Neural development in Xenopus
University of Portsmouth UK
Sharpe, Colin R.
Primary Neurones in Xenopus Embryos: Amphibian and fish embryos develop two nervous systems. The embryonic primary nervous system forms rapidly and is active in little more than one day following fertilisation. It consists of a small number of large neurones that control the simple behaviour associated with a free swimming lifestyle. These features have made the embryonic primary neurones a popular model system to examine vertebrate neurone formation. Embryonic primary neurones are readily identified by the expression of the neural specific type-II beta-tubulin (NST) gene or the peripherin (XIF3) gene. The secondary neurones begin to differentiate several days later and populate the larval and adult nervous system. We have found that signalling by retinoic acid is required for the formation of primary neurones.
Retinoic acid (RA) is a derivative of vitamin A that acts as a signalling molecule. It binds the RAR/RXR nuclear receptor heterodimer, which then activates target gene expression. We have shown that retinoid signalling is essential for embryonic primary neurone formation. When RS is inhibited, primary neurones fail to form, but when RS is elevated, more embryonic primary neurones form than normal. The current aims of the laboratory are to determine how RS is regulated in the embryo and how RS controls primary neurogenesis. To do this we are using an animal cap system. Animal caps taken from embryos injected with noggin mRNA become neuralised but do not form differentiated primary neurons. However, the co-injection of noggin and retinoid receptor mRNA and the addition of RA to the animal caps results in primary neurone formation.