A bone marrow fraction enriched for granulocyte-macrophage progenitors gives rise to osteoclasts in vitro

Studies involving bone marrow transplantation of osteopetrotic rodents have provided evidence for the lineage of the osteoclast. Recent investigations have demonstrated that isolates of bone marrow containing an enriched population of granulocyte-colony-forming cells (G-CFC) or granulocyte macrophage-colony-forming cells (GM-CFC) from normal animals cure the skeletal sclerosis and result in the formation of normal osteoclasts when transplanted into ia osteopetrotic rats. Macrophage-colony-forming cell isolates were ineffective in this transplant system. A criticism of these findings is that the microenvironment of the osteopetrotic bone and the bone marrow compartment may be unique in their ability to induce the differentiation of these stem cells into osteoclasts. To test this hypothesis, G-CFC, GM-CFC, and M-CFC were co-cultured with fetal metatarsal bones form normal animals. The CFC were isolated from normal bone marrow using FITC-labeled monoclonal antibodies directed against rat Thy 1.1 and fluorescence-activated cell sorting. The isolates were evaluated in soft agar culture; granulocyte isolates generated 74% G-CFC of all colonies formed and were enhanced 30 times over unfractionated cells. Mixed isolates generated 60% GM-CFC of the colonies formed and were 12 times enhanced, while macrophage isolates were 77% M-CFC with an enhancement factor of 28. The various CFC isolates or whole mononuclear bone marrow were co-cultured with 20-day fetal rat metatarsal rudiments for 7 days and then prepared for light and electron microscopy. The number of osteoclasts generated in vitro by each isolate was determined.(ABSTRACT TRUNCATED AT 250 WORDS)