The morphogenesis of the neuromuscular junction in the chick skeletal muscle was followed by the improved histochemical AChE method (KARNOVSKY and ROOTS, 1964; KARNOVSKY, 1964) and the silver impregnation method (SCHEFTHALER and MAYET, 1958). Specimens used were chick embryos from 5 to 21 days in ovo (the proximal portion of the lower limb), chicks 1 to 2 days ex ovo (M. sartorius) and adult fowls (M. sartorius and M. pectoralis major). A majority of the work was carried out by the light microscope, but a part of the enzymatic study was done with an electron microscope.
Under the light microscope, the configuration of the enzymatically active structure became gradually more complicated in appearance and larger in size as the neuromuscular junction reaches maturity. The earliest appearance of the junctional AChE activity in the skeletal muscle was seen in embryos of as early as 7 days of incubation (stage 30) in the form of a single curved line, and afterwards the subsequent differentiation of the formed junction takes place. The morphogenesis of the neuromuscular junction in the chick skeletal muscle, studied enzymatically, could be divided into three main stages; the stage of initial formation (7 to 13 days in ovo), the stage of subsequent development (13 to 18 days in ovo) and the stage of nearly maturation which occurs at around 20 days of incubation.
When observed under the electron microscope, the localization of the final product (copper ferrocyanide) of the AChE reaction was mainly in the synaptic cleft and the interspace between the axolemma and the plama membrane of the Schwann cell.