part_of part of part of capable of developmentally induced by regulation of mesenchymal cell proliferation negative regulation of cartilage development ectoderm embryonic structure mesenchyme apical ectodermal ridge http://upload.wikimedia.org/wikipedia/commons/5/56/Limbbuddiagram.jpg note that media, dorsal fins etc have AERs. maintaining mesenchyme in plastic proliferating state; maintaining expression of A-P axis genes; interacting with D-V proteins. Genes: AER induced by Fgf10 in mesenchyme. AER secretes Fgf8, which stimulates mitosis in the mesenchyme causing Fgf10 production - positive feedback loop paired limb/fin bud