A model of neocortical histogenesis based on Cajal's principles of homotropism and neurocladism.
Generation and regeneration of the Nervous System are considered essentially similar processes since the pioneering studies of Cajal (15). The present paper is a hypothetical attempt to explain the puzzling internal-to-external histogenetic pattern of the mammalian neocortex(4), largely on the basis of various observations made by Cajal on the developing and regenerating Central and Peripheral Nervous System. Fundamental to our model are the principles of excitatory neurocladism and of reciprocal homotropism as described by Cajal. By assuming that both latter principles are valid for the early stages of neocortical development, an explanation can be given for most of the main events of neocortical histogenesis that is consistent with known facts. Our main conclusion is that neocortical neurogenesis differs from that of any other area of the neural tube due to the interposition of the corticipetal fiber system in between the cortical matrix layer and the cortical plate. The mechanical obstacle thus posed to neuroblast migration by this fiber system appears responsible - by activating the operation of the aforementioned principles of Cajal - for both the cellular and topographical neurogenetic sequences of neocortical development, as well as the growth predominantly in width, all features that are unique to the neocortex as compared to the rest of the neural tube.
D000818:Animals; D002454:Cell Differentiation; D002465:Cell Movement; D002540:Cerebral Cortex / Q000166:cytology / Q000196:embryology*; D008959:Models, Neurological; D009416:Nerve Regeneration; D009457:Neuroglia / Q000502:physiology; D009474:Neurons / Q000502:physiology; D011817:Rabbits; D012038:Regeneration