Fibroblasts (IMAGE) University of North Carolina Health Care Caption Fibroblasts (red) express endothelial markers (green), making the heart cells in mice appear yellow. The green blood vessels running horizontally are partly composed of red/yellow cells, suggesting that fibroblasts are incorporated into blood vessels. Credit Eric Ubil, PhD 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.