B lymphocytes simultaneously expressing surface and cytoplasmic IgM were first detected in the bursa of Fabricius from 12-day embryos lacking in detectable Ig-positive cells elsewhere. Bursal lymphocytes showed characteristic patterns of cIgM4 distribution which were related both to age and anatomic localization within the bursal follicles. Cells with diffuse cytoplasmic IgM (type 1) were the first to appear and later became the predominant cell type in the cortex. Cells containing cIgM localized in perinuclear and in Golgi regions (type 2) were the most abundant cell type in the bursa between the 16th day of incubation and the 3rd week after hatching. Cells with dispersed IgM-containing vesicles (type 3) appeared soon after hatching, and were prevalent in the medulla of mature follicles; release of IgM from type 3 cells observed at physiologic temperature was prevented by cold and by cytochalasin B. A rough correlation between the immunofluorescence patterns of cIgM distribution and the susceptibility of sIgM to be modulated by anti-µ antibodies was demonstrated. Type 1 cells required lower concentrations of anti-µ antibody for modulation of their sIgM than did type 2 and 3 cells. Except in very young birds, cells having the characteristics of bursal lymphocytes were not found in extrabursal tissues, including the bone marrow.

These results suggest that during normal development of the chicken, B cells are generated exclusively within the bursa and at least a subpopulation of B cells undergoes an unusual IgM secretory stage in differentiation. The data also suggest that tolerance or clonal abortion of B cells should be most easily accomplished in ovo.

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This work was supported by National Cancer Institute Grants CA 16673 and 13148.

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