A number of disease states arise from the loss or destruction of tissue, e.g., loss of pancreatic islets cells results in Type I Diabetes and loss of adrenal cortical cells results in Addison's Disease. The ultimate treatment of these conditions would involve (1) reversal of the destructive process and (2) restoration of normal tissue. Tissue regeneration is the process by which lost or damaged tissue is restored to its normal state. The mechanisms, which regulate this dynamic and essential process, remain incompletely understood. Two major mechanisms include the regulation of (1) tissue-specific adult stem cells and (2) lineage development, which likely involves cellular differentiation, de-differentiation and trans-differentiation. Employing a classic model of tissue regeneration, the mouse adrenal cortex, we will investigate these fundamental processes. Using fluorescence activate cell sorting, we will identify, isolate and characterize adrenal progenitor cells from animals undergoing a variety of controlled manipulations. Progenitor cells will then be examined for their differentiation potential in transplantation model systems. Using a gene targeting approach and Cre-LoxP technology we will genetically modify mouse adrenal cells in order to perform lineage tracing and tissue-specific knockout experiments. These experiments are designed to elucidate fundamental mechanisms of adrenal development and zonation and begin to define the molecular pathway of adrenal regeneration. This work is being performed in a highly supportive environment with expertise in each major area proposed for study. Furthermore, this work builds upon the prior experiences of the PI, generating transgenic animals to study gene expression and lineage development, by incorporating the modern tools of gene targeting and Cre-LoxP technology. Finally, by combining this work with limited clinical responsibility (10% time) as a Pediatric Endocrinologist, the PI will continue to benefit from clinical encounters, which often lead to novel insights into the mechanisms of disease. It is expected that this work will be enable the PI to transition to a career as an independent researcher.