| Research Abstract |
Analysis of cell cycle regulation in neocortical neuronogenesis has been undertaken in vitro in the embryonic day 13 mouse. Explants of the dorsomedial region of the cerebral wall, implanted in collagen gel, were incubated for 1 hr in minimal medium and then in minimal medium containing BUdR.After 30 mm with BUdR (1.5 hr in vitro), the labeling index (LI, labeled/total cells) was 0.41 as compared to the previously determined LI of 0.38 in vivo. The in vitro LI remained at 0.4 over the next 7 hr of continued exposure to BUdR and then ascended linearly to asymptote at 0.73. These observations establish (1) that there is a flux of cells from G1 to S for the first 1.5 hr in vitro, (2) that this G1 to S flux is then interrupted for the following 7 hr but (3) that G1 to S flux is subsequently reestablished. In this same system, mitotic figures were first labeled only after a 5 hr exposure to BUdR but then over 2 more hours all mitoses became labeled. In vivo, by contrast, all mitoses become
… More
labeled during 2 hr of BUdR exposure. By implication there is a transient arrest in the progression from S to M phase which is evidently overcome within 5 hr of explantation. The duration of the reestablished cell cycle and those of G1, Sand combined G2-M phases were estimated to be 19.2, 6.3 -8.3, 8.8, and 2.0 -4.0 hr, respectively. The leaving (Q) fraction of the cycle (0,64) was twice the in vivo value. Two thirds of the Q fraction cells remained in the ventricular epithelium, resulting in a substantially low growth fraction of 0.73 as compared with 1.0 in vivo.
Also examined in a cerebral wall explant in vitro was the consequences of basic fibroblast growth factor (b-FGF) and the gap junction uncoupling agent, 1-octanol. LI was determined in the sector of the medial and lateral cortical zones, corresponding to areas 1 and 40, respectively. In area 40, the overall LI in b-FGF-exposed explants was higher and that in 1-octanol-exposed explants was lower than the LI in control explants. Those observations suggest that b-FGF reduces Q fraction while 1-octanol increases the fraction. Less
|
