Cell fate and signalling in chick limb bud development

The chick limb develops from a bud of apparently homogeneous mesenchyme cells encased in ectoderm. Mesenchymal-epithelial interactions and mesenchymal signalling pathways are important in controlling the patterning and positioning of the skeletal elements and other tissues of the limb. I have produced detailed and comprehensive fatemaps for mesenchyme and apical ridge of a stage 20 chick wing bud. The mesenchyme fate maps show that the majority of the limb including the digits arises from posterior subapical mesenchyme. Fatemaps of the apical ridge show that apical ridge and mesenchyme do not remain in concert as development proceeds; the apical ridge expands more anteriorly than mesenchyme. The detailed behaviour of cells in the fate maps is not consistent with the model of vertebrate limb evolution which suggests an anterior bending of the posterior metapterygial axis led to the evolution of the hand plate. I have used these fate maps to investigate the relationship between cell lineage and gene expression. I show that the Hoxa-13 and Hoxd-13 expression domains expand by different mechanisms. I investigated activation of 5' HoxA genes in response to FGF-4, and show that Hoxa-13 gene expression may require FGF-4 for elaboration but not initiation. I have also used the fate maps to investigate cell fate in regulating and 'regenerating' limbs. I show that regulation and regeneration can only operate in the progress zone. Furthermore I show polarising signals act over a very short distance in reprogramming cells in the anterior tip to produce posterior digits. Finally I investigated expression of genes in the Notch signalling pathway in chick limb outgrowth and patterning. I show that genes in the Notch signalling pathway are expressed at different stages of chick limb development, and are implicated in bud outgrowth, formation of the musculature and vasculature and in digit spacing and positioning