Another 'Clast'

Metaphyseal side of physis, 1-day-old mouse (C57BL/6), nondecalcified section. The chondroclast (red circle) is a multinucleated cell responsible for removing calcified cartilage matrices (especially the horizontal septa) and defunct chondrocytes (more of a macrophage function). Chondroclasts may be able to bind to and resorb non-mineralized cartilage. Chondroclasts share a similar morphology and lineage to the better known osteoclast. Functionally, the two cells are quite different. For example, osteoclasts seal to bone matrices with integrins, create an acidic extracellular vacuole beneath a prominent ruffled border, and internalize a HIGH concentration of Ca2+ (up to 40 mM!) But are chondroclasts and osteoclasts the same? That's controversial. The two cells do share many morphologic, biochemical, and ultrastructural similarities. Chondroclasts and osteoclasts may be distinct but closely related cell lineages in the macrophage family. Consider pathologic conditions affecting osteoclast function (conversation with Steven Weisbrode, professor emeritus, OSU CVM), such as congenital osteopetrosis or bisphosphenate treatment in juvenile animals. Remodeling of bone is affected, but the physis/endochondral ossification (where chondroclasts are found) are normal! This suggests osteoclasts and chondroclasts are different cells. Or maybe chondrocytes are not that important to physeal function