Retinoic acid (RA), the active metabolite of vitamin A, is essential for vertebrate development. Vitamin A is maintained through dietary means, which is metabolized in a two-step process into RA. RA plays multiple roles during development including patterning of the body axis, cell growth, and cell differentiation. It again plays a role in tissue regeneration because inhibiting RA synthesis during limb regeneration will block limb regeneration. Along with Dr. Malcolm Maden, we are trying to understand the molecular mechanism of RA signaling during development and regeneration and whether RA plays the same role in both processes.

Below is an image of a transgenic RA reporter axolotl (A) and mouse (B) that fluoresce wherever RA signaling is taking place. Notice the two fluorescing areas in the anterior and posterior spinal cord of both animals. This example demonstrated the high degree of conservation in RA signaling across vertebrates.

RARE_glowing juvenile


Visualization of retinoic acid signaling in transgenic axolotls during limb development and regeneration.
Monaghan JR and Maden M.
Dev Biol. 368(1):63-75.


Retinoic acid (RA) plays a necessary role in limb development and regeneration, but the precise mechanism by which it acts during these processes is unclear. The role of RA in limb regeneration was first highlighted by the remarkable effect that it has on respecifying the proximodistal axis of the regenerating limb so that serially repeated limbs are produced. To facilitate the study of RA signaling during development and then during regeneration of the same structure we have turned to the axolotl, the master of vertebrate regeneration, and generated transgenic animals that fluorescently report RA signaling in vivo. Characterization of these animals identified an anterior segment of the developing embryo where RA signaling occurs revealing conserved features of the early vertebrate embryo. During limb development RA signaling was present in the developing forelimb bud mesenchyme, but was not detected during hindlimb development. During limb regeneration, RA signaling was surprisingly almost exclusively observed in the apical epithelium suggesting a different role of RA during limb regeneration. After the addition of supplemental RA to regenerating limbs that leads to pattern duplications, the fibroblast stem cells of the blastema responded showing that they are capable of transcriptionally responding to RA. These findings are significant because it means that RA signaling may play a multifunctional role during forelimb development and regeneration and that the fibroblast stem cells that regulate proximodistal limb patterning during regeneration are targets of RA signaling.