Induced regeneration and gene expression in the chick limb bud
Description
Because of its relative simplicity and high accessibility, the limb has long been the organ of choice for studying the establishment of pattern during development and regeneration. Among the tetrapod vertebrates, the urodele amphibians (newts and salamanders) alone can completely regenerate amputated limbs at maturity. The anurans (frogs) and mammals exhibit considerable regenerative ability during development that declines as development progresses. Embryonic chicken limbs regenerate only at early stages of development, immediately after the initial appearance of the limb bud. This limited regenerative capacity makes the developing chick limb an ideal candidate for examination of the barriers to regeneration in higher vertebrates The studies presented in this dissertation identify and overcome an early barrier to chick limb regeneration, the failure to regenerate a specialized apical epidermis that is required for limb bud outgrowth. A temporal window was identified during which this barrier can be at least partially overcome by exposing the limb bud stump cells to either an ectopic source of FGF-2, a member of the fibroblast growth factor family, or an apical ectodermal ridge (AER), a specialized ectodermal structure that rims the apex of the developing limb bud. Successful regeneration was associated with the restoration of expression domains of Msx1, HoxD11 and HoxD13, apical homeobox-containing genes that are involved in the formation of pattern. These results also suggest that the observed stage-related decline in regenerative response may rise as a result of a breakdown of apical mesodermal/ectodermal interactions that are required for limb bud outgrowth and patterning The results of these studies formed the foundation for Sequential Regenerative Potential Model, wherein regenerative failure in higher vertebrate limbs is proposed to be the result of multiple barriers, each associated with a distinct developmental stage. Through the systematic examination of changes in regenerative ability associated with tissue maturity, these individual barriers can be identified. To successfully induce regeneration in higher vertebrates, all of the individual barriers, which act cumulatively to suppress regeneration postnatally, have to be overcome. This model serves as a conceptual framework for dissecting the problem of regenerative failure in higher vertebrates