EARLY ONTOGENY OF THE SECONDARY PROLIFERATIVE POPULATION OF THE EMBRYONIC MURINE CEREBRAL WALL

The present report is an analysis of the proliferative behavior of the secondary proliferative population (SPP) of the dorsomedial region of the embryonic mouse cerebral wall. It is based upon experiments under taken on embryonic days 14-16 (E14-E16) and exploits methods in which proliferative cells are labeled in S phase with either or both bromode oxyuridine and tritiated thymidine. The SPP, which arises from the PVE by E13, is principally the progenitor population to the neuroglial po pulation of the mature neocortex and subjacent cerebral wall. By the e nd of E14 the SPP comes to be distributed diffusely from the outer mar gin of the ventricular zone throughout subventricular zone and interme diate zone. The length of the cell cycle of the SPP is constant at app roximately 15 hr throughout this interval; thus, this population under goes 1.6 cell cycles/24 hr or 3.2 cycles in the course of the 48 hr pe riod, E14-E16. Over this 48 hr period, the SPP increases from 11% to 3 5% of the total proliferative population of the dorsomedial cerebral w all. The absolute size of the SPP increases nearly sixfold. With these values taken together it may be estimated that approximately 87% of p ostmitotic cells of the SPP reenter S phase after each cell division i n this interval which means that only approximately 13% of the prolife rative population exits the cycle. These findings illustrate the massi ve expansion of the SPP antecedent to the explosive diffusion of glial cells through the neocortex and subjacent cerebral wall as neuronal m igration comes to completion and neocortical growth and differentiatio n accelerate.