Reappraisal of the structure of arterial Tunica adventitia and its involvement in atherosclerosis
Tunica adventitia was previously considered an inert fibrous layer only involved in nutritional and physical support of the arterial wall. Recent studies reveal that it is an important dynamic layer actively involved in the regulation of vascular structure, function, response to injury and disease processes especially atherosclerosis. Many anatomical studies on arteries, however, still make only peripheral reference to it, without elucidating its detailed structure. Knowledge of the latter is important in understanding pathobiology and interventional approaches to atherosclerosis. This review, therefore, aims at consolidating contemporary literature on the structure and clinical significance of the arterial tunica adventitia. Google literature search was done using the key words tunica adventitia combined with: artery, aorta, cells, cell types, collagen, elastic fibres, vasa vasora, lymphatics, nerves, atherosclerosis. There is overwhelming evidence that the tunica adventitia comprises multiple types of collagen and elastic fibres arranged in various directions, a wide variety of cells including fibroblasts, smooth muscle, pericytes, myofibroblasts, leukocytes, mononuclear phagocytic, defence, mesenchymal stem and mast cells; vasa vasora including microvasculature, lymphatics and neural elements. The exact cellular, fibre composition and orientation vary between various arteries and regions of the same vessel. Its components are involved in the initiation, progression and complications of atherosclerosis. In conclusion, the tunica adventitia is an active dynamic layer which, besides mechanical and nutritive functions; has metabolic, regulatory and defence roles that are critical in arterial homeostasis and atherosclerosis. Vascular studies should always include detailed analysis of the biology of the tunica adventitia.
Key words: Tunica adventitia, cells, fibres, vessels, nerves, atherosclerosis