INTERKINETIC AND MIGRATORY BEHAVIOR OF A COHORT OF NEOCORTICAL NEURONS ARISING IN THE EARLY EMBRYONIC MURINE CEREBRAL WALL

Neocortical neuronogenesis occurs in the pseudostratified ventricular epithelium (PVE) where nuclei of proliferative cells undergo interkine tic nuclear movement. A fraction of daughter cells exits the cell cycl e as neurons (the quiescent, or Q, fraction), whereas a complementary fraction remains in the cell cycle (the proliferative, or P, fraction) . By means of sequential thymidine and bromodeoxyuridine injections in mouse on embryonic day 14, we have monitored the proliferative and po st-mitotic migratory behaviors of 1 and 2 hr cohorts of PVE cells defi ned by the injection protocols. Soon after mitosis, the Q fraction par titions into a rapidly exiting (up to 50 mu m/hr) subpopulation (Q(r)) and a more slowly exiting (6 mu m/hr) subpopulation (Q(s)). Q(r) and Q(s) are separated as two distributions on exit from the ventricular z one with an interpeak distance of similar to 40 mu m. Cells in Q(r) an d Q(s) migrate through the intermediate zone with no significant chang e in the interpeak distance, suggesting that they migrate at approxima tely the same velocities. The rate of migration increases with ascent through the intermediate zone (average 2-6.4 mu m/hr) slowing only tra nsiently on entry into the developing cortex. Within the cortex, Q(r) and Q(s) merge to form a single distribution most concentrated over la yer V.