T cell progenitors emerge earlier than B cell progenitors in the murine fetal liver

The developmental potential of individual cells in the Lin(-)c-kit(+)CD45()IL-7R(+) (IL-7R(+)) population from murine fetal liver was investigated us ing a clonal assay capable of determining the potential of a progenitor to give rise to myeloid, T, and B cells. Unipotent progenitors generating T ce lls (p-T) or B cells (p-B) but not other types of progenitors were found in the IL-7R(+) population. A large proportion of progenitors at day 12 of ge station are p-T, whereas the frequency of p-T dramatically decreases with g estational age. In marked contrast, p-B are very rare by day 12, but they r apidly increase thereafter. These findings strongly suggest that the commit ment of multipotent progenitors to T and B cell lineages occurs independent ly.