Blood cells are constantly produced in the bone marrow (BM) of adult mammals. This
constant turnover ultimately depends on a rare population of progenitors that displays self-
renewal and multilineage differentiation potential, the hematopoietic stem cells (HSCs). It is
generally accepted that HSCs are generated during embryonic development and
sequentially colonize the fetal liver, the spleen, and finally the BM. Here we discuss the
experimental evidence that argues for the extrinsic origin of HSCs and the potential …