Traditional view considers that the arterial wall is composed of three concentric tissue coats (tunicae): intima, media, and adventitia. However, large- and medium-sized arteries, where usually atherosclerosis develops, are consistently surrounded by periadventitial adipose tissue (PAAT). Here we update growing information about PAAT, and conceptualize it as the fourth coat of arterial wall, that is, tunica adiposa (in brief, adiposa, like intima, media, adventitia). Recent evidence has revealed that adipose tissue expresses not only metabolic, but also secretory (endo- and paracrine) phenotype, producing/releasing a large number of signaling proteins collectively termed adipokines. Through paracrine ("vasocrine") way, adiposa-derived mediators may contribute to various arterial functions such as contraction-relaxation, smooth muscle cell growth, inflammation, hemostasis, and innervation, hence to "outside-in" signaling pathway of atherogenesis.
Biomedical Reviews 2009; 20: 41-44.
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