B-CELLS ARE GENERATED THROUGHOUT LIFE IN HUMANS

This analysis of B cell development as a function of age reveals a rel atively widespread distribution of progenitor B (pro-B), pre-B, and B cells in fetal tissues, and thus supports the idea of a multifocal ori gin of B lineage cells during embryonic development, From mid-gestatio n onward, the bone marrow is the major site of B cell generation in hu mans, A relatively constant ratio of bone marrow precursors to B cells of immature phenotype (CD24(high)CD10(+)CD20(low)IgD(-)) is maintaine d from mid-gestation through the eighth decade of life, The persistenc e of recombinase gene activity in pro-B cells further attests the sust ained production of B cells in bone marrow, Interestingly, a subpopula tion of B cells with mature phenotype (CD24(low)CD10(-)CD20(high)IgD()) accumulates in the bone marrow during childhood, and this becomes t he predominant B cell subpopulation in adult bone marrow, This mature population of bone marrow B cells may represent a subpopulation of rec irculating B cells that have undergone selection in the periphery.