The establishment and regulation of human germ cell lineage

Abstract

The specification of primordial germ cells (PGCs) during early embryogenesis initiates the development of the germ cell lineage that ensures the perpetuation of genetic and epigenetic information from parents to offspring. Defects in germ cell development may lead to infertility or birth defects. Historically, our understanding of human PGCs (hPGCs) regulation has primarily been derived from studies in mice, given the ethical restrictions and practical limitations of human embryos at the stage of PGC specification. However, recent studies have increasingly highlighted significant mechanistic differences for PGC development in humans and mice. The past decade has witnessed the establishment of human pluripotent stem cell (hPSC)-derived hPGC-like cells (hPGCLCs) as new models for studying hPGC fate specification and differentiation. In this review, we systematically summarize the current hPSC-derived models for hPGCLC induction, and how these studies uncover the regulatory machinery for human germ cell fate specification and differentiation, forming the basis for reconstituting gametogenesis in vitro from hPSCs for clinical applications and disease modeling.

Fig. 1

The cycle of human germ cell lineage Life begins with the fertilization of the egg by sperm, mature germ cells for reproduction. The fertilized egg develops into a blastocyst with epiblasts that will differentiate into the embryo proper. Human primordial germ cells (hPGCs) (colored red) are specified around the time of gastrulation (nascent hPGCs). After that, these hPGCs migrate along the hindgut (migrating hPGCs) to meet the gonadal somatic precursor cells to form the embryonic gonads (gonadal hPGCs), where they keep differentiation and maturation until they form sperm and eggs in the adult to initiate a new cycle

Fig. 2

Summary of hPSC-derived hPGCLC induction methods (A) Schematic illustration of hPGCLC induction in floating 3D aggregates systems, including the hPGCLCs induced from 4i hESCs, iMeLCs, formative and resetting hPSCs. hPGCLCs are shown in red. (B) Schematic illustration of hPGCLC induction in 2D adhesion systems, including the BMEx, monolayer, and micropatterned methods. (C) Schematic illustration of hPGCLC induction by hPSC-derived embryo-like models, including the Gel-3D and microfluidic methods

Fig. 3

Regulation network for hPGC specification and differentiation ACTA/WNT/NODAL activates the expression of EOMES, which is critical for germline competency. TET1/2/3 maintains the expression of NANOG which inhibits the expression of OTX2 to ensure competency for hPGCLC induction. BMP4 and ACTA/WNT/NODAL signaling pathways promote hPGCLC specification by regulating TFAP2C and SOX17, the key regulators of hPGCLC specification. SOX17 is activated by EOMES and TET1/2/3 and inhibited by NANOG. On the other hand, TFAP2C and SOX17 function downstream of GATA3 and upstream of PRDM1. OCT4 functions downstream of PAX5 and TFAP2C, while PRDM1 functions downstream of PAX5 and TFAP2C, and upstream of PRDM14 to composite the regulator network for hPGCLC specification. Finally, TFAP2C activates SOX15 to upregulate ETV5, PRDM14, and NANOG to maintain the hPGCLC fate. Arrows and blunt arrows indicate positive and negative regulation, respectively. Dashed lines indicate possible regulation