Differential serum dependence of cultured osteoclastic and osteoblastic bone cells

Sequential collagenase digestion of mice calvariae provides populations of bone cells that express either osteoclasts (OC) or osteoblastic (OB) activities after growth for 6 days in similar culture conditions consisting of minimal essential medium supplemented with 10% fetal calf serum (FCS). The OC characteristics (acid phosphatase activity and hyaluronate synthesis, and their stimulation by PTH) were recovered in the cell populations released early from calvariae, but these also contained OB cells and numerous spindle-shaped alkaline phosphatase positive cells that resembled fibroblasts. We have attempted to select for growth of OC cells in these early populations by exploiting differences in growth requirements of OC, OB, and fibroblastic cells. We find that after growth for 6 days in low serum (2% FCS), OC cell populations demonstrated a threefold increase in OC activity/cell, and cell yield was reduced to one-third of that obtained in 10% FCS. Spindle-shaped cells were absent in 2% FCS and OB marker activities (alkaline phosphatase and citrate decarboxylation) were reduced threefold. In contrast to OC cells, high serum (10% FCS) favored the growth and phenotypic expression of OB cells (late populations). Cell yield and OB marker activities/cell were twofold higher in OB cells grown in 10% FCS vs 2% FCS, whereas growth but not phenotypic expression was retained at 5% FCS. These data suggest that differential serum dependence of OC and OB cells may provide a basis for further enrichment for each cell type following sequential digestion.