Retinoic acid-induced thymic abnormalities in the mouse are associated with altered pharyngeal morphology, thymocyte maturation defects, and altered expression of Hoxa3 and Pax1

Exogenous retinoic acid is teratogenic in animals and man, causing a spectr um of abnormalities termed retinoic acid embryopathy. Using a mouse model o f retinoic acid embryopathy, our results show that exposure to all-trans re tinoic acid (RA) on gestational day (gd) 9 results in thymic ectopia, hypop lasia, and thymocyte maturational defects. Immunohistochemical and flow cyt ometric analyses showed aberrant expression of stromal and thymocyte marker s, and abnormalities in thymocyte development. RNA in situ hybridization fo r the transcription factors Hoxa3 and Pax1 was used to investigate the basi s of this defect. Hoxa3 and Pax1 have been shown to be required for normal thymus development, and are normally expressed in the cells of the third ph aryngeal pouch and third and fourth pharyngeal arches, involved in thymus o rganogenesis. RA-exposed embryos showed an increased level of Hoxa3 express ion in the neural tube and caudal pharyngeal arches as soon as 6 hr after e xposure. The Pax1 expression pattern, in conjunction with analysis of the e xternal pharyngeal morphology, showed that the development and structure of the third pharyngeal pouch and cleft were disrupted, resulting in a reduce d third pharyngeal arch and/or fusion of the third and fourth arches, Chang es in the expression of cellular retinoic acid binding protein (CRABP) and in the morphology of the cranial ganglia were consistent with altered neura l crest cell migration from the caudal hindbrain after RA exposure. Togethe r, our findings suggest that the teratogenic effects of RA on thymus develo pment include changes in both the cranial neural crest and pharyngeal endod erm that contribute to thymus development. Further, the observed defects in thymus development may be mediated by RA-induced alterations in the expres sion of Hoxa3. (C) 1998 Wiley-Liss, Inc.