A host contribution to the regeneration of muscle grafts

Isoenzymic forms of NADP-dependent malate dehydrogenase (MDH) have been used as markers in experiments to discover whether the source(s) of the new muscle fibres which form in grafts are of host, donor or mixed origin in regenerating muscle. Grafts of minced tibialis anterior muscle were made from CBA donor mice into C3H hosts. These 2 strains produce different isoenzymic forms of NADP-MDH. C3H hosts were made tolerant to the donor strain by neonatal injection of CBA × C3H lymphoid cells. Some 73% of grafts of CBA muscle made into tolerant C3H hosts contained newly formed muscle fibres. Pools of such grafts yielded on extraction MDH isoenzymes which were shown by electrophoresis to be largely of host type: traces were found of donor-type but not of hybrid isoenzymes. Histochemical and other evidence suggested that MDH activity was derived mainly from the muscle component of grafts (but in part from fibrous connective tissue). The absence from 150–220 day-old grafts of hybrid isoenzymes suggests that few, if any, muscle fibres had formed by fusion of host with donor cells. The predominance of host isoenzymes suggests that little or none of the muscle formed was of donor origin.