CD44 expression in human bone: a novel marker of osteocytic differentiation

CD44 is a transmembrane glycoprotein with cell-cell and cell-matrix adhesion functions that is expressed by a wide variety of cell types and has a number of known biologic functions. Because of its ability to bind matrix macromolecules, such as fibronectin, collagen, and hyaluronate, we investigated the possibility that it is expressed by the cells of bone, the matrix receptors of which are largely unknown. Immunohistochemical study of a variety of sources of human bone was carried out using a panel of six well-characterized anti-CD44 monoclonal antibodies. Osteocytes strongly expressed CD44, whereas osteoblasts and lining cells were negative. Osteoclasts and periosteal cells also expressed CD44, although not as strongly as osteocytes. These patterns of staining were observed with all six antibodies. These results demonstrate that acquisition of CD44 immunoreactivity is a sensitive marker of osteocytic differentiation and raise the possibility that CD44 acts as a cell matrix receptor in bone.