Graphical Abstract (IMAGE) AlphaMed Press Caption Biological aging of tracheobronchial tissue specific stem cells (TSC) and their trophic unit, the pseudostratified conducting airway epithelium. Each injury activates a subset of TSC which proliferate and undergo terminal differentiation. Thus, each injury cycle depletes the TSC pool and many injuries compromise epithelial regeneration. Over time, biological age exceeds chronological age and increases the risk of chronic lung disease. Credit AlphaMed Press Usage Restrictions None License Original 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.