Morphological changes in skeletal muscle after transplantation. A light and electron-microscopic study of the initial phases of degeneration and regeneration.

The morphological changes in subcutaneously implanted muscle homografts in mice were studied by light and electron microscopy 1-23 days after transplantation. The initial degenerative changes were identical in isografts and allografts and were essentially thesame as those found in muscle explants in tissue culture and aftervarious forms of muscle injury. Regnerative changes were prominent at the periphery of graftsby 48-72 hr beforeevidence of graft revascularization could be demonstated by India ink perfusion. Active regeneration occurred in isografts and in allografts during the first week resulting in the formation of a new population of muscle fibres. Rejection subsequently occurredin allografts between days 7-14. Regeneration was retarded or completely inhibitied by exposure of the donor animal to 1,500 rad of X-irradiation 1-2 hours prior to transplantationsuggesting that regeneration is brough about by cells dervied from the graft. Eectron-microscopic observations in 48-72 hr grafts suggested that primitive mononucleated myogenic cellsmay form within degeneratiang muscle fibres by a process of myonuclear sequestration. Inactive satellite cells were present in the muscle from which grafts were taken in some donoranimals but no definite conclusions could be reached as to the role of these cells in graftsregeneration.