Correlation of an osteoclast antigen and ruffled border on giant cells formed in response to resorbable substrates

The osteoclast is the specialized multinucleated cell primarily responsible for the degradation of the organic and inorganic components of bone matrix. The functional and developmental relationship between osteoclasts and foreign body giant cells is unclear. The osteoclast plasma membrane ruffled border juxtaposed to the bone surface is a unique morphologic characteristic of active osteoclasts. In the studies reported here giant cell formation was induced in response to a variety of materials implanted onto the richly vascularized chick chorioallantoic membrane. Light and electron microscopic techniques were used to examine the morphologic characteristics of the giant cells. In addition, immunohistochemical methods were used to demonstrate the appearance of a 150 kD cell surface antigen on chicken osteoclasts recognized by monoclonal antibody 121F. Giant cells that formed in response to mineralized bone particles exhibited ruffled borders and stained positively with the 121F antibody. Many giant cells that formed in response to hydroxyapatite possessed ruffled borders similar to but not as extensive as those observed on giant cells formed on bone. Immunohistochemical localization of the 121F antigen on these cells suggested that the antigen was present, but staining intensity was reduced compared to that of bone-associated giant cells. The formation of mineral matrix complexes by the adsorption to hydroxyapatite of bone extract or osteocalcin enhanced ruffled borders and the presence of the 121F antigen on elicited giant cells. In contrast, giant cells that formed on non-resorbable materials, such as Sepharose beads, mica, and methacrylate, lacked ruffled borders and were negative for the 121F antigen. It appears that expression of the 121F osteoclast antigen correlates with the appearance and extent of ruffled membranes on giant cells. Furthermore, it appears that giant cell ruffled membrane development and the presence of the 121F osteoclast antigen are related to giant cell formation in response to resorbable materials that are subject to extracellular dissolution. Expression of this antigen may be indicative of the developmental and/or functional state of giant cells (osteoclasts) that form on resorbable substrates. In addition, components of the bone matrix, including osteocalcin, in association with bone mineral, lead to elevated levels of this osteoclast antigen.