Date of Award


Document Type

Campus Access Dissertation

Degree Name

Doctor of Philosophy (PhD)


Biology/Molecular, Cellular, and Organismal Biology

First Advisor

Catherine D. McCusker


The recent conflicts in the Middle East have resulted in over 1600 amputations among American service members, which have significant implications for veteran independence and mental health. Though humans cannot regenerate from such injuries, the Mexican axolotl (Ambystoma mexicanum) has this capacity, and it has fascinated researchers for hundreds of years. Axolotl limb regeneration is an amazing process where terminally differentiated cells revert to a progenitor cell-like state, known as a blastema cell, and rebuild the missing limb pattern. For this new pattern to be reconstructed, the pattern information within these “old cells” that contribute to the blastema must become competent to generate new pattern, a property we call positional plasticity. Although signals from the nerve have been shown to induce and maintain positional plasticity, the mechanisms mediating this process are not well understood. Using a new surgical based assay that allows us to detect when limb cells become positionally plastic I showed that blastema cells become positionally plastic once they begin proliferating and re-expressing limb development genes. Through the use of regenerative assays I developed, I identified signaling molecules downstream of the limb nerves that are required and sufficient to induce positional plasticity and that part of this mechanism involves largescale downstream modifications to chromatin structure. Using next generation sequencing assays, I identified genes that play key roles in the regeneration and positional memory located within regions of chromatin that are altered by nerve-dependent signaling. Finally, using novel regenerative assays, I uncovered a new state in which the blastema exhibits properties of both positional plasticity and stability called quasi-plasticity. I demonstrated that this quasi-plastic state appears to be part of the normal development of the blastema. As we begin to parse out the mechanisms controlling pattern generation and positional memory in regenerating systems, we are beginning to understand how various cell signaling pathways guide the development of limb-progenitors into functional pattern. These findings bring us one step closer to understanding how to one day induce limb regeneration in humans.