Outer Subventricular Zone
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The human brain, and subsequently the structure most commonly associated with intelligence (the neocorte)x, contains more neurons than any other primate species. The mechanism which drives this process arises during development as stem cells (progenitors), which give rise to neurons and other non-neuronal cells, go through a prolonged period of replication. The particular ontological development of cortical layers, particularly the degree of expansion of a zone known as the outer subventricular zone, is thought to contribute to human uniqueness.
During cortical development, progenitor cells form via symmetrical and asymmetrical division from the ventricle lining to populate the ventricular zone (VZ) and subventricular zone (SVZ). In rodents progenitors are confined strictly to the VZ and SVZ (where the distinction between the two zones may be arbitrary). In primates, the VZ gives rise to deeper cortical layers whose neurons make connections to deeper brain structures. The SVZ gives rise to superficial cortical layers whose neurons makes intracortical connections and an additional outer zone of neural precursors has been identified, the outer subventricular zone (oSVZ). While the VZ drives corticogenesis in rodents, the VZ declines rapidly in primates in conjunction with the appearance of the SVZ and oSVZ. Whereas in rodents the SVZ is only partially self -sustaining and must receive a supply of precursors from the VZ, the primate SVZ and oSVZ has a self-sustaining, amplifying population. Enlargement of the SVZ precursor pool in primates ensures an increase in neuron production necessary to form a more complex cortex and in turn facilitate communication between different cortical areas. The population of oSVZ is drastically overrepresented, even compared with other primates, in living humans. This in turn leads to a dramatic overpopulation of the superficial layers of cortex when compared to other species and may underlie increased cortico-cortical connectivity giving humans an increased capacity for intermodal learning and creativity. It also partially contributes to differences in brain size and is thought to underlie gyrification of the brain.
The existence of these distinctions remain unknown in other hominids as they are not directly preserved in the fossil record. However, basic inferences about their existence can be made based upon existence in our closest living relatives and the brain expansion rates of hominids which can be identified in the fossil record. This would suggest, along with changes in brain size, H. neaderthalensis and H. sapiens underwent a differentiation event which lead to a drastic expansion of brain size, likely through cortical expansion and thus presumably oSVZ proliferation.
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