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 1.5p, 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.

No abstract

Basal lamina pores may provide a communication route between the lymphoid follicles and the external environment via the FAE cells. Also, the close association between the FAE cells of the epithelial domes, the epithelial pores, the capillary complex of the previously described bursal--blood barrier, and the subepithelial lymphoid follicles could represent a morphological "pore complex" that matures early in posthatching development and may be related to the immunological function of the bursa.

No abstract

In New Hampshire chickens, the primary clinical symptom of dystrophy is limitation of wing motility. Examination of the brachial-level motor unit in chick embryos homozygous for dystrophy reveals abnormalities in both muscular and neural components. Wing motility in these embryos is abnormal as early as six days, and there is a corresponding lack of differentiation of the pectoralis major muscle. The findings suggest that delayed development of brachial-level neuronal pathways is responsible for the decreased wing motility and early degeneration of the pectoral muscle.