GRADIENT IN THE DURATION OF THE G1 PHASE IN THE MURINE NEOCORTICAL PROLIFERATIVE EPITHELIUM

Neuronogenesis in the neocortical pseudostratified ventricular epithel ium (PVE) is initiated rostrolaterally acid progresses caudo-medially as development progresses. Here we have measured the cytokinetic param eters and the fractional neuronal output parameter, a, of laterally lo cated early-maturing regions over the principal embryonic days (E12-E1 5) of neocortical neuronogenesis in the mouse. These measures are comp ared with ones previously made of a medial, late-maturing portion of f ire PVE. Laterally, as medially, the duration of the neuronogenetic in terval is 6 days and comprises 11 integer cell cycles. Also, in both l ateral acid medial areas the length of G1 phase (T-G1) increases nearl y 4-fold and is the only cell cycle parameter to change, a progresses essentially identically laterally and medially with respect to the suc cession of integer cell cycles. Most importantly, from E12 to E13 ther e is a steeply declining lateral to medial gradient in T-G1. The gradi ent is due both to the lateral to medial graded stage of neuronogenesi s and to the stepwise increase in T-G1 with each integer cycle during the neuronogenetic interval. To our knowledge this gradient in T-G1 of the cerebral PVE is the first cell biological gradient to be demonstr ated experimentally in such an extensive proliferative epithelial shee t. We suggest that this gradient in T-G1 is the cellular mechanism for positionally encoding a protomap of the neocortex within the PVE.