A critical period for retinoic acid teratogenesis and loss of neurophilic migration of pontine nuclei neurons

Abnormalities in the pontine nuclei (PN) and inferior olive are hallmarks of human retinoic acid (RA) teratogenesis. This study shows that RA exposure of the mouse at a specific embryonic stage alters morphological structures that derive from the wall of the IVth ventricle to form components of the precerebellar system (the inferior olivary nucleus and the PN). The study employs both normal and a RAREhspLacZ transgenic RA reporter mouse. It is shown that abnormalities in the PN and inferior olive result from exposure at a critical period of embryonic day 9.5 and 10.5. The abnormalities in the PN are due to a failure in their usual neurophilic migration. The compact stream of cells that leads from the anterior rhombic lip to the ventral pons is instead scattered widely over the anterior medulla. Given that the RA exposure occurs after the resolution of rhombomere identity this suggests that teratogenic RA interferes with a regulatory event that overlays this original pattern.