Bone marrow cells were examined by double immunofluorescent labeling techniques to detect determinants for the B lineage monoclonal antibody, 14.8, the nuclear enzyme, terminal deoxynucleotidyl transferase (TdT), cytoplasmic mu chains (c mu), and surface mu (s mu). In 8-9-wk-old C3H/HeJ mice, 14.8+ cells totalled 22.2% of all marrow cells (35 X 10(5) cells/femur). While many 14.8+ cells were c mu+ s mu- pre-B cells and s mu+ B lymphocytes (17.0%), the remainder (5.2%) were large cells lacking mu chains. After injecting vincristine sulfate, these 14.8+ mu- cells accumulated in mitosis at a rate of 13.5%/h (turnover time, 7.4 h). Their calculated total production rate (41 X 10(6) cells/whole marrow/d) exceeded that previously determined for large pre-B cells, suggesting some cell loss from the B lineage. TdT+ cells made up 1.8% of marrow cells and were mainly medium-sized cells. They all lacked mu chains, but half (0.9%) bound 14.8 antibody at low to medium intensity. Three discrete cell populations were thus defined, differing in mean cell diameter TdT+ 14.8- mu-, 9.5 micron; TdT+ 14.8+ mu-, 10 microns; and TdT- 14.8+ mu-, 11.5 micron, presumptively representing a sequence of cell stages preceding the expression of mu chains in large pre-B cells (TdT- 14.8+ c mu+ s mu-, 11.5 microns). This work provides a tentative model of early progenitor cells and their proliferation in normal marrow as a basis for studies of perturbations and the control of B lymphocytopoiesis.

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