Tunica externa (adventitia)
Tunica externa (adventitia)
Also known as: adventitia, tunica adventitia, vasa vasorum
The tunica externa (adventitia) is the outermost layer of the vessel wall. It is made mainly of type I collagen and elastic fibres with fibroblasts. It anchors the vessel and carries the vessel's own blood vessels (vasa vasorum) and nerves (nervi vasorum).
Overview
The adventitia is loose to dense connective tissue that blends into the surrounding tissue and holds the vessel in place. In arteries it is usually thinner than the media. In veins it is usually the thickest layer, and large veins contain longitudinal smooth muscle bundles in it.
Vasa vasorum. These small vessels supply the outer part of the wall in vessels too thick to be nourished from the lumen alone. In the thoracic aorta they reach the outer half or two-thirds of the media. Veins have more vasa vasorum than arteries, because the blood in their lumen is low in oxygen.
Nervi vasorum. These are mainly sympathetic fibres. Their varicosities lie at the border between the adventitia and the media, where they release noradrenaline onto the smooth muscle.
The adventitia also contains lymphatics, and its fibroblasts and progenitor cells take part in remodelling and inflammation of the vessel wall.
Anatomy
- Histology
Collagen (mostly type I) and elastic fibres, fibroblasts, vasa vasorum, nerves and lymphatics. Large veins also contain longitudinal smooth muscle.
Function
- Anchors the vessel to the surrounding tissue and limits over-distension.
- Carries the vessel's own blood supply (vasa vasorum) and nerves (nervi vasorum).
- Provides the tensile strength that holds sutures in vascular surgery.
Clinical significance
Syphilitic aortitis
Tertiary syphilis inflames and narrows the vasa vasorum of the ascending aorta (obliterative endarteritis). The media is starved of blood and weakens, producing a "tree-bark" ascending aortic aneurysm and aortic regurgitation.
Large-vessel vasculitis
In giant cell arteritis and Takayasu arteritis, inflammation begins in the adventitia and vasa vasorum and then spreads inward through the wall.
Compiled from standard anatomy and physiology references — see the Cardiovascular System sources. For education only; not medical advice.