Cell proliferation plays an important role in various differ

Cell proliferation plays an important role in various differentiation processes and needs to be tightly regulated. For example, lengthening G1 phase increases differentiation of neural stem SL-327 into neurons (Lange and Calegari, 2010; Lange et al., 2009); cyclin D in human embryonic stem cells controls balance between neuroectoderm and endoderm specification (Pauklin and Vallier, 2013); and deletion of p27 cell-cycle inhibitor prevents specification of hematopoietic cells from the yolk sac endothelium (Marcelo et al., 2013). The temporal kinetics of pre-HSC proliferation is well controlled: although mature fetal liver HSCs expand, their proliferative activity decreases compared with the AGM region and subsequently, in the adult bone marrow, switches into quiescence associated with low c-KIT expression, which is necessary to prevent exhaustion of the HSC pool (Thorén et al., 2008; Matsuoka et al., 2011; Shin et al., 2014). Our data indicate that pre-HSC expansion within the AGM region is driven by proliferation. It needs to be elucidated in future whether stage-specific proliferative changes per se play a role in HSC maturation.
Experimental Procedures
Author Contributions
Acknowledgments The authors thank J. Verth, C. Manson, J. Agnew, and R. McInnis for assistance with mouse maintenance and breeding; C. Watt., C. Flockhart, C. Forrest, A. Dyer for irradiations; O. Rodriguez, F. Rossi, and C. Cryer for cell sorting; and V. Berno and B. Vernay for help with microscopy. We thank A. Sakaue-Sawano for providing Fucci reporter mice. We thank S. Zuyev for helpful discussions on the correlation analysis. We thank S. Gordon-Keylock, A. McGarvey, J. Easterbrook, E. Crosse, and S. Heinrichs for helpful comments. This work was supported by Bloodwise (former LLR), BBSRC, MRC, and the Wellcome Trust.
Introduction During embryonic development, hematopoietic cells arise from transient endothelial-like precursors known as hemogenic endothelium (HE), which are found initially in the yolk sac vascular plexus and later in various intraembryonic and extraembryonic vessels (Frame et al., 2016; Gordon-Keylock et al., 2013; Yzaguirre and Speck, 2016). An expanding number of studies have revealed multiple overlapping waves of hematopoietic progenitors generated from HE, which give rise to increasingly diverse hematopoietic lineages, including progenitors with erythroid, myeloid, and lymphoid potential detected prior to and independently of hematopoietic stem cells (HSCs) (Frame et al., 2013; Hadland et al., 2004; Kobayashi et al., 2014; Lin et al., 2014; Yoshimoto, 2015; Yoshimoto et al., 2012). Our group previously identified B-restricted progenitors that arise prior to HSCs with distinct B cell potential that includes B-1a cells, a lineage of tissue-resident B cells implicated in innate immune responses and autoimmunity (Yoshimoto et al., 2011). Further, mature B-1a cells are serially transplantable, suggesting an embryonic origin for life-long B-1a cells independent of HSCs and conventional (B-2) B cells (Kobayashi et al., 2014). Separate studies demonstrated that highly purified long-term HSCs from the adult bone marrow, transplanted to adult recipients, do not contribute significantly to the B-1a subset of B lymphocytes (Ghosn et al., 2012), further supporting the concept of a separate embryonic origin of the innate B-1a lineage. However, a recent study in which HSCs were defined functionally by long-term contribution to multilineage hematopoiesis using cellular barcoding techniques indicated contribution to both B-2 and B-1a cells by clonal fetal stage HSCs, whereas two separate studies examining whether highly purified fetal liver HSCs contribute to B-1a cells following transplantation produced conflicting results, potentially related to differences in phenotypic markers used for sorting HSCs from the fetal liver (Ghosn et al., 2016; Kristiansen et al., 2016). These studies suggest the possibility of heterogeneity within the fetal HSC pool with regard to B-1a potential and the need to determine the existence of a common precursor for HSCs and B-1a cells during early HSC development.