Development of B lymphocytes-1,2.

A rough outline of the development pathway of stem cells to B lymphocytes in mammals is beginning to emerge. The stem cell is most probably either identical to or closely related to the spleen colony-forming unit or hemopoietic stem cell. The stem cell appears not to be specificity-restricted and to lack Ig surface receptors. Whether part of B-cell development in mammals takes place in a specialized lymphoid environment, analogous to the bursa of Fabricius in birds, is still unresolved. The bone marrow does not form such an environment. The bone marrow and spleen, but not the lymph nodes, of mice contain a nondividing B-cell precursor population at an intermediate stage of development between the stem cell and B cell. This population has Ig surface receptors and is specificity-restricted. It cannot be initiated into differentiating into antibody-producing cells. However, under appropriate conditions it can give rise to functional B cells within 3 days. Newly-formed B cells (designated B1) are physically different from mature B cells (designated B2) and appear to be a dividing population of cells differentiating from the B-cell precursor to B2 nondividing cell populations.