TFAP2C Safeguards Weismann's Barrier to Preserve Human Germline Identity During Primordial Germ Cell Specification from Lineage Primed Progenitors

The germline is fundamental to reproduction. How germline cell precursors, called primordial germ cells (PGCs), establish and maintain germline identity during peri-implantation development in humans remains elusive. Here, we show using human embryo attachment culture that human primordial germ cell (hPGC) specification begins at day 12 post fertilization. Next, using single cell RNA-sequencing of more than 100,000 cells during differentiation into hPGC-like cells (hPGCLCs), we show that hPGCLC specification in vitro involves first resetting pluripotency towards a new transitional state of pluripotency with shared characteristics between naive and primed, followed by differentiation into lineage primed TFAP2A+ progenitors that serve as a common precursor to amnion gastrulating cells and hPGCLCs. At the point of hPGCLC specification, TFAP2C functions upstream of SOX17 to protect the germline cells from crossing the Weismann’s barrier to adopt somatic cell fates. Keeping cells within Weismann’s barrier ensures the transmission of hereditary material to future generations.