Timing of major ontogenetic events in the visual cortex of the rhesus monkey

The place and time of origin, the migration and eventual disposition of neurons of the monkey visual cortex were studied by autoradiography in animals killed at various intervals after 3H-thymidine pulse labeling at embryonic (E) and early postnatal (P) stages. All neurons destined for the visual cortex are generated during about a 2-month period between E45 and E102. Neuron position in the cortical laminae correlates systematically with time of cell origin; neurons destined for deeper cortical positions are generated earlier, and more superficial ones progressively later. Thus, most neurons in layer VI are born between E45 and E60, in layer V between E60 and E70, in layer IV between E70 and E80, and in layers III and II between E80 and E102. No neurons, but numerous glia, are generated within the cortical plate itself. Initially young neurons are produced almost exclusively in the ventricular zone. Later they are probably generated in both ventricular and subventricular zones, and by the end of the proliferative period the subventricular zone becomes the predominant source of new cells. At the time when all neurons destined for the monkey visual cortex have already been produced, primary fissures are barely indicated on the cerebral surface and no secondary fissure appears as yet. Autoradiographic analyses indicate that at early stages young neurons move to the cortical plate relatively synchronously and at a fast rate, whereas at later stages there are considerable differences in the rates of cell migration. At early stages when the migration pathway is relatively short, the external process of the ventricular cell may stretch across almost the entire migratory distance. It is possible that nuclei move without interruption within their own cylinders of cytoplasm, a mechanism which might account for the rapid, synchronous movement of cell bodies as seen in the autoradiographic material. During later stages when young neurons, mostly of subventricular origin, move across a distance which is more than 10 times the length of their leading process, migrating cells follow radial glial guides across the widened intermediate zone and through densely packed cortical plate. This type of cell displacement seems to proceed less synchronously and require more time. It is proposed that two different mechanisms of cell displacement might exist: one in operation at early stages of cortical formation when mostly ventricular cells migrate, and another utilized by subventricular cells at later stages.