Cell-stroma interactions in monocytopoiesis

We have used immunohistochemistry to distinguish monocytes from early granulocyte precursors in trephine biopsies, in order to determine the distribution of monocytopoiesis within bone marrow. Developing granulocytes and monocytes have extensively overlapping immunophenotypes, but differential expression of calgranulin by monocytes and granulocytes during their maturation permitted the use of this antigen as a marker of bone marrow monocytes. In addition to morphologically normal bone marrow biopsies, in which monocyte numbers are relatively low, we studied pathological conditions in which either monocytopoiesis or granulopoiesis is selectively increased. By contrast with the highly zonal distribution of developing granulocytes, we found that monocytes were dispersed singly throughout the bone marrow. There was no evidence of preferential localisation of monocytes to particular stromal compartments. We hypothesise that developing monocytes are highly mobile within the bone marrow stroma and are relatively independent of physical stromal contacts for differentiation signals.