Cellular stages and molecular steps of murine B-cell development.

Development of B cells in fetal liver occurs in one synchronous wave and involves probably no more than four critical divisions. This leads us to suggest that the main pool of proliferating progenitors that replenish the peripheral B-cell pool are progenitors before Ig gene rearrangement, that the four Ig gene rearrangements (DH to JH, VH to DHJH, VK to JK, and V lambda to J lambda) might occur in four critical divisions, and that a stromal-cell-dependent phase of pre-B development in which all rearrangements are made is succeeded by a stromal-cell-independent phase of sIG+ pre-B-cell maturation to mature mitogen-reactive B cells. We speculate on the molecular nature of the tightly controlled steps of Ig rearrangements during pre-B-cell development that might involve the pre-B-cell specific genes Vpre-B and lambda 5 and the B-lineage-specific gene mb-1 in interactions with stromal cells.