Abnormalities of neural tube formation in pre-spina bifida splotch-delayed mouse embryos.

The splotch-delayed homozygous mutant (Spd/Spd) develops spina bifida with or without exencephaly, has spinal ganglia abnormalities, and delays in posterior neuropore closure and neural crest cell emigration. The heterozygote (Spd/+) has a pigmentation defect, and occasionally neural tube defects. To investigate the underlying mechanisms, we compared the neuroepithelium in the posterior neuropore region of cytogenetically identified 15-18 somite pair Spd/Spd, Spd/+, and +/+ mouse embryos by transmission electron and light microscopy. The notochordal area and cell number in the non-fused neuroepithelium region of Spd/Spd and Spd/+ embryos were significantly reduced compared to those of normal (+/+) embryos, which suggests an abnormality in notochord elongation. In the mesoderm, the mean cell number and mean ratio of cell number to area in the non-fused region were significantly lower in the Spd/Spd compared with +/+ embryos. The distance of exposed neuroepithelium above the mesoderm in the just-fused region was significantly lower in the Spd/Spd versus +/+ embryos, which may indicate an insufficient force exerted by the mesoderm during neural tube closure. Within the neuroepithelium, significantly more intercellular space was found in Spd/Spd than in +/+ embryos indicating disorganization. The basal lamina was poorly organized and the formation delayed around the neural tube in Spd/Spd and Spd/+ embryos. All together, these results suggest an early abnormality in interactions among the neuroepithelium, mesoderm, and notochord, which may lead to the delay or inhibition of neural tube closure observed in Spd/Spd mutants.