ZebRA: An overview of retinoic acid signaling during zebrafish development.

Retinoic acid (RA), the main active vitamin A derivative, is crucial for embryo development, regulating cellular processes, embryo patterning and organogenesis. Many studies performed in mammalian or avian models have successfully undertaken the investigation of the role played by RA during embryogenesis. Since the early 1980s, the zebrafish (Danio rerio) has emerged as a powerful developmental model to study the in vivo role of RA during embryogenesis. Unlike mammalian models, zebrafish embryogenesis is external, not only allowing the observation of the translucent embryo from the earliest steps but also providing an easily accessible system for pharmacological treatment or genetic approaches. Therefore, zebrafish research largely participates in deciphering the role of RA during development. This review aims at illustrating different concepts of RA signaling based on the research performed on zebrafish. Indeed, RA action relies on a multitude of cross-talk with other signaling pathways and requires a coordinated, dynamic and fine-regulation of its level and activity in both temporal and spatial dimensions. This review also highlights major advances that have been discovered using zebrafish such as the observation of the RA gradient in vivo for the first time, the effects of RA signaling in brain patterning, its role in establishing left-right asymmetry and its effects on the development of a variety of organs and tissues including the heart, blood, bone and fat. This review demonstrates that the zebrafish is a convenient and powerful model to study retinoic acid signaling during vertebrate embryogenesis. This article is part of a Special Issue entitled: Nuclear receptors in animal development.