Ontogenic studies of lymphoid progenitor cells in human bone marrow

The enzyme terminal deoxynucleotidyl transferase (TdT) is considered a useful marker of human bone marrow lymphoid progenitor cells (i.e., those cells that lack surface immunoglobulin and sheep erythrocyte receptors). The cell surface molecule p24 (identified by monoclonal antibody BA-2) has also been identified on a significant number of bone marrow lymphoid progenitor cells. The current study was undertaken to analyze adult, pediatric, and fetal bone marrow lymphoid cells for the expression of p24 and TdT. We document, for the first time, that fetal bone marrow represents a highly enriched source of TdT+ cells. We also show that dramatic quantitative differences exist in the percentage of p24+ and TdT+ cells from human bone marrow at different stages of ontogeny, with highest numbers in fetal marrow, intermediate numbers in pediatric marrow, and lowest numbers in adult marrow. Such differences are consistent with the hypothesis that marrow lymphopoiesis is an active process early in ontogeny that decreases with age.