Characterization of the neural crest defect in Splotch (Sp1H) mutant mice using a lacZ transgene.

We have reinvestigated the neural crest defect of Splotch (Sp1H) mutant embryos using the tissue specific expression of lacZ by the HCMV-IEP-lacZ (CMZ) transgene as a marker. The CMZ transgene was backcrossed onto the Sp1H mutant background, which has been shown to carry mutations in the Pax-3 gene. The CMZ transgene has previously been shown to be expressed in some neural crest-derived neural tissues of midgestation embryos. The pattern of CMZ expression in Splotch mutants is not caused by alterations of transgene transcription, but demonstrates morphological deviations of neural crest development. The gradual size reduction of spinal ganglia along a rostrocaudal gradient is shown to occur concomitantly with a size reduction of the sympathetic ganglia. CMZ expression also reveals the total absence of sympathetic ganglion cells in thoracic and lumbar segments of Sp1H homozygotes, which is confirmed in serial sections. Observations in whole mounts of CMZ transgenic homozygotes suggest that cranial nerve ganglia develop normally in these embryos. CMZ is expressed in epithelial cells around the neural tube defect in Splotch mutants at the epidermal/neuroepithelial boundary. It is proposed that this expression represents premigratory neural crest cells that remain within the epithelial layer around the neural tube defect. These observations are discussed with reference to the normal pattern of Pax-3 expression.