Neurons destined for the cerebral neocortex are formed in the pseudostratified ventricular epithelium (PVE) lining the ventricular cavity of the developing cerebral wall. The present study, based upon cumulative S-phase labeling with bromodeoxyuridine, is an analysis of cell cycle parameters of the PVE. It is undertaken in the dorsomedial cerebral wall of mouse embryos from the eleventh to the seventeenth gestational day (E11-E17, day of conception = E0) corresponding to the complete period of neuronogenesis. The growth fraction (fraction of cells in the population which is proliferating) is virtually 1.0 from E11 through E16. The length of the cell cycle increases from 8.1 to 18.4 hr, which corresponds to a sequence of 11 integer cell cycles over the course of neuronal cytogenesis in mice. The increase in the length of the cell cycle is due essentially to a fourfold increase in the length of G1 phase which is the only phase of the cell cycle which varies systematically. Thus, the G1 phase is most likely to be the phase of the cell cycle which is modulated by extrinsically and intrinsically acting mechanisms involved in the regulation of neuronal cytogenesis.

A cumulative labelling protocol using 5-bromo-2'-deoxyuridine (BUdR) was followed to determine: (1) the growth fraction (i.e., the proportion of cells that comprise the proliferating population), (2) the length of the cell cycle, and (3) the length of the DNA-synthetic phase (S-phase) for proliferative cells in the dentate gyrus of the mouse. On postnatal day 20 (P20), C57BL/6J mice were injected with BUdR at two hour intervals for a total period of 12 hours. Animals were sacrificed at selected intervals, and the brains were processed for immunohistochemistry using a monoclonal antibody directed against single-stranded DNA containing BUdR. The numbers of BUdR-labelled and unlabelled cells in sections through the hilus of the dentate gyrus were counted. The number of BUdR-labelled cells increased linearly from an initial value of about 12% of the total number of cells to a maximum value of just over 24% of the total. These findings indicate that, at P20, a maximum of 24.2 +/- 1.2% of the cells in the dentate hilus are part of the proliferating population. The calculated length of the cell cycle of the cells comprising the intrahilar proliferative zone was estimated to be 16.1 +/- 0.8 h. The length of the S-phase was estimated at 8.0 +/- 0.4 h. In addition, mathematical analysis, using one and two population models, indicates that over 90% of the proliferating cells in the dentate hilus at this age comprise a single population at least in terms of the lengths of the cell cycle and the S-phase.(ABSTRACT TRUNCATED AT 250 WORDS)

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