A band of F-actin containing podosomes is involved in bone resorption by osteoclasts.

Isolated rabbit osteoclasts cultured on devitalized thin bone slices excavate resorption lacunae that can be visualized with brightfield or phase-contrast microscopy. Superimposition of the brightfield images of such resorption lacunae and the fluorescence images of the corresponding osteoclasts after fixation and staining with rhodamine-conjugated phalloidin revealed that a bright fluorescent band of F-actin-containing podosomes precisely outlined the resorption lacunae in stationary osteoclasts. When the resorption lacunae were being extended laterally, the clearly delineated band of podosomes corresponded to the advancing edge of the resorbing osteoclast and the most recently excavated part of the lacunae. Reshaping and reorganization of the bright bands preceded development of the lateral boundary of the lacunae. Podosomes forming these bands were highly dynamic, changed in size and location, and appeared and disappeared continuously. Their lifespan varied between 2 and 12 min. Similar bands were also seen in vivo in bone-resorbing osteoclasts on the endocranial surface of growing calvariae. Podosomes disappeared in osteoclasts treated with calcitonin, resulting in the disruption of the fluorescent bands. Our results suggest that podosomes are an essential part of the resorption apparatus of osteoclasts.