Cohn, Martin J; (1997) Initiation of vertebrate limb development. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Development of paired appendages at appropriate levels along the body axis characterizes the jawed vertebrate body plan. Molecular networks that operate within limb buds have received much attention, although very little is known about how limb budding is initiated. Here I show that beads soaked in Fibroblast growth factors (FGFs) and placed in prospective flank of chick embryos induce formation of ectopic limb buds, which contain their own signaling regions and develop into complete limbs. Application of FGF to anterior flank induces ectopic wings, and FGF applied to posterior flank induces ectopic legs. Hox genes are good candidates for encoding position in lateral plate mesoderm along the body axis, and thus determining where limbs form. If particular combinations of Hox gene expression determine where wings and legs develop, then formation of additional limbs from flank should involve changes in Hox gene expression which reflect the type of limb induced. Here I show that the same population of flank cells can be induced to form wing or leg. Induction of ectopic limbs is accompanied by specific changes in expression of 3 Hox genes in lateral plate mesoderm which reproduce, in the flank, expression patterns found at normal limb levels. Together, these results suggest that local production of FGF may initiate limb development, and that Hox gene expression may specify limb position. Pythons lack forelimbs, have vestigial hindlimbs, and regional specialization of the axial skeleton has been lost. I have examined patterns of Hox gene expression in python embryos, and show that the uniform pattern of vertebrae is associated with uniform Hox gene expression. Transplantation experiments and analysis of gene expression in python hindlimbs suggest that limb development is arrested due to failed apical ridge formation. Absence of a ridge in pythons may result from failure of limb ectoderm to respond to mesenchymal signals.

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