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Fusion mechanism of the myoblasts in the myotome of the chick embryo.

We have studied the fusion process of myoblasts in the mytome corresponding to H.H. stages 22, 23 and 24 from calcitonin-treated chick embryos and their untreated controls. The micrograph images of this process were studied to detect the ultrastructural changes in myoblast morphology that could be associated with the known hormonal and biochemical changes that take place in preparation of fusion. Once actin and myosin myofilament differentiation and sarcomerogenesis had begun, the myotome myoblasts fused in bundles of 10-14 myoblasts, and the following was observed during this process: plasmatic membrane interdigitation and invagination; the appearance of cytoplasmic flaps covering other myoblasts and fading at the point of contact; plasmatic membranes that fade and disintegrate; membrane rupturing; double closed lamina; small ringed lamina; large disperse vesicles; small vesicles (liposomes), and semi-dense amorphous material. Seven stages were established: 1--Membrane rupture; 2--Double closed and elongated lamina; 3--Small ringed lamina aligned lengthwise; 4--Large dispersed vesicles; 5--Imprecise boundaries with amorphous material in diffuse areas; 6--Cytoplasm fusion; and 7--Prefunctional syncitium. Answers to the questions: "how", "when", "where", "why" and "for what purpose" the fusion of myoblasts takes place are suggested.