Enzyme histochemical characteristics of osteoblasts and mononucleated osteoclasts in a teleost fish with acellular bone (Oreochromis niloticus, Cichlidae)

Bone resorption by mononucleated cells was studied in the acellular bone of a teleost fish (Oreochromis niloticus) by histological and enzyme histochemical observations and by transmission electron microscopy. Bone resorbing cells (osteoclasts) were identified by their location at the sites of bone resorption, their frequent association with a band of concentrated activity of tartrate-resistant acid phosphatase at the bone surface and by the presence or lack of certain enzymes. Tartrate-resistant acid phosphatase was used as a marker for osteoclasts, and alkaline phosphatase as a marker for osteoblasts. Osteoclasts in O. niloticus are not multinucleated; however, during intense bone resorption, they form cell aggregations that resemble multinucleated giant cells in mammals. Conversely, during less intense bone degradation, osteoclasts are flat, have long narrow cytoplasmic processes and resemble the bone-lining cells of mammals. All bone-resorbing cells in O. niloticus are mononucleated and lack a ruffled border. Similarities to and differences from bone resorption by mononucleated cells in mammals are discussed.