B cell development in the bone marrow.

Recent years have produced considerable progress in defining stages in the development of B cells in vivo, and in revealing interactions with regulatory molecules and cells. Studies of the phenotype and population dynamics of precursor B cells in mouse bone marrow have quantitated cell production at sequential steps of differentiation and have also indicated a substantial cell death. The proliferation of precursor B cells is influenced both by systemic factors and by cytokines derived from bone marrow stromal cells. In situ immunolabeling has revealed that early precursor B cells are closely associated with subosteal stromal cells, aberrant B lineage cells appear to be deleted by macrophages and terminal B cells mature within the lumen of vascular sinusoids before being released. The findings lead to working models of the in vivo differentiation, regulation and microenvironmental organization of B cell genesis in the bone marrow.