The fetal liver counterpart of adult common lymphoid progenitors gives rise to all lymphoid lineages, CD45⁺CD4⁺CD3^- cells, as well as macrophages

We identified an IL-7Rα⁺Sca-1^lowc-Kit^low population in E14 fetal liver, which is the phenotypical analog of common lymphoid progenitors (CLP) in adult bone marrow. After transfer into newborn mice, the IL-7Rα⁺Sca-1^lowc-Kit^low population rapidly differentiated into CD45⁺CD4⁺CD3^- cells, which are candidate cells for initiating lymph node and Peyer's patch formation. In addition, this population also gave rise to B, T, NK, and CD8α⁺ and CD8α^- dendritic cells. The fetal liver precursors expressed a significantly lower level of the myeloid-suppressing transcription factor Pax-5, than adult CLP, and retained differentiation activity for macrophages in vitro. We propose that the transition from fetal liver IL-7Rα⁺Sca-1^lowc-Kit^low cells to adult CLP involves a regulated restriction of their developmental potential, controlled, at least in part, by Pax-5 expression.