Genesis of B lymphocytes in the bone marrow: Extravascular and intravascular localization of surface IgM‐bearing cells in mouse bone marrow detected by electron‐microscope radioautography after in vivo perfusion of 125I anti‐IgM antibody

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Abstract
The role of mammalian bone marrow in generating surface IgM (sIgM)-bearing B lymphocytes is reviewed. Precursor cells in the marrow give rise to large, rapidly dividing cells bearing free cytoplasmic .mu. chains (c.mu.). The progeny of the large c.mu.+ cells form a population of small, nondividing c.mu.+ cells that mature into small lymphocytes, progressively expressing sIgM and other B-cell surface membrane components. Newly formed sIgM+ cells soon migrate through the bloodstream to the spleen and other lymphoid tissues, where they may die after a short lifespan or be activated to produce antibody molecules. The large-scale lymphocytopoiesis in the bone marrow thus maintains a population of rapidly renewed virgin B lymphocytes in the peripheral lymphoid tissues. This process continuously creates and selects B cell clones with the wide range of antibody specificities necessary to mediate primary humoral immune responses through postnatal life. A technique for perfusing radiolabeled anti-IgM antibodies in young mice has now permitted sIgM+ cells to be detected radioautographically in histological preparations of bone marrow under the EM. Small sIgM+ lymphocytes are situated either singly or in small groups throughout the extravascular hemopoietic compartment of the bone marrow, often near sinusoid walls adjacent to late erythroblasts and reticular cells. Some regional concentrations of sIgM+ cells are apparent. sIgM+ cells also appear in transit through the sinusoidal endothelium and are markedly concentrated in the lumen of some sinusoids. Intrasinusoidal sIgM+ small lymphocytes have high densities of sIgM and long microvilli, on which sIgM molecules are concentrated. The localization and cell associations of specifically identified sIgM+ small lymphocytes in the extravascular marrow compartment are revealed. These cells may also undergo a transient intravascular storage and maturation phase. Use of this in vivo immunolabeling technique to detect other cell-surface markers may further elucidate the microenvironmental basis of B lymphocyte genesis in the bone marrow.