Regulation of normal proliferation in the developing cerebrum potential actions of trophic factors.

We review here a computational model of neocortical histogenesis based upon experiments in the developing cerebral wall of the mouse. Though based upon experiments in mouse, commonalities of developmental history and structure of neocortex across mammalian species suggest that the principles which support this model will be generally applicable to neocortical evolution and development across species. In its scope the model spans the successive histogenetic events: cell proliferation, cell migration, and the positioning of cell somata in neocortical layers following migration. Neurons are produced in a pseudostratified epithelium (PVE) which lines the ventricular cavaties of the embryonic cerebrum. The parameters which determine the rate and total number of neurons produced in the PVE are (1) the size of the founder population, (2) the number of integer cell cycles executed by the founder population and its progeny in the course of the neuronogenetic interval, (3) the growth fraction, and (4) the fraction of cells which exits the cycle (Q fraction) with each integer cycle. There is a systematic relationship between the integer cycle of origin and the sequence of cell migration, position in the cortex, and the extent to which a set of postmigratory neurons will be diluted in the cortex by the combined effects of tissue growth and cell death. Variation across species in the number of integer cell cycles as a function of the rate of progression of Q may be expected to modulate profoundly the total numbers of neurons that are produced but not the relative proportions of neurons assigned to the major neocortical layers.