Retinoic acid induces down-regulation of Wnt-3a, apoptosis and diversion of tail bud cells to a neural fate in the mouse embryo

The tail bud comprises the caudal extremity of the vertebrate embryo, conta ining a pool of pluripotent mesenchymal stem cells that gives rise to almos t all the tissues of the sacro-caudal region. Treatment of pregnant mice wi th 100 mg/kg all-trans retinoic acid at 9.5 days post coitum induces severe truncation of the body axis, providing a model system for studying the mec hanisms underlying development of caudal agenesis. In the present study, we find that retinoic acid treatment causes extensive apoptosis of tail bud c ells 24 h after treatment. Once the apoptotic cells have been removed, the remaining mesenchymal cells differentiate into an extensive network of ecto pic tubules, radially arranged around the notochord, These tubules express Pax-3 and Pax-6 in a regionally-restricted pattern that closely resembles e xpression in the definitive neural tube, Neurofilament-positive neurons sub sequently grow out from the ectopic tubules, Thus. the tail bud cells remai ning after retinoic acid-induced apoptosis appear to adopt a neural fate. W nt-3a, a gene that has been shown to be essential for tail bud formation, i s specifically down-regulated in the tail bud of retinoic acid-treated embr yos, as early as 2 h after retinoic acid treatment and Wnt-3a transcripts b ecome undetectable by 10 h, In contrast, Wnt-5a and RAR-gamma are still det ectable in the tail bud at that time. Extensive cell death also occurs in t he tail bud of embryos homozygous for the vestigial rail mutation, in which there is a marked reduction in Wnt-3a expression, These embryos go on to d evelop multiple neural tubes in their truncated caudal region. These result s suggest that retinoic acid induces down-regulation of Wnt-3a which may pl ay an important role in the pathogenesis of axial truncation, involving ind uction of widespread apoptosis, followed by an alteration of tail bud cell fate to form multiple ectopic neural tubes. (C) 1999 Elsevier Science Irela nd Ltd. All rights reserved.