Evidence for the fusion of myoblasts in amphibian embryos. II. Xenoplastic transplantations of somitic cells from anuran to urodele embryos

In order to further support the thesis that myoblast fusions lead to the formation of multinucleate myotubes during early amphibian development, prospective myoblasts from frog embryos ( Rana pipiens; Xenopus laevis ) were orthotopically and heterotopically transplanted into embryonic salamanders ( Ambystoma maculatum; A. tigrinum ). The stages employed ranged from gastrula to early tail bud. After the hosts had developed to more advanced stages, they were fixed, sectioned in the frontal plane, and prepared for histological observation. Of special interest in these xenoplastic combinations are the nuclear and cytoplasmic differences between anuran and urodele embryonic cells. In most cases, transplanted cells tended to form a tissue complex distinct from the host tissues. This naturally decreased the opportunity for cell intermingling and consequently reduced the chances for fusions between graft and host myoblasts. Thus, there was a correlation between the extent of dispersion of the transplanted cells and the frequency of chimeric myotubes. Nevertheless, in each experimental series, a number of multinucleate myotubes emerged consisting of nuclear and cytoplasmic components of both graft and host cells. These chimeric myotubes, which must have originated by multiple cell fusions of anuran and urodele cells, (1) retained their distinct nuclear differences, (2) displayed resorption of both anuran and urodele yolk platelets, and (3) contained cross‐striated myofibrils. The function of these chimeric cells, their synthesis of myofibrils, and the apparent lack of free intermingling at the cellular level, are discussed.