Factors which affect the fusion of allogeneic muscle precursor cells in vivo.

Skeletal muscle allografts in mice have been used to study the factors which promote the fusion in vivo of muscle precursor cells derived from different strains of mice. Tolerance of host mice to allogeneic donor tissues has been induced either by the injection of donor spleen cells into newborn recipients, or by the irradiation and reconstitution, with donor bone marrow cells, of the haemopoietic system of the thymectomized recipient. The isoenzymes of glucose-6-phosphate isomerase (GPI) have been used as markers to distinguish the contribution of host and donor tissues to these allografts. In 174 of 204 such grafts evidence of both host and donor tissues was found. In 57 grafts which yielded a "hybrid' isoenzyme of GPI, it was inferred that host and donor muscle cells had fused to form mosaic fibres. The following factors appear to influence the fusion of muscle precursor cells: a the presence within grafts of adequate numbers of precursor cells of both the host and the donor strains; b the particular strain-combinations used; c the immunological state of the host.