Possible differences in unperturbed in vivo fate and transplantability. At key developmental time points, noninvasive fate mapping studies and phenotypic analyses indicate the presence of far more HSC-fated precursors than are capable of engraftment in transplantation assays. At E10 in the aortic lumen and E12 to E14 in the FL, a few rare HSC-committed hematopoietic stem and progenitor cells (HSPCs) are already sufficiently mature to be transplantable (green), but most HSC-fated HSPCs are not (brown). A small transplantable pool of HSPCs may also be present that would never realize an adult HSC fate without transplantation (blue circles). Differences in unperturbed in vivo fate vs transplantation potential may arise from immaturity of HSPCs at the time of transplantation, stresses associated with ex vivo manipulations, and niche availability.