While some vertebrates regenerate complex tissues such as limbs and eyes, the ability to restore damaged tissues and organs in humans is largely limited. In contrast, some invertebrates such as planarian flatworms possess extraordinary abilities to regenerate complete animals from small tissue fragments. However, in contrast to other planarian species, Procotyla fluviatilis is limited in its ability to restore lost structures, failing to regenerate heads when injured in the posterior tissues. The goal of this project is to elucidate the cellular and molecular factors that allow some planarian tissues, but not others, to regenerate missing parts. To identify the critical mechanistic failure in P. fluviatilis regeneration, the early stages of wound healing, cell proliferation, and the re-establishment of axial polarity will be compared in tissues with differential regeneration potentials. Subsequently, next generation sequencing technology will be utilized to examine and compare global changes in gene expression following amputation in regeneration-proficient and regeneration-deficient tissues. Transcripts differentially expressed in regenerating and non-regenerating tissues will be validated by quantitative PCR and in situ hybridization and the role of these genes in regeneration will be determined by RNA interference. Performing similar investigations in the regeneration-competent planarian Schmidtea mediterranea will allow a comprehensive comparative analysis of genes critically involved in regeneration between closely related organisms with different regeneration potentials. These studies will identify the permissive and/or inhibitory factors at work in tissues with different regeneration abilities and lay the foundation for studies of regeneration loss in other organisms. Understanding the factors leading to regeneration proficiency and deficiency is a crucial step in advancing the field of regenerative medicine, which attempts to confer restorative abilities to non-regenerating tissues.