Is the epithelium vascularized? How do nutrients and oxygen reach the epithelium? Why is this feature an important evolutionary acquisition?
Epithelia are not vascularized (capillaries do not directly reach their cells). The epithelium exchanges substances by diffusion with the connective tissue situated under it. Since the epithelia are not vascularized minuscule skin injuries or scratches that happen all the time do not trigger bleeding and do not expose the blood to contamination from external agents. This is an important protective strategy discovered by evolution.
