By 14 days after transplantation revascularisation and myogenesis were complete with only an occasional labelled nucleus seen. Autoradiographically labelled premitotic muscle nuclei (presumptive satellite cells) were observed in transplants from 48 hours after transplantation. These results show that revascularisation of the transplanted muscles is not necessary for the activation of myogenic cells, but that activation is probably due to some other stimulus, possibly the diffusion of nutrients from blood vessels of the adjacent tissues.