|Micrograph showing cystic medial degeneration. The tunica adventitia (yellow at bottom of image) with vaso vasorum is also seen. Movat's stain.|
The vasa vasorum (Latin, "the vessels of the vessels") is a network of small blood vessels that supply the walls of large blood vessels.
The vasa vasorum are found in large arteries and veins such as the aorta and its branches. These small vessels serve to provide blood supply and nourishment for tunica adventitia and outer parts of tunica media of large vessels.
Studies conducted with 3-dimensional microcomputed tomography (3D Micro-CT) on porcine and human arteries from different vascular beds have shown that there are three different types of vasa vasorum:
- Vasa vasorum internae, that originate directly from the main lumen of the artery and then branch into the vessel wall.
- Vasa vasorum externae, that originate from branches of the main artery and then dive back into the vessel wall of the main artery.
- Venous vasa vasorae, that originate within the vessel wall of the artery but then drain into the main lumen or branches of concomitant vein.
Depending on the type of vasa vasorum, it penetrates the vessel wall starting at the intimal layer (vasa vasorum interna) or the adventitial layer (vasa vasorum externa). Due to higher radial and circumferential pressures within the vessel wall layers closer to the main lumen of the artery, vasa vasorum externa cannot perfuse these regions of the vessel wall (occlusive pressure).
The structure of the vasa vasorum varies with the size, function and location of the vessels. Cells need to be within a few cell-widths of a capillary to stay alive. In the largest vessels, the vasa vasorum penetrates the outer (tunica adventitia) layer and middle (tunica media) layer almost to the inner (tunica intima) layer. In smaller vessels it penetrates only the outer layer. In the smallest vessels, the vessels' own circulation nourishes the walls directly and they have no vasa vasorum at all.
Vasa vasorum are more frequent in veins than arteries.  Some authorities hypothesize that the vasa vasorum would be more abundant in large veins, as partial oxygen pressure and osmotic pressure is lower in veins. This would lead to more vasa vasorum needed to supply the vessels sufficiently. The converse argument is that generally artery walls are thicker and more muscular than veins as the blood passing through is of a higher pressure. This means that it would take longer for any oxygen to diffuse through to the cells in the tunica adventitia and the tunica media, causing them to need a more extensive vasa vasorum.
Associated pathologic conditions
- An interesting point of fact is that, in the human descending aorta, vasa vasorum cease to supply the arterial walls with oxygenated blood at the level of the renal arteries. Thus, below this point, the aorta is dependent on diffusion for its metabolic needs, and is necessarily markedly thinner. This leads to an increased likelihood of aneurysm at this location, especially in the presence of atherosclerotic plaques. Other species, such as dogs, do have vasa vasorum below their renal vasculature, and aneurysms at this site are substantially less likely. Cerebral blood vessels are devoid of vasa vasorum; however, these vessels have rete vasorum, which have similar function to vasa vasorum.
- Small vessels like vasa vasorum and vasa nervorum are particularly susceptible to external mechanical compression.  and thus are involved in pathogenesis of peripheral vascular and nerve diseases.
- A tear in vasa vasorum situated in tunica media layer of aorta may start pathologic cascade of events leading to aortic dissection.
- Presence of corkscrew collateral vessels in vasa vasorum is a hallmark of Buerger's disease and distinguishes it from Raynaud's phenomenon.
- Inflammation and subsequent destruction of the vasa vasorum is the cause of thoracic aortic aneurysms in tertiary syphilis. (Obliteration of the vasa vasorum results in ischemia and weakening of the aortic adventitia, leading to aneurysm.)
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