Fluorescently Labeled Chicken Embryo Heart (IMAGE) University of Pennsylvania Caption Researchers from the University of Pennsylvania used fluorescently labeled embryonic chicken hearts to study the role collagen plays in the stress of a heartbeat. By treating the heart tissue with chemicals that that either made the collagen stiffer or softer and measuring how much individual cells moved on each beat, they could see there is an optimal stiffness "sweet spot" that changes as the heart develops. Credit University of Pennsylvania Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.