Somites are formed progressively from the presomitic mesoderm (PSM) in a highly regulated process according to a strict periodicity driven by an oscillatory mechanism. The Notch and Wnt pathways are key components in the regulation of this somitic oscillator and data from Xenopus and zebrafish embryos indicate that the Notch-downstream target Nrarp participates in the regulation of both activities. We have analyzed Nrarp/nrarp-a expression in the PSM of chick, mouse and zebrafish embryos, and we show that it cycles in synchrony with other Notch regulated cyclic genes. In the mouse its transcription is both Wnt- and Notch-dependent, whereas in the chick and fish embryo it is simply Notch-dependent. Despite oscillating mRNA levels, Nrarp protein does not oscillate in the PSM. Finally, neither gain nor loss of Nrarp function interferes with the normal expression of Notch-related cyclic genes.
Cellular adhesion is fundamental to the behaviour of cell populations during embryonic development and serves to establish correct tissue pattern and architecture. The cadherin superfamily of cell adhesion proteins regulates cellular organization and additionally influences intracellular signalling cascades. Here we present for the first time a detailed account of chick Fat-1 gene expression during embryogenesis visualised by whole-mount in situ hybridisation. In part, we focus on the expression pattern in limb buds that has not been accurately documented. While Fat-1 is generally expressed in epithelial tissues and its Drosophila counterpart Fat-like regulates formation of ectodermally-derived organs, in limb buds we have found that chick Fat-1 is uniquely restricted to mesenchyme. This Fat-1 expression pattern is remarkably dynamic throughout tissue differentiation, limb maturation and pattern formation. Diffuse expression of Fat-1 begins at stage HH17 as the limb bud is forming. It then becomes more proximal as the limb bud grows and is expressed within both tendon and muscle progenitors in the dorsal and ventral subectodermal mesenchyme. Later, Fat-1 transcripts were more abundant in anterior and posterior domains of the limb bud. During hand plate formation, Fat-1 transcripts were expressed in the mesenchyme adjacent to the wrist joint zone and in the interdigit mesenchyme.
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