In the course of an extensive comparative, structural and developmental study of the cranial and postcranial dermal skeleton (teeth and scales) in osteichthyan fishes, we have undertaken investigations on scale development in zebrafish (Danio (Brachydanio) rerio) using alizarin red staining, and light and transmission electron microscopy. The main goal was to know whether zebrafish scales can be used as a model for further research on the processes controlling the development of the dermal skeleton in general, especially epithelialmesenchymal interactions. Growth series of laboratory bred specimens were used to study in detail : (1) the relationship of scale appearance with size and age ; (2) the squamation pattern ; and (3) the events taking place in the epidermis and in the dermis, before and during scale initiation and formation, with the aim of searching for morphological indications of epithelial-mesenchymal interactions. Scales form late in ontogeny, generally when zebrafish are more than 8n0 mm in standard length. Within a population of zebrafish of the same age scale appearance is related to standard length, but when comparing populations of different age the size of the fish at scale appearance is also related to age. Scales always appear first in the posterior region of the body and the squamation then 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 formation clearly indicate that the basal epidermal cells differentiate first, before any evidence of differentiation of the progenitors of the scale-forming cells in the dermis. This strongly suggests that scale differentiation could be initiated by the epidermal basal layer cells which probably produce a molecular signal towards the dermis below. Subsequently dermal cells accumulate close to the epidermis, and differentiate to form scale papillae. The late formation of the scales during ontogeny is due to a late colonisation of the dermis by the progenitors of the scale-forming cells. Because of their late formation during ontogeny and of their regular pattern of development, scales in zebrafish represent a good model for further investigations on the general mechanisms of epithelial-mesenchymal interactions during dermal skeleton development, and in particular for the study of the gene expression patterns.
The vertebrate skull vault forms almost entirely by the direct mineralisation of mesenchyme, without the formation of a cartilaginous template, a mechanism called membranous ossification. Dlx5 gene mutation leads to cranial dismorphogenesis which differs from the previously studied craniosynostosis syndromes [Development 126 (1999), 3795; Development 126 (1999), 3831]. In avians, little is known about the genetic regulation of cranial vault development. In this study, we analyze Dlx5 expression and regulation during skull formation in the chick embryo. We compare Dlx5 expression pattern with that of several genes involved in mouse cranial suture regulation. This provides an initial description of the expression in the developing skull of the genes encoding the secreted molecules BMP 2, BMP 4, BMP 7, the transmembrane FGF receptors FGFR 1, FGFR 2, FGFR 4, the transcription factors Msx1, Msx2, and Twist, as well as Goosecoid and the early membranous bone differentiation marker osteopontin. We show that Dlx5 is activated in proliferating osteoblast precursors, before osteoblast differentiation. High levels of Dlx5 transcripts are observed at the osteogenic fronts (OFs) and at the edges of the suture mesenchyme, but not in the suture itself. Dlx5 expression is initiated in areas where Bmp4 and Bmp7 genes become coexpressed. In a calvarial explant culture system, Dlx5 transcription is upregulated by BMPs and inhibited by the BMP-antagonist Noggin. In addition, FGF4 activates Bmp4 but not Bmp7 gene transcription and is not sufficient to induce ectopic Dlx5 expression in the immature calvarial mesenchyme. From these data, we propose a model for the regulatory network implicated in early steps of chick calvarial development.
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