The unobtrusive majority: mononucleated bone resorbing cells in teleost fish and mammals

P>The canonical view of a mammalian (usually shown as human) bone resorbing cell is that of a giant macrophage-like cell (osteoclast) that dissolves bone minerals and digests bone matrix proteins. The cells' presence and activity is easily recognised based on three distinct morphological features: (i) multinuclearity, (ii) a multiply folded apical cell membrane (ruffled border), and (iii) deep lacunae (Howship's lacunae) that the cells eroded into the bone surface. Mononucleated osteoclasts without these features are considered to be inactive precursors. We challenge the view that bone resorbing cells must be multinucleated giant cells, based on our comparative studies on the teleost skeleton, on what is currently known - but often disregarded - about mononucleated mammalian osteoclasts, and on what is know about osteocytic osteolysis.