Scale development in fish: a review, with description of sonic hedgehog (shh) expression in the zebrafish (Danio rerio).

In the first part of this paper we review current knowledge regarding fish scales, focusing on elasmoid scales, the only type found in two model species, the zebrafish and the medaka. After reviewing the structure of scales and their evolutionary origin, we describe the formation of the squamation pattern. The regularity of this process suggests a pre-patterning of the skin before scale initiation. We then summarise the dynamics of scale development on the basis of morphological observations. In the absence of molecular data, these observations support the existence of genetic cascades involved in the control of scale development. In the second part of this paper, we illustrate the potential that scale development offers as a model to study organogenesis mediated by epithelial-mesenchymal interactions. Using the zebrafish (Danio rerio), we have combined alizarin red staining, light and transmission electron microscopy and in situ hybridisation using an anti-sense RNA probe for the sonic hedgehog (shh) gene. Scales develop late in ontogeny (30 days post-fertilisation) and close to the epidermal cover. Only cells of the basal epidermal layer express shh. Transcripts are first detected after the scale papillae have formed. Thus, shh is not involved in the mechanisms controlling squamation patterning and scale initiation. As the scales enlarge, shh expression is progressively restricted to a subset of basal epidermal cells located in the region that overlies their posterior field. This pattern of expression suggests that shh may be involved in the control of scale morphogenesis and differentiation in relationship with the formation of the epidermal fold in the posterior region.