Pathogenesis of retinoid-induced hindbrain malformations in an experimental model.

Among the findings associated with the human Retinoic Acid Embryopathy are hindbrain defects including the Arnold-Chiari malformation. The human Arnold-Chiari malformation (ACM) is a malformation complex where the cardinal feature is herniation of the caudal hindbrain into the vertebral column; it is frequently accompanied by lumbosacral myelorachischisis and hydrocephalus. Mice exposed to all-trans-retinoic acid or etretinate on day 8.25 of pregnancy, produce offspring with hindbrain herniation and caudal lumbosacral myelorachischisis in addition to a variety of other craniofacial and caudal malformations. Several experimental animals were observed to lack the caudal myelorachischisis proving that this lesion is not required to generate hindbrain herniation. We provide evidence that the cranial malformations, including hindbrain herniation, result from primary damage to the neural crest and the rhombencephalon. The vulnerability of these sites appears to be correlated with the presence of normal physiological cell death. While these experimental animals differ in many respects from the typical human Arnold-Chiari malformation, they may provide some insight into the pathogenesis of the latter.