Engraftment potential of human fetal hematopoietic cells in NOD/SCID mice is not restricted to mitotically quiescent cells.

During fetal development, there is a continued demand for large numbers of primitive and mature hematopoietic cells. This demand may require that all potential hematopoietic stem cells (HSCs) migrate effectively to emerging hematopoietic sites and subsequently contribute to blood cell production, regardless of their cell cycle status. We recently established that umbilical cord blood cells in the G(1) phase of the cell cycle have a repopulating potential similar to cells in G(0), suggesting that cycling prenatal and neonatal HSCs may have the same functional capabilities described for quiescent, but not cycling, cells from adult sources. To establish the relationship between cell cycle status and hematopoietic potential at early stages of human ontogeny, the in vivo engraftment potential of mitotically defined fetal liver (FL) and fetal bone marrow (FBM) cells were examined in NOD/SCID recipients. Following transplantation of the same numbers of G(0), G(1), or S/G(2)+M CD34(+) cells from FL, equivalent percentages of recipient mice were chimeric (55%, 60%, and 60%, respectively). FBM-derived CD34(+) cells in all phases of the cell cycle engrafted in conditioned recipients and sustained human hematopoiesis, albeit at lower levels than their FL-derived counterparts. Multilineage differentiation was evident in all transplanted mice independent of the source or cell cycle status of graft cells. In addition, levels of chimerism in mice transplanted with fetal blood-derived G(0) or G(1) CD34(+) lineage-depleted cells were similar. These results support the assertion that mitotically quiescent and cycling fetal hematopoietic cells contain marrow-repopulating stem cells capable of multilineage engraftment in NOD/SCID mouse recipients.