Getting Stiffer (IMAGE) University of Pennsylvania School of Medicine Caption During early development in an embryo, the progenitor cells of many types of musculoskeletal tissue start out in close contact to each other and over time transition into an organized network of individual cells surrounded by an extracellular matrix. Throughout the course of embryo development, it gets stiffer due to increased amounts of matrix material and crosslinking. Credit The lab of Robert Mauck, Perelman School of Medicine, 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.