2-cell block
PRE-GASTRULATION DEVELOPMENT
Although the embryos of all mammals, except those of the monotremes, eventually implant as blastocysts into the uterine tissue of the mother, during the first stages of development they are in fact free-floating in the oviduct and uterus. From fertilization of the oocyte to the blastocyst stage, the oviduct and uterus provide a perfect environment for the embryo to grow and develop. In this period, the embryo undergoes its first differentiation, while the uterus is simultaneously prepared for blastocyst implantation.
Most studies on mammalian development concern the mouse (Mus musculus) despite the fact that embryogenesis in rodents differs to some extent from that in other mammals. First cleavage of the mouse embryo is programmed by maternal mRNA, but before the second cleavage a transition from dependence on maternal mRNA to embryonic mRNA takes place and from then on embryonic gene expression regulates development and growth. Although after the two-cell stage maternal mRNA is rapidly degraded, the store of proteins already produced from the maternal messages may be detected for much longer periods.
The first differentiation event occurs in the morula, when the outer cells form trophectoderm, a cell layer mediating initial contact with the uterine wall, implantation, and invasion of uterine tissue. The trophectoderm contributes exclusively to extraembryonic structures. The inner cells of the morula remain as a group of undifferentiated cells known as the inner cell mass (ICM); these give rise to the three germ layers of the embryo proper and the extraembryonic endoderm. At embryonic day 3.5 (E3.5) a fluid-filled cavity entirely covered by the trophectoderm is formed, with the ICM present eccentrically at one pole of the embryo. Up to this stage, the embryo is surrounded by a thick membrane known as the zona pellucida, which protects it during passage through the oviduct to the uterus. On the fourth day of development, at the blastocyst stage, the embryo hatches from the zona pellucida using proteases secreted from the trophectoderm. The embryo is now ready to implant (Hogan et al., 1994