In mice, around embryonic day (E) 6.0, at the time when gastrulation begins, mouse PGCs (mPGCs) are specified as about 10 cells in the most posterior part of epiblast, the cup-shaped simple pluripotent epithelium, from which all of the embryonic portion are derived (Fig. 1.1). This process occurs in response to bone morphogenetic protein 4 (BMP4) [25] produced by the extraembryonic ectoderm (ExE), an extraembryonic part of conceptus in intimate contact with the edge of the epiblast (Fig. 1.1).　https://www.sciencedirect.com/topics/engineering/extraembryonic-ectoderm