Marrow Regulating Factors (MRF) and Radiation Chimeras : A Model for Bone Marrow-Directed Immunity

The bone marrow microenvironment still constitutes a completely unexplored continent in spite of the fact that, in adult organisms, the bone marrow is the principal hemopoietic organ and that lympho-hemopoiesis represents the vital and ultimate result of a finely modulated course of proliferative and differentiative events. The bone marrow is populated by a huge variety of cells of very different embryologic origin (bone, nerve, endothelial, reticular, hemopoietic) which live in close contact and show very different stages of maturation. Signals of disparate origin and character (neural, hormonal, chemical mediators, blood pressure variations, temperature, oxygen tension, ionic changes, pH, a.o.) are all detected, elaborated and transmitted in the bone marrow. We know practically nothing of the complexity, significance and functions of this ancient organ, except that it delivers mature cells to the lymphatic tissues and to the blood and that some of these cells (B cells) are relevant for synthesis of antibodies (Kelemen et al., 1979; Wolf, 1979; Lichtman, 1981).