The effect of the splotch gene on the parameters of the cell cycle in the brain and spinal cord of mouse embryos.

The parameters of the cell cycle were determined in 10-day-old mouse embryos, homozygous for the splotch (Sp) gene, by the radioautographic method with H-3-thymidine. Normal embryos (+/+)with a similar genetic environment were used as the control. In the brain and spinal cord of Sp/Sp embryos the mitotic indices were more than twice as high as those in +/+ embryos. The rate of cell proliferation increased identically in both the dorsal and ventral parts of the neural tube. The generation time of the brain and spinal cord cells of Sp/Sp embryos was 8 h and that of normal embryos 9 h. The Sp gene decreased the duration of the S phase 1.5 h (25 percent). In addition, this gene increased the duration of the G(1) phase 0.5 h. The Sp gene evidently acts in the G(1) phase, as a result of which the duration of this phase increases while that of the S phase decreases considerably. This leads to an increase in the rate of cell proliferation and to the development of abnormalities of the CNS of homozygous embryos.