Electron microscopic study of the developing neuroblast of the dorsal root ganglion of the rabbit embryo

The differentiation of the neuroblast of the spinal ganglion of the embryonic rabbit has been studied by phase, light, and electron microscopy from the period of migration of the cells from the neural crest until their transformation into unipolar neurons. The relatively undifferentiated migratory neuroblast, which has single primitive neurite, contains fine filaments and large numbers of ribonucleoprotein particles, but very little membranous endoplasmic reticulum. The presence in the Golgi‐centriolar region of large numbers of neurotubules, which probably originate from the centriole, is one of the first signs of further maturation of the neuroblast. The neurotubles migrate into the neurite, where they become a prominent feature during development, particularly in the distal processes. Elements of the immature Golgi complex appear to participate directly in the formation of the membranous portion of the primitive Nissl substance, which develops during the bipolar period. A difference was noted in the fine structure of the processes of the transitional bipolar neuroblast; i.e., one neurite resembles an axon; whereas, the other contains granular endoplasmic reticulum typical of dendrites. The findings of this study are in accord with the concept that the unipolar process arises by constriction and elongation of perikaryal cytoplasm. Two neural cell types, a small cell having randomly disposed organelles, and a larger cell having large clumps of endoplasmic reticulum at the periphery, can be distinguished. The application of the Cajal reduced silver technique to embryonic ganglia, which were subsequently examined with the electron microscope, resulted in a random distribution of silver deposit throughout neural perikarya.