Multiple waves of fetal-derived immune cells constitute adult immune system.

It has been over three decades since Drs. Herzenberg and Herzenberg proposed the layered immune system hypothesis, suggesting that different types of stem cells with distinct hematopoietic potential produce specific immune cells. This layering of immune system development is now supported by recent studies showing the presence of fetal-derived immune cells that function in adults. It has been shown that various immune cells arise at different embryonic ages via multiple waves of hematopoiesis from special endothelial cells (ECs), referred to as hemogenic ECs. However, it remains unknown whether these fetal-derived immune cells are produced by hematopoietic stem cells (HSCs) during the fetal to neonatal period. To address this question, many advanced tools have been used, including lineage-tracing mouse models, cellular barcoding techniques, clonal assays, and transplantation assays at the single-cell level. In this review, we will review the history of the search for the origins of HSCs, B-1a progenitors, and mast cells in the mouse embryo. HSCs can produce both B-1a and mast cells within a very limited time window, and this ability declines after embryonic day (E) 14.5. Furthermore, the latest data have revealed that HSC-independent adaptive immune cells exist in adult mice, which implies more complicated developmental pathways of immune cells. We propose revised road maps of immune cell development.