adventitia

The adventitia is the outermost of the three layers (tunica intima, tunica media, tunica adventitia) composing the wall of arteries and veins. Histologically, it is composed of loose connective tissue containing fibroblasts, collagen fibers, and elastic fibers, and in larger vessels it contains the vasa vasorum (small vessels supplying the vessel wall itself) and nerve fibers (nervi vascularis) that regulate vasomotor tone. In large arteries like the aorta, the adventitia also anchors the vessel to surrounding structures and resists overdistension during systole. Clinically, the adventitia is relevant in vascular pathology: aortic dissection can involve tearing through the intima with blood tracking into or through the media, and rupture ultimately breaches the adventitia leading to catastrophic hemorrhage. Vasa vasorum disruption (e.g., in tertiary syphilis affecting the vasa vasorum of the aortic adventitia) leads to medial ischemia, weakening, and aneurysm formation (syphilitic/tertiary aortitis causing thoracic aortic aneurysm). The term adventitia is also used more broadly outside vasculature to describe the outer connective tissue layer of organs lacking a serosa (e.g., esophagus, some retroperitoneal organs), where it blends with surrounding connective tissue, as opposed to organs with a serosal layer (visceral peritoneum) which have a smooth mesothelial covering. Distinguishing adventitia (fibrous, blends with surrounding tissue) from serosa (mesothelial, smooth, secretes serous fluid) is a classic histology/anatomy board question, particularly regarding the esophagus (upper 2/3 has adventitia; the abdominal segment has serosa).