Retinoic acid recapitulates the action of the somites on neural differentiation of the developing caudal neural plate in chick embryo

During vertebrate neurogenesis, the caudal neural plate (CNP)-consisting of a stem zone- remains open. It adopts with the extension of the body to form the developing spinal cord. In the caudal-most part of the stem zone, the cell population has a dual neural/mesenchymal fate, depending on whether they move to the cranial part of the stem zone or gastrulate to form the underlying presomitic mesoderm. Due to FGF signaling in presomitic mesoderm, the cells residing in the stem zone are in a proliferative state. When the cells migrate cranially to the closing part of the neural tube, they differentiate into neurons. Here, they are exposed to retinoic acid (RA), secreted from the somites. The aim of this study was to evaluate whether RA mimics the somite function in neural differentiation of the CNP in culture conditions. The CNP explants from the chick embryos were dissected, cultured and treated with RA. The untreated CNP was considered as the control group. Using immunostaining and RT-PCR analysis, we examined the MAP2 protein expression and the different genes involved in neural differentiation in both groups. Upon expansion, the CNP-derived cells expressed MAP2 protein. The RT-PCR assessment showed that they also expressed Sox2 and Delta1, as well as Brachyury. Subsequently, the RA induced neural differentiation in the CNP cells through the upregulation of Sox2 and NeuroM and the downregulation of Delta1 and Brachyury. The CNP cells showed a neural progenitor/mesenchymal fate by expressing Sox2, Delta1 and Brachyury. The RA downregulated the Brachyury expression and recapitulated the somite neurogenic function through differentiating the CNP cells into neural-like cells expressing NeuroM.