WITTEN, P. E.; HUYSSEUNE, A.; HALL, B. K.; nstitute of Marine Research at the University of Kiel (Germany); Biology Department, Ghent University (Belgium); Department of Biology, Dalhousie University (Halifax): Acellular bone and mononucleated osteoclasts in advanced teleosts: New characters or adaptations resulting from heterochrony?

The lack of osteocytes (acellular bone) and bone resorption by mononucleated osteoclasts typifies the bone of advanced teleosts (e.g. tilapia, medaka). In contrast the bone of basal osteichthyans and less advanced teleosts contains osteocytes (cellular bone) and is resorbed by multinucleated osteoclasts (e.g. salmon, carp, zebrafish). So far, a relation between the presence/absence of osteocytes and the type of osteoclasts (mono-/multinucleated) has not been established. Both acellular bone and mononucleated osteoclasts might be new characters reflecting the low metabolic activity of bone which evolved in a stable mineral-rich marine environment, favouring the uncoupling of the skeleton from the animals’ mineral metabolism. Our ontogenetic studies on several teleost species with cellular bone reveal that acellular bone and bone resorption by mononucleated osteoclasts precede the development of cellular bone and multinucleated osteoclasts. Furthermore, the situation observed in juvenile tilapia (acellular bone) resembles that of postembryonic zebrafish, where bone resorption is still performed by many mononucleated cells. In contrast, juvenile carp of similar size (cellular bone) perform bone resorption with few multinucleated cells. We hypothesise that acellular bone and mononucleated osteoclasts are not new characters of advanced teleosts but represent a heterochronic shift, resulting from the retardation of early skeletal features well into adult life. Further support for our hypothesis comes from the occasional reappearance of cellular bone and multinucleated osteoclasts in some advanced teleosts.