Printed in Great Britain DNA synthesis in the preimplantation mouse embryo

Strain PO preimplantation embryos were labelled with [3H]thymidine. The incorporation of the label was studied by autoradiography of air-dried and serially sectioned embryos. DNA amounts were measured with a microdensito-meter. The following observations were made at the 5- to 16-cell stages. All nuclei con-tained 2C-4C amounts of DNA and all could eventually synthesize DNA. However, after short labelling intervals, unlabelled nuclei were found with 2C and AC amounts of DNA. We concluded that both the Gx and the G2 phases of the cell cycle were present at this time. Embryos were found in which the S phase of the 4th and the 5th cell cycles post fertilization overlapped. In 12- to 15-cell embryos which contained inside cells it was found that the inside cells were produced by one of the first four 8-cell-stage blastomeres to divide. The inside cells of 9- to 256-cell embryos had a significantly higher labelling index than the outside cells and the number of inside cells increased faster than the number of outside cells during development. We concluded that the inside cells were dividing faster than the outside cells. Blastocysts which had developed in vivo or in vitro contained nuclei with greater than AC amounts of DNA. We concluded that the development of excess DNA amounts does not depend on the maternal environment. These nuclei which con-tained greater than AC amounts of DNA were labelled after short exposures to radioactivity. We concluded that it was likely that they were becoming polyploid