Background: Gametogenesis starts with the specification of human primordial germ cells (hPGCs) in the early post-implantation embryo. Subsequently, hPGCs migrate through the primitive hindgut and dorsal mesentery into the developing gonads. After sex determination, hPGCs start to differentiate into gonocytes or oogonia in the developing testis or ovary, respectively. Proposal and aims of the project: In this project, we propose to select and characterize the most competent hPGCLCs (chPGCLCs) for robust and faithful in vitro reconstitution of the human germline. At first, we purpose to characterize hPGCLCs specified from resetting precursors. After, we purpose to co-culture chPGCLCs with human hindgut organoids to model the maturation process occurring during migration. Finally, chPGCLCs will be transferred to gonadal organoid co-cultures to progress their maturation into gonocytes or oogonia. Impact of our research: Fertility rates are decreasing in developed countries and increasing in developing ones. Soon, there will be a pronounced regional imbalance in human fertility rates around the globe, which will lead to demographic problems and social-politic challenges. Therefore, basic research on in vitro systems that model human gametogenesis will be valuable tools to restore the fertility of patients affected by, for example, infertility caused by gonadotoxic treatments, low sperm counts, reduced and aged pool of oocytes, and genetic diseases. Instead, the establishment of models for in vitro spermatogenesis and oogenesis will create unprecedented platforms to investigate the mechanisms and efficiency of candidate contraceptive compounds and biomolecules.