FUSION MECHANISM OF THE MYOBLASTS IN THE MYOTOME OF THE CHICK-EMBRYO

We have studied the fusion process of myoblasts in the mytome correspo nding to H.H. stages 22, 23 and 24 from calcitonin-treated chick embry os and their untreated controls. The micrograph images of this process were studied to detect the ultrastructural changes in myoblast morpho logy that could be associated with the known hormonal and biochemical changes that take place in preparation of fusion. Once actin and myosi n myofilament differentiation and sarcomerogenesis had begun, the myot ome myoblasts fused in bundles of 10-14 myoblasts, and the following w as observed during this process: plasmatic membrane interdigitation an d invagination; the appearance of cytoplasmic flaps covering other myo blasts and fading at the point of contact; plasmatic membranes that fa de and disintegrate; membrane rupturing; double closed lamina; small r inged lamina; large disperse vesicles; small vesicles (liposomes), and semi-dense amorphous material. Seven stages were established: 1.- Mem brane rupture; 2.- Double closed and elongated lamina; 3.- Small ringe d lamina aligned lengthwise; 4.- Large dispersed vesicles; 5.- Impreci se boundaries with amorphous material in diffuse areas; 6.- Cytoplasm fusion; and 7.- Prefunctional syncitium. Answers to the questions: <<h ow>>, <<when>>, <<where>>, <<why>> and <<for what purpose>> the fusion of myoblasts takes place are suggested.