Preimplantation development of the mammalian embryo.

The bases for the differentiation of an adult individual are founded during the early development of the mammalian embryo before its implantation in the uterus. After the first cleavage, which is under maternal genome control, the segmentation continues under the control of the embryonic genome. At this time polarization and flattening of the blastomeres and the formation of specialized junctions among them, characterize the foundation of two different cell populations. These will give rise to the inner cell mass and the trophectoderm, which are finally segregated during cavitation. The last step of the preimplantation development is the blastocyst hatching from the zona pellucida, that prevented the implantation until now. During the preimplantation period the embryo may be exposed to environmental chemicals via mother genital fluid and teratogenic effects have been reported during this period. Even if the differentiation of the blastomeres begins very early in the development, the embryo seems to conserve its developmental plasticity, suggesting that the mechanisms of this teratogenicity are eventually to be found at DNA level.