Osteoclast

Osteoclasts are large multinucleated cells that dissolve bone matrix and release its calcium into the circulation. Bone is continuously remodelled by osteoclasts removing old tissue and osteoblasts replacing it, with the whole skeleton turned over across roughly a decade.

It matters because bone loss is a balance problem, not simply a failure to build. When osteoclast activity outpaces osteoblast activity — as it does after menopause, in prolonged inactivity, or with certain medications — net density falls. Several osteoporosis drugs work specifically by suppressing osteoclasts.

In practice this is why unloading is so costly to bone: without mechanical strain, the removal side keeps working while the building signal fades. Bed rest and spaceflight both produce rapid bone loss for exactly this reason.