In Vitro Cellular Systems for Studying OC Function and Differentiation: Primary OC Cultures and the FLG 29.1 Model

Several pieces of evidence have shown that osteoclasts (OCs) are derived from progenitors originating from hemopoietic stem cells (1 –3 ). More specifically, early OC precursors seem to be closely related to the colony-forming unit for granulocytes and macrophages (CFU-GM) (3 –5 . However, compared with other bone or marrow cells, OCs are found in extremely low numbers in normal adult bone. In addition, active OCs are strongly adherent to the bone surface. For these reasons, it is impossible to obtain pure or highly enriched cultures of intact OCs, although it is possible to obtain large numbers of OCs if good source tissue is available. OCs are found in large numbers only in bone undergoing extensive physiological remodeling (e.g., fetal bone and growing bone metaphyses) or pathological osteolysis (e g., fracture callus) Since human tissue is often difficult to obtain, most OC research has employed animal models, notably rabbit, rat, and chick.