On the origin of macrophage heterogeneity: a hypothesis.

Data are summarized suggesting an inherent heterogeneity of bone marrow macrophage precursor cells. The possible relationship of this heterogeneity with the functional heterogeneity of blood monocytes and tissue macrophages under normal conditions and at an inflammatory site is discussed. A hypothesis suggesting that the bone marrow compartment is involved in the generation of macrophage heterogeneity is presented.