Retinoic acid acts during peri-implantational development to alter axial and brain formation

All-trans retinoid acid (RA) induces a stereotypic spectrum of stage-specific malformations in vertebrate conceptuses. The present work evaluated the anatomic and biochemical effects of exposure to RA in mouse embryos at a peri-implantational stage of development - gestational day (GD) 5. The RA receptors (RARs) beta and gamma, the retinoid X receptors (RXRs) alpha and beta, and the cellular retinoid acid binding proteins (CRABPs) I and II were detected by RT-PCR in both control and treated individual GD 5 decidua/embryo complexes 3 h after RA injection, indicating the presence of the mRNAs coding for the proteins that mediate the effects of RA. In contrast, the RAR alpha mRNA was detected in some but not all decidua/embryo complexes, both control and treated, suggesting that its expression is initiated at approximately GD 5, while RXR gamma mRNA was not detected. Examination of the control and RA-exposed embryos on GD 10, 12, or 17 showed that greater than 50% of the RA-exposed embryos were adversely affected, many with defects found only after serial histopathological examination. The malformations were localized primarily in the central nervous system, the branchial arches, and their derivatives. These terata included excessive folding and elevation of the neural tube floor plate, exencephaly (with detachment of the cephalic neuroepithelium and rarefied cephalic mesenchyme), persistent patency of Rathke's pouch, small trigeminal ganglia, neural diverticula (chiefly from the spinal cord), and/or various optic and otic defects. Unexpectedly, limb reduplications were not apparent in RA-exposed fetuses. Those litters examined on GD 17 had a high percentage of resorbed or malformed implantations, and the few apparently normal fetuses were developmentally delayed with respect to bone ossification. These data confirm that the development of neural- and neural crest-derived structures are severely disrupted by RA exposure prior to initial specification of the neural plate and suggest that many of the proteins that regulate RA signaling are available in early vertebrate embryos at this developmental stage.