Figure 2.

Simplified timeline of mouse embryonic and germline development. Cells of the germ lineage are depicted in green. Clockwise from upper left: fertilization of a mature oocyte by a sperm cell gives rise to the single‐cell zygote (embryonic day 1.0 [E1.0], which progresses through cleavage divisions and the morula stage to form the blastocyst [≈E4.0]). The inner cell mass (ICM) of the blastocyst is composed of pluripotent cells that give rise to the embryo. During the process of gastrulation, these pluripotent embryonic cells differentate to form the three germ layers (endoderm, ectoderm, and mesoderm). At ≈E6.5, a founding population of primordial germ cells (PGCs) are specified, then migrate (≈E7.5–11.5) to colonize the genital ridges by ≈E12.5, followed by formation of the embryonic gonads. Following birth and sexual maturation, completion of meiosis in adult animals generates mature sperm and oocytes.