Developmental control of human preimplantation embryos: a comparative approach.

The period for which oocyte-derived factors are engaged in the control of human embryonic development involves at least the first four cell cycles after fertilization. The maternal-embryonic transition in humans 8- to 16-cell embryos is a relatively vulnerable process, the failure of which entails developmental arrest of the given blastomere. The very early cellular differentiative events in human embryos, including blastomere surface polarization and segregation of the inner cell mass and trophectoderm cell lineages, appear to be dependent largely on the maternal genetic program. However, the embryonic genome is required for the formation of the blastocyst cavity, which is necessary to allow further differentiation of the first two embryonic tissues. Blastomeres with major developmental defects are removed by fragmentation and their loss is compensated by proliferation of remaining normal blastomeres. This mechanism is also mainly responsible for the regulation of ploidy through elimination of aneuploid blastomeres. The data presented suggest that embryos of individual mammalian species may differ in the timing of relevant developmental changes at the cellular and molecular levels. This should be taken into account when findings obtained on embryos of one species are used to anticipate the behavior of embryos of another species under identical conditions.