Gradient in the duration of the G1 phase in the murine neocortical proliferative epithelium

Neuronogenesis in the neocortical pseudostratified ventricular epithelium (PVE) is initiated rostrolaterally acid progresses caudo-medially as development progresses. Here we have measured the cytokinetic parameters and the fractional neuronal output parameter, a, of laterally located early-maturing regions over the principal embryonic days (E12-E15) of neocortical neuronogenesis in the mouse. These measures are compared with ones previously made of a medial, late-maturing portion of fire PVE. Laterally, as medially, the duration of the neuronogenetic interval is 6 days and comprises 11 integer cell cycles. Also, in both lateral acid medial areas the length of G1 phase (T-G1) increases nearly 4-fold and is the only cell cycle parameter to change, a progresses essentially identically laterally and medially with respect to the succession of integer cell cycles. Most importantly, from E12 to E13 there is a steeply declining lateral to medial gradient in T-G1. The gradient is due both to the lateral to medial graded stage of neuronogenesis 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 demonstrated experimentally in such an extensive proliferative epithelial sheet. We suggest that this gradient in T-G1 is the cellular mechanism for positionally encoding a protomap of the neocortex within the PVE.