Osteoclasts: origin and bone resorption

Osteoclasts are the cells that clear bone away. They are large cells with many nuclei (multinucleated), and they sit on bone surfaces. Their job is resorption, the breaking down of bone tissue.

An osteoclast presses onto the bone and seals off a small pocket beneath itself. The ruffled border is the folded part of the membrane facing the bone, and it is the surface that does the resorbing. Into the pocket the cell releases acid, which dissolves hydroxyapatite (the hard calcium-containing crystals of bone), and collagenase, an enzyme that digests the collagen framework (the organic matrix) of bone.

Origin sets osteoclasts apart from the bone-forming cells, the osteoblasts. Osteoclasts come from haemopoietic stem cells, the blood-forming cells of the bone marrow, by way of monocytes and macrophages, which are types of white blood cell. Osteoblasts, the bone-forming cells, come from mesenchymal stem cells instead. Osteocytes are mature bone cells: they come from osteoblasts that have become trapped in the matrix they made.

Resorption matters because bone is continually remodelled: old or damaged bone is broken down and rebuilt. It also releases calcium into the blood, so osteoclasts help to regulate blood calcium. In a fracture that heals through callus formation (secondary healing; the callus is the new tissue that forms around the break), osteoclasts remove the temporary woven bone (immature bone with poor mechanical strength) and osteoblasts then lay down lamellar bone (mature bone arranged in organised layers). When breakdown runs ahead of formation, bone density falls, which is the basis of osteoporosis.