The process of myoblast fusion was observed in embryonic chick skeletal muscle cells grown in monolayer cultures at the fine structural level. At the first step, the sarcolemmas of cells destined to fuse are closely applied to each other. They are linked in some places by fasciae adherentes; in other places, engulfment of small processes of one cell by another is seen. At a somewhat more advanced stage of myogenesis, vesicles and tubules are formed between the adjacent cytoplasms; presumably, the apposed membranes have opened at several points and their edges have fused to each other. Finally, remnants of cell membranes (vesicles and tubules) disappear completely, and the confluent cytoplasm is formed. The cytoplasmic contents of the multinucleated cells are often poorly admixed, giving the cytoplasm a mosaic appearance in which different zones can be designated as arising from separate cells. This observation suggests, however, that there is slow diffusion of myoblast contents (ribosomes and, possibly, other materials) into the myotube. In agreement with the previous works at the light microscopic level, the present study suggests the occurence of fusion between mononucleated cells, between mononucleated cells and multinucleated myotubes, and between nascent multinucleated myotubes.

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