First ultrastructural differentiation of myoblasts of chicken embryos: appearance of the initial filaments.

A study is made of the structure and ultrastructure of myoblasts located in the myotome in chicken embryos, between stages 18 and 19, for electron microscope observation of the occurrence of the first myogenetic filamentous molecules. We suggest a hypothesis for the formation of initial filaments consisting of an initial synthesis of actin globular molecules, carried out in the centre of the myoblast, near the nucleus, with the participation of RNA and the ribosomes. These molecules accumulate peripherically in areas below the plasmatic membrane where they polymerize into actin filamentous molecules which form the initial filaments. These move towards the sharp ends of the myoblast, under the plasmatic membrane, and thence to the interior of the cytoplasm, where they are evenly distributed. This genesis of initial filaments is independent of the influence of the nervous system. We postulate the existence of a single type of myoblast, of fibrillar form, with a dark central area containing the nucleus and the cell organelles, and two sharp, light end zones which contain only the initial filaments in a very light cytoplasmic matrix.