Characterizing the Developmental Origins and Clonal Organization of the Hematopoietic System

Abstract

It is generally thought that, at the clonal level, blood is constructed from a small number of embryonic-born hematopoietic stem cells (HSCs). These HSCs arise within the large arteries of the embryo as part of hematopoietic clusters derived from underlying hemogenic endothelium. They then migrate to the fetal liver, where they mature and amplify, and, some time after, generate the progenitors that will establish and maintain the adult blood system. This view, however, is largely based on assays that measure cellular lifespan and lineage output after embryonic disruption and transplantation; how the hematopoietic system is clonally constructed and when progenitor specification occurs in the native state are unknown. Here, we use in situ transposon tagging to characterize the embryonic origins of adult hematopoiesis. Our work defines the numbers of embryonic precursors born at multiple developmental milestones—of which there are several hundred during the definitive wave—and delineates a clonal map of their lineage output. Moreover, through clonal analysis and genetic fate mapping, we demonstrate that a large fraction of lifelong blood production derives from a population of embryonic multipotent progenitors (eMPPs), independently of traditional, adult HSCs. This specification initiates within intra-arterial hematopoietic clusters as early as embryonic day 10.5, and represents a unique, previously unappreciated molecular trajectory of emerging cluster cells. Our results thus identify eMPPs as an important source of adult blood and provide new insight into the layered construction of the mammalian hematopoietic system.