SCALE DEVELOPMENT IN ZEBRAFISH (DANIO-RERIO)

In the course of an extensive comparative, structural and developmenta l study of the cranial and postcranial dermal skeleton (teeth and scal es) in osteichthyan fishes, we have undertaken investigations on scale development in zebrafish (Danio (Brachydanio) rerio) using alizarin r ed staining, and light and transmission electron microscopy. The main goal was to know whether zebrafish scales can be used as a model for f urther research on the processes controlling the development of the de rmal skeleton in general, especially epithelial-mesenchymal interactio ns. Growth series of laboratory bred specimens were used to study in d etail: (1) the relationship of scale appearance with size and age; (2) the squamation pattern; and (3) the events taking place in the epider mis and in the dermis, before and during scale initiation and formatio n, with the aim of searching for morphological indications of epitheli al-mesenchymal interactions. Scales form late in ontogeny, generally w hen zebrafish are more than 8.0 mm in standard length. Within a popula tion of zebrafish of the same age scale appearance is related to stand ard length, but when comparing populations of different age the size o f the fish at scale appearance is also related to age. Scales always a ppear first in the posterior region of the body and the squamation the n extends anteriorly. Scales develop in the dermis but closely apposed to the epidermal-dermal boundary. Cellular modifications occurring in the basal layer of the epidermis and in the dermis before scale forma tion clearly indicate that the basal epidermal cells differentiate fir st, before any evidence of differentiation of the progenitors of the s cale-forming cells in the dermis. This strongly suggests that scale di fferentiation could be initiated by the epidermal basal layer cells wh ich probably produce a molecular signal towards the dermis below. Subs equently dermal cells accumulate close to the epidermis, and different iate to form scale papillae. The late formation of the scales during o ntogeny is due to a late colonisation of the dermis by the progenitors of the scale-forming cells. Because of their late formation during on togeny and of their regular pattern of development, scales in zebrafis h represent a good model for further investigations on the general mec hanisms of epithelial-mesenchymal interactions during dermal skeleton development, and in particular for the study of the gene expression pa tterns.