VARIABILITY AND PARTIAL SYNCHRONY OF THE CELL-CYCLE IN THE GERMINAL ZONE OF THE EARLY EMBRYONIC CEREBRAL-CORTEX

Cell cycle parameters were estimated using the cumulative H-3-thymidin e S-phase labeling and percentage of labeled mitoses methods in the em bryonic day 14 and 15 germinal zone of the rat cerebral cortex. The sh ortest cell cycle time was seen in the dorsal neocortex and the longes t in the lateral neocortex and fimbria (the latter also had a low grow th fraction). No differences were observed in cell cycle times between the cells in the ventricular and subventricular zone in the same neoc ortical region. The results suggest gradients of lengthening cell cycl e times extending ventrolaterally and ventromedially from the dorsomed ial neocortex. Although a majority of proliferating cells in individua l cortical regions seem to belong to one population in terms of cell k inetics, several pieces of evidence suggest some heterogeneity: the as ymmetric shapes of the percentages of labeled mitoses curves, the smal l population of noncycling neuroepithelial cells in the neocortex and mesocortex, and small population of cells that become pyknotic. Groups of DNA-synthesizing nuclei that were ectopically located in the inner half of the ventricular zone also indicate the existence of different subpopulations of neuroepithelial cells. In addition, after a pulse i njection of H-3-thymidine the germinal zone is characterized by altern ating clusters of heavily and lightly labeled cell nuclei that may ref lect the simultaneous passage of a cluster of cells through the same p ortion of S-phase. We suggest that partial cell cycle synchrony within groups of ventricular cells may explain the presence of these iterati ve cell kinetic patterns in the developing cortex. (C) 1995 Wiley-Liss , Inc.