PROJECT SUMMARY The pericardium is a mesothelial sac that encapsulates the heart to support its development and maintain cardiac homeostasis over time. Throughout the process of heart looping, the pericardium serves as a cell source to form various cardiac structures, making it a crucial player in heart morphogenesis. However, little is known about its developmental origins and how it acquires its unique properties in cardiac development, homeostasis, and regeneration post-injury. While early mesothelial origins overall have only been recently described in detail using the zebrafish, the pericardium has been particularly difficult to study due to its dynamic development and a lack of genetic markers. Our lab has mapped mesothelial progenitors to the lateral plate mesoderm expressing the transcription factor Hand2, and further linked Hand2 function to mesothelium and pericardium formation. These conceptual and technical advances in mesothelial biology enable us to investigate how pericardial and cardiac structures come together to form a single functional organ system during development. My expansion of this data suggests that pericardial and cardiac progenitors are distinct progenitor populations that later come together to fuse together during embryogenesis. The objective of this proposal is to uncover the developmental mechanisms governing progenitor patterning into the heart and pericardium and identify the role of canonical Wnt signaling that controls the pericardial emergence. In Aim 1, I will use a combination of transgenic zebrafish lines that label the developing heart and pericardium, in vivo time-lapse imaging, stable genetic knockdowns of cardiac components, and multi-color lineage tracing reporters to examine the cellular origins and dynamics of the developing pericardium. In Aim 2, I will investigate the interplay between Hand2 and canonical Wnt signaling by documenting the expression dynamics of Wnt targets in Hand2 mutant zebrafish, overexpressing Wnt components, and using CRISPR-Cas9 mutagenesis to mutate Hand2 binding sites. Together, this project will be the first to fully characterize how pericardial development proceeds and incorporates the heart and the interplay between Hand2 and canonical Wnt signaling in controlling pericardium development.