Changes in S-phase associated with differentiation of mouse embryos in culture from blastocyst to early somite stage

Mouse embryos, cultured for 8 days from the blastocyst (stage 5 of Theiler, 1972) to the 10--12 somite stage (stage 13) were labeled at daily intervals with 5 muCi/ml 3H-thymidine for 2 h, sectioned, and processed for autoradiography. The frequency of labeled cells, as well as the intensity of labeling, exhibited a progressive decline during the period of growth of embryos in culture. Endoderm and ectoderm cells were characterized by significant differences in the labelling index and the rate of DNA syntheses. These changes indicate that, during mammalian embryogenesis, the relative duration of the S-phase declines and that the absolute duration of the S-period expands.