Figure 1.

The development of early mammalian embryos. The embryo development starts from a zygote. Then, it enters the cleavage process and divides into two cells, four cells, eight cells, etc., and to morula stage. After that, it will form a blastocyst, consisting of trophoblasts, blastocyst cavity, and the ICM. Blastomeres isolated from the ICM of mammalian embryos could be cultured in vitro to generate pluripotent ESCs. Upon implantation, ICM from the blastocyst will differentiate into primitive endoderm and epiblasts, with the latter further developing into the three-germ layers of an embryo, while the trophoblasts will become the placenta.