In mice, expression of Oct4 in germ cells has been studied using a transgene in which expression of green fluorescent protein was driven by an 18-kilobase fragment encompassing the Oct-4 gene. In the fetal testes, OCT4-posi-tive germ cells were mitotically active and expressed C-KIT at Embryonic Day 11.5. By Embryonic Day 16.5, mitotic activity had ceased, and C-KIT expression was down-regulated. On the day of birth, all the OCT4-positive germ cells were C-KIT-negative. These results suggest that in contrast to those obtained during the present study, in mice, expression of C-KIT is not correlated with expression of OCT4.
A number of investigators have examined germ cell proliferation in the human testis. For example, Bendsen et al. reported that between 6- and 9-wk postconception (811 wk of gestation), the number of germ cells in the testis increased from approximately 3000 to 30 000, although the ratio of germ cells to somatic cells remained constant. In third-trimester samples, the number of germ cells has been estimated to be 3.8 X 106, which is consistent with further mitotic activity in the second trimester.