Preimplantation development of the mammalian embryo consists of stages that include formation of the zygote, blastocyst formation and implantation of the embryo into the uterus. Depending on the animal, first few cleavages of the early embryo is fully supported by translation of maternal transcripts and use of maternal proteins. After this period, the preimplantation embryo starts to transcribe from its own genome and produce products, which are necessary for further development. Eventually, differential gene expression results in production of three cell types in the preimplantation embryo; an outer transporting polarized epithelium (trophoblast) and two cell types of primitive endoderm (hypoblast), and epiblast in the inner cell mass. After implantation, the trophoblast and hypoblast give rise to extra-embryonic tissues and epiblast cells form primarily the embryo proper. Expression of maternal and embryonic transcripts and proteins, and differential expression of these products that lead to differentiation of embryonic cells are all highly coordinated events, which need to be temporally and spatially regulated during this period of development. In this review article mechanisms and paradigms that may define and regulate these cellular activities leading to the first cellular differentiation of life are presented. Considering the abundance of research data on the preimplantation development of rodents, in this review we will mainly focus on the mouse model.

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