ABSTRACT
The cell cycle of mesencephalic ventricular cells was studied by means of tritiated thymidine radioautography during normal and abnormal development in the looptail (Lp) mutant mouse. The total generation time, DNA-synthetic (S), premitotic (G2), mitotic (M), and postmitotic (G1) periods were compared in looptail homozygotes (Lp/Lp) which exhibit neural dysraphism and in their normal littermates ( + / + ) at 10 and 11 days’ gestation. Both normal and abnormal embryos showed a chronological lengthening of the generation time between the 10th and 11th day. However, the generation time in the 10-day abnormal brains was 4·5 h longer than that in normal littermates, and the difference was the result of an increase mainly in the M and G1 periods. At 11 days of gestation the generation time in the abnormal brains increased by 5·0 h over that of the normal brains. Since the cell cycle was actually prolonged in the defective brains, the increased numbers of mitotic figures which characterize the looptail homozygote brain during early development appear to reflect the lengthening of the mitotic period rather than increased proliferation.
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