LOCAL HOMOGENEITY OF CELL-CYCLE LENGTH IN DEVELOPING MOUSE CORTEX

We have measured the amount of variation in the length of the cell cyc le for cells in the pseudostratified ventricular epithelium (PVE) of t he developing cortex of mice on embryonic day 14. Our measurements wer e made in three cortical regions (i.e., the neocortex, archicortex, an d periarchicortex) using three different methods: the cumulative label ing method (CLM), the percent labeled mitoses (PLM) method, and a comp arison of the time needed for the PLM to ascend from 0 to 100% with th e time needed for the PLM to descend from 100 to 0%. These 3 different techniques provide different perspectives on the cytokinetic paramete rs. Theoretically, CLM gives an estimate for a maximum value of the to tal length of the cell cycle (T-C), whereas PLM gives an estimate of a minimum value of T-C. The difference between these two estimates indi cates that the range for T-C is +/-1% of the mean T-C for periarchicor tex, +/-7% for neocortex, and +/-8% for archicortex. This was confirme d by a lengthening of the PLM descent time in comparison with its asce nt time. The sharpness of the transitions and the flatness of the plat eau of the PLM curves indicate that 99% of the proliferating cells are within this narrow estimated range for T-C; hence, only similar to 1% deviate outside of a relatively restricted range from the average T-C of the population. In the context of the possible existence within th e cortical PVE of two populations with markedly dissimilar cell cycle kinetics from the mean, one such population must comprise similar to 9 9% of the total population, and the other, if it exists, is only simil ar to 1% of the total. This seems to be true for all three cortical re gions. The narrow range of T-C indicates a homogeneity in the cell cyc le length for proliferating cells in three different cortical regions, despite the fact that progenitor cells of different lineages may be p resent. It further predicts the existence of almost synchronous interk inetic nuclear movements of the proliferating cells in the ventricular zone during early development of the cerebral cortex.