NEUROCYTOGENESIS .I. NEUROFIBRILS NEUROFILAMENTS AND TERMINAL MITOTIC CYCLE

Initial cytodifferentiation of neuroblasts in the retina of chick embryos was investigated by means of silver impregnation, H3‐thymidine autoradiography, and electron microscopy. Utilization of various combined procedures with the same initial fixative demonstrated that the results of neurofibrillar staining are complementary, not contradictory, to information obtained from both autoradiography and electron microscopy. The question of a homogeneous versus a non‐homogeneous cell population within the neural epithelium was clarified by the closely related appearance of neurofibrils to the terminal mitotic cycle of some presumptive neuroblasts. Neurofibrils were observed in certain apolar pre‐division cells as well as in pairs of transitional apolar and bipolar daughter cells which apparently do not remain in the neural epithelium. At the ultrastructural level, aggregates of 60 Å neurofilaments were observed in configurations that corresponded to neurofibrillar networks. As the bipolar neuroblast moves into the ganglion cell layer, it would appear that much of the neurofibrillar substance is transported within the elongating peripheral process which becomes the optic nerve fiber. Further investigation of the relationship of neurofilaments to neurotubules in the neuroblast undergoing cytodifferentiation is now in progress. Copyright © 1969 Wiley‐Liss, Inc.