The germ cell fate of cynomolgus monkeys is specified in the amnion

The germ cell lineage ensures reproduction, heredity and evolution. However, the mechanism for germ cell specification in primates, including humans, has been unknown. In primates, the pluripotent epiblast segregates the amnion upon implantation and thereafter initiates gastrulation to generate three germ layers. Here, we show that in cynomolgus monkeys, the SOX17/BLIMP1/TFAP2C-positive primordial germ cells (cyPGCs) arise from the dorsal amnion at embryonic day (E) 11. cyPGCs then migrate down beneath the epiblast and expand their numbers, through proliferation and continuous recruitment from the posterior amnion, around posterior yolk sac endoderm by E17. Remarkably, the amnion itself expresses BMP4, a cytokine potentially critical for PGC specification, thereby inducing cyPGCs in an autocrine fashion. Consistently, the amnion expresses T, a key mesodermal factor, prior to the onset of gastrulation. Our study demonstrates the origin of cyPGCs in the amnion, which undergoes a unique morphogenetic event prior to and independent from the gastrulation. Overall design: Single cell transcriptome analysis of cynomolgus monkey embryo E13-20 using SC3-seq technology.