The developmental journey of human HSCs. Schematic diagram showing human HSCs throughout gestation as they migrate between intra and extraembryonic niches. HSCs are generated in the lateral plate mesoderm–derived AGM region, where they show an undifferentiated and developmentally immature phenotype. As HSCs emerge from the IAHCs, they follow circulation to the placenta and the yolk through the main arterial outlets of the aorta, the umbilical and vitelline arteries. In extraembryonic sites, HSCs suppress endothelial identity and show transcriptional priming for erythro-megakaryocytic fates. HSCs return to the embryo after CS17 as they colonize the liver for developmental maturation and initiate lineage differentiation. Over time, HSCs acquire multilineage differentiation ability as they switch from erythro-megakaryocytic bias to multilineage potential, including myeloid and lymphoid lineages. They express fetal HSC surface marker GPI80 and acquire HSC maturity markers (PROM1/CD133 and antigen presentation machinery). They gradually lose intrinsic fetal properties and boost HSC self-renewal program, while transitioning toward homeostatic state. During the second trimester, HSCs transition to the BM, where they transition to deeper quiescence and complete developmental maturation. HSC shuttling between intra- and extraembryonic sites (dashed arrows) continues throughout gestation as evidenced by their presence in the placental umbilical cord blood at birth.