The development of hippocampal and dorsolateral pallial regions of the cerebral hemisphere in fetal rabbits. VI. Ninety millimeter stage, cortical differentiation

The structure of the cerebral hemisphere of 90 mm rabbit embryos was studied with aniline and Golgi stains and a comparison of cortical plate morphology was made with earlier stages. Neuronal differentiation was inferred to be a continuous process since cells forming the base of the cortical plate gradually increase in size from 29 mm through 90 mm stages. However, differentiation also appeared to be most rapid among cells situated near the marginal lamina. Analysis of the development of large pyramids of layer V in the pallial region provided the most convincing proof for this conclusion. This mode of cortical development suggests the possibility that embryonic synapses are estabilished sequentially along the apical dendrites of pyramidal cells as the cortical plate grows through the addition of new cells. This process is postulated to occur in a “critical zone” at the boundary of the marginal lamina with the cortical plate. The development of interneuronal neuropile in the lower part of the cortical plate of 90 mm embryos seems to be correlated not only with the growth of basal dendrites and axon collaterals, but also with the simultaneous appearance of true astrocytes and oligodendrocytes.