The human germ cell lineage originates from primordial germ cells (PGCs), which are specified at approximately the third week of development. Our understanding of the signaling pathways that control this event has significantly increased in recent years and that has enabled the generation of PGC-like cells (PGCLCs) from pluripotent stem cells in vitro. However, the signaling pathways that drive the transition of PGCs into gonia (prospermatogonia in males or premeiotic oogonia in females) remain unclear, and we are presently unable to mimic this step in vitro in the absence of gonadal tissue. Therefore, we have analyzed single-cell transcriptomics data of human fetal gonads to map the molecular interactions during the sex-specific transition from PGCs to gonia. The CellPhoneDB algorithm was used to identify significant ligand–receptor interactions between germ cells and their sex-specific neighboring gonadal somatic cells, focusing on four major signaling pathways WNT, NOTCH, TGFβ/BMP, and receptor tyrosine kinases (RTK). Subsequently, the expression and intracellular localization of key effectors for these pathways were validated in human fetal gonads by immunostaining. This approach provided a systematic analysis of the signaling environment in developing human gonads and revealed sex-specific signaling pathways during human premeiotic germ cell development. This work serves as a foundation to understand the transition from PGCs to premeiotic oogonia or prospermatogonia and identifies sex-specific signaling pathways that are of interest in the step-by-step reconstitution of human gametogenesis in vitro.

The second trimester of human development is marked by asynchronous gonadal development hampering the isolation of homogenous populations of early and late fetal germ cells (FGCs). We evaluated the feasibility of using surface markers TNAP, PDPN, EPCAM and ITGA6 to isolate FGCs as well as human primordial germ cell-like cells (hPGCLCs) derived from embryonic stem cells (hESCs) from both sexes by fluorescence-activated cell sorting (FACS). Our results suggest that a combination of TNAP and PDPN was sufficient to separate populations of premeiotic FGCs and hPGCLCs in both sexes. This combination of antibodies also proved efficient in separating ‘mitotic’ from ‘retinoic-acid responsive’ female FGCs. Furthermore, we report that the differentiation efficiency of TNAP+PDPN+ hPGCLCs from hESCs was sex-independent, but the ability to propagate differed considerably between the sexes. In contrast to male, female hPGCLCs retained their characteristics and exhibited robust colony-forming ability when cultured for five days in medium containing LIF, forskolin and FGF2. We conclude that marked sex differences exist in the isolation and propagation of human FGCs and hPGCLCs. Our study provides novel insights relevant for the optimization of in vitro gametogenesis in humans.

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Human pluripotent stem cells (hPSCs) are not only a promising tool to investigate differentiation to many cell types, including the germline, but are also a potential source of cells to use for regenerative medicine purposes in the future. However, current in vitro models to generate human primordial germ cell-like cells (hPGCLCs) have revealed high variability regarding differentiation efficiency depending on the hPSC lines used. Here, we investigated whether differences in X chromosome inactivation (XCI) in female hPSCs could contribute to the variability of hPGCLC differentiation efficiency during embryoid body (EB) formation. For this, we first characterized the XCI state in different hPSC lines by investigating the expression of XIST and H3K27me3, followed by differentiation and quantification of hPGCLCs. We observed that the XCI state did not influence the efficiency to differentiate to hPGCLCs; rather, hPSCs derived from cells isolated from urine showed an increased trend towards hPGCLCs differentiation compared to skin-derived hPSCs. In addition, we also characterized the XCI state in the generated hPGCLCs. Interestingly, we observed that independent of the XCI state of the hPSCs used, both hPGCLCs and soma cells in the EBs acquired XIST expression, indicative of an inactive X chromosome. In fact, culture conditions for EB formation seemed to promote XIST expression. Together, our results contribute to understanding how epigenetic properties of hPSCs influence differentiation and to optimize differentiation methods to obtain higher numbers of hPGCLCs, the first step to achieve human in vitro gametogenesis.

During development of the male reproductive tract, the human rete functions as a bridging structure between the seminiferous tubes and the efferent ducts of the epididymis. However, despite its significant contribution to human sex-specific gonadogenesis and future male fertility, the rete testis remains poorly understood. To investigate fetal rete testis development, we have performed single-cell transcriptomics on human fetal testes (and ovaries) and mesonephros/epididymis from first and second trimester. By revealing KRT19 and PAX8 as rete markers, we were able to identify the molecular signatures of the rete epithelial cells and mesonephros/epididymis epithelial cells. In the process, we have identified a population of Sertoli cells and germ cells in the rete testis. Moreover, we also revealed a small population of epithelial cells in the rete ovarii, comparable to the epithelial cells in the rete testis. Together, our study provides insights into human fetal sex-specific gonadogenesis and development of the male reproductive tract beyond the gonads.

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Current human primordial germ cell like cells (hPGCLCs) differentiation methods from human pluripotent stem cells (hPSCs) are inefficient, and it is challenging to generate sufficient hPGCLCs to optimize in vitro gametogenesis. We present a new differentiation method that uses diluted basement membrane extract (BME) and low BMP4 concentration to efficiently induce hPGCLC differentiation in scalable 2D cell culture. We show that BME overlay potentiated BMP/SMAD signaling, induced lumenogenesis and increased expression of key hPGCLC-progenitor markers such as TFAP2A and EOMES. These findings highlight the importance of (factors in) BME during hPGCLC differentiation, and demonstrate the potential of the BME-overlay method to interrogate the formation of PGCs and amnion in humans as well to investigate the next steps to achieve in vitro gametogenesis.