Abstract Current models of engineered human skeletal muscle mainly consist of myogenic cells and fibroblasts which are grown in various types of natural or synthetic biomaterial scaffolds. The simplified cellular makeup of these tissues can limit their utility in disease modeling and regenerative therapies, which can be improved by incorporating additional muscle-resident cell types such as vascular cells. However, to date, no studies have demonstrated successful in vitro vascularization of mature functional engineered muscle without loss of contractile function. The Bursac lab has been the first to engineer contractile human skeletal muscle tissues (“myobundles”) made of primary human myoblasts or induced pluripotent stem cell-derived muscle progenitors. My preliminary results show that under optimized conditions, mixing myoblasts with 5% endothelial progenitor cells (EPCs) at the time of tissue formation produces vascularized tissues with contractile function comparable to that of muscle-only controls. Importantly, with time of culture, robust vascular networks formed throughout the myobundle volume undergo early lumen formation. Building on these promising results, I will test the hypotheses that engineering dense capillary networks inside highly functional engineered human muscle will: 1) support the maintenance of the muscle stem cell (satellite cell, SC) niche and enhance in vitro regenerative capacity of myobundles and 2) accelerate vascularization and perfusion of myobundle implants to improve their survival and therapeutic efficacy in volumetric muscle loss (VML) injury model in vivo. Specifically, I will characterize the effect of myobundle-EPC coculture on the transcriptomic profile of resident SCs using single cell RNA-sequencing and will investigate vascularization-induced changes in SC activation and muscle regeneration in response to a toxin injury. In immunocompromised mice in vivo, I will analyze how establishment of blood flow through pre- vascularized myobundle implants affects SC phenotype and will further assess potential of implanted myobundles to induce repair of VML injury in the tibialis anterior muscle. If successful, this work will establish the first biomimetic model of highly functional, vascularized human skeletal muscle tissue and will provide a foundation for future pursuits of engineered muscle therapies for VML.