MICROVESSEL ENDOTHELIAL-CELLS AND PERICYTES INCREASE PROLIFERATION AND REPRESS OSTEOBLAST PHENOTYPIC MARKERS IN RAT CALVARIAL BONE CELL-CULTURES

To investigate the influence of microvessel cells on osteoblasts, we e xposed osteoblast-enriched cultures of rat calvarial cells to cultured endothelial cells and pericytes using feeder-layer co-cultures, co-cu lture dish inserts, and conditioned media experiments. When co-culture d with growth-arrested feeder-layers of endothelial cells or pericytes for 10 days, bone cell cultures showed an increase in cell number and reduction in alkaline phosphatase activity. The response of bone cell s to endothelial cells was nearly twice their response to pericytes. A similar response was demonstrated by exposure to microvessel cells in co-culture dish inserts and by exposure to media conditioned by micro vessel cells. In long-term cultures of bone cells, the levels of osteo calcin and the number of mineralized nodules both were reduced by expo sure to media conditioned by the microvessel cells. Transient exposure to conditioned media from the microvessel cell cultures for 3 days, d uring the period from initial plating to cell confluence, produced nea rly the same effect on the cultures of bone cells as did continuous ex posure to these conditioned media. The influence of isolated microvess el cells on osteoblast-enriched calvarial cells was found to be primar ily mitogenic, mediated by soluble factors, independent of cell contac t, and a cause of prolonged reduction in the expression of early and l ate markers of the osteoblast phenotype.