CUTTING EDGE - PRESENCE OF PROGENITORS RESTRICTED TO T-LINEAGE, B-LINEAGE, OR MYELOID-LINEAGE, BUT ABSENCE OF MULTIPOTENT STEM-CELLS, IN THE MURINE FETAL THYMUS

The most immature population of fetal thymus (FT) cells has been shown to generate not only T but also B and myeloid cells. The present stud y was undertaken to clarify whether such a multipotent activity of the earliest population of FT cells is attributed to multipotent hemopoie tic progenitors or to a mixture of lineage-restricted progenitors, Exa mination of individual FT progenitors by a recently established clonal assay system, which is able to determine the developmental potential of each progenitor toward T, B, and myeloid lineages, elucidated that a large majority of progenitors in FT were restricted to the T cell li neage. Presence of a small number of B or myeloid lineage-restricted p rogenitors was also disclosed. No multipotent progenitors, however, we re detected in FT. These results are consistent with our recent findin g that restriction of hemopoietic stem cells to T, B, and myeloid line ages takes place in the fetal liver.