Ultrastructure of developing muscle cells in the chick embryo

Abstract Sectioned as well as negatively stained preparations of developing myotomal cells from the chick embryo were studied with the electron microscope. The application of these two techniques to similar material made it possible to observe developing muscle cells in successive stages of differentiation and at the same time visualize some of the isolated components making up these cells. The thick and thin filaments observed in the embryonic muscle were morphologically indistinguishable from those observed in adult muscle. The thin filaments, found randomly dispersed in the cytoplasm of the early cells, were present in large concentrations before the first appearance of thick filaments. The first observed thick filaments had lengths of 1.5 μ, equal to that of adult thick filaments. The appearance of large polyribosomes was correlated with the first appearance of the thick filaments. Single and small groups of ribosomes were always present. Early formed thick and thin filament aggregates were found in the same hexagonal array that is typical to the organization of myofilaments in adult myofibrils. Non‐striated myofibrils, associated with a periodic transverse tubular system, were found before striated myofibrils could be observed. Some preliminary observations were made concerning the appearance of glycogen particles.