Immunolocalization of CD44 and the ERM family in bone cells of mouse tibiae.

We studied the immunohistochemical localization of CD44, hyaluronate receptor, and the ezrin-radixin-moesin (ERM) family, actin binding proteins, in bone cells using confocal laser scanning microscopy and transmission electron microscopy to clarify the mechanism of the organization of their cytoskeletons. In osteoclasts, intense immunoreactivity to CD44 could be detected on their basolateral plasma membranes. There was less reactivity observed in the area of the plasma membrane in direct contact with the bone surface. The immunogold electron-microscopical method revealed that CD44 was mainly localized on the microvilli of the basolateral plasma membrane. The plasma membrane of the clear zone and the ruffled border were not immunolabeled with CD44. As for the ERM family, the basolateral plasma membrane of osteoclasts was stained with antimoesin monoclonal antibody, but not with ezrin or radixin. In osteoblasts attached to the bone surface, immunoreactivity to CD44 was restricted to their cytoplasmic processes. They showed immunoreactivities to radixin and moesin on the cytoplasmic side of their plasma membrane when in contact with each other. However, although osteocytes in the bone matrix demonstrate an intense immunolabeling with CD44 on their plasma membrane, they scarcely show immunoreactivity to the ERM family. These findings suggest that: (1) the CD44-moesin-actin filament system is involved in the organization of cytoskeletons in the basolateral plasma membrane of osteoclasts; and (2) other mechanisms, rather than the CD44 and the ERM family, may be involved in the cells of osteoblast lineage.