Influence of monocyte-macrophage lineage cells on alkaline phosphatase activity of developing osteoblasts derived from rat bone marrow stromal cells.

We have studied the osteogenic changes of the marrow stromal cells cultured from bone marrow cells of Wistar rats. After the stromal cells became confluent, several clusters of cuboidal cells appeared. These cuboidal cells showed strong alkaline phosphatase (ALP) activity and revealed some osteoblastic features electron-microscopically. In the culture, monocyte-macrophage-lineage cells (MMLC) proliferated in contact with the stromal cells. The MMLC exist near the bone cells in vivo. Yet the exact role of the MMLC is not clear. In order to evaluate how the MMLC influence the stromal cells, the ALP activity of the stromal cells in the culture condition containing a large number of MMLC (group A) was compared with that in the condition in a small number of MMLC (group B). The ALP activity in group A (11.28 +/- 1.04 Units/mg protein) was significantly higher than that in group B (6.58 +/- 0.38 Units/mg protein). This suggests that the MMLC stimulate the osteogenic differentiation of the stromal cells.