The genesis of human hematopoietic stem cells.

Developmental hematopoiesis consists of multiple, partially overlapping hematopoietic waves that generate the differentiated blood cells required for embryonic development while establishing a pool of undifferentiated hematopoietic stem cells (HSC) for post-natal life. This multi-layered design where active hematopoiesis migrates through diverse extra- and intra-embryonic tissues has made it difficult to define a roadmap for generating HSCs versus non-self-renewing progenitors, especially in human. Recent single cell studies have helped identify the rare human HSCs at stages when functional assays are unsuitable to distinguish them from progenitors. This approach has made it possible to track the origin of human HSCs to unique type of arterial endothelium in the aorta-gonad-mesonephros (AGM) region, and document novel benchmarks for HSC migration and maturation in the conceptus. These studies have delivered new insights into the intricate process of HSC generation and provided tools to inform the in vitro efforts to replicate the physiological developmental journey from pluripotent stem cells (PSC) via distinct mesodermal and endothelial intermediates to HSCs.