Fgf-signaling is compartmentalized within the mesenchyme and controls proliferation during salamander limb development

Abstract

Although decades of studies have produced a generalized model for tetrapod limb development, urodeles deviate from anurans and amniotes in at least two key respects: their limbs exhibit preaxial skeletal differentiation and do not develop an apical ectodermal ridge (AER). Here, we investigated how <i>Sonic hedgehog</i> (<i>Shh</i>) and <i>Fibroblast growth factor</i> (<i>Fgf</i>) signaling regulate limb development in the axolotl. We found that <i>Shh</i>-expressing cells contributed to the most posterior digit, and that inhibiting Shh-signaling inhibited <i>Fgf8</i> expression, anteroposterior patterning, and distal cell proliferation. In addition to lack of a morphological AER, we found that salamander limbs also lack a molecular AER. We found that amniote and anuran AER-specific <i>Fgfs</i> and their cognate receptors were expressed entirely in the mesenchyme. Broad inhibition of Fgf-signaling demonstrated that this pathway regulates cell proliferation across all three limb axes, in contrast to anurans and amniotes where Fgf-signaling regulates cell survival and proximodistal patterning.