Expression patterns of runx2, sparc, and bgp during scale regeneration in the goldfish Carassius auratus

Teleost fish scale is a dermal skeleton equipped with a strong regenerative ability. Owing to this regenerative ability, teleost fish scale can be used as a model for the regeneration of the dermal skeleton. However, there is insufficient fundamental knowledge of the regeneration, and this limits the usage of fish scale. In this study, as a first step toward understanding the molecular mechanism of the cellular differentiation during scale regeneration, we cloned the cDNAs for osteoblast-related proteins (Runx2, Sparc, and Bgp) in goldfish, and analyzed their expressions during scale regeneration. The expression profiles of these genes during scale regeneration were similar to those during mammalian osteoblastic differentiation. Specifically, runx2 expression was increased at the earliest time point, followed by sparc expression and then bgp expression. In the earlier stages, these genes were expressed in cells that formed cellular condensations and the flat cells surrounding them in the scale pocket. As the regeneration proceeded, the expressions became restricted to the episquamal, hyposquamal, and marginal scleroblasts and the cells around the marginal area of the regenerating scale. These results strongly suggest that (1) the differentiation mechanism of scleroblasts is similar to that of mammalian osteoblasts and odontoblasts, (2) scleroblast differentiation occurs around the cellular condensations at the early regeneration stage and is restricted to the marginal area of the scale at the later stage, and (3) the differentiation mechanisms are similar between the episquamal scleroblasts that produce the external layer and the hyposquamal scleroblasts that produce the basal plate.