Adventitia

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Close up view of an artery and a vein in cross-section.

The term adventitia can typically refer to the outermost layer of the blood vessel wall or the outer connective tissue layer of organs devoid of mesothelium, that blends into the surrounding tissues.

In relation to blood vessels, the adventitia is sometimes called the tunica adventitia or tunica externa. It is mainly composed of type 1 collagen and few elastic fibers. Generally, it consists of connective tissue containing nerves (nervi vasorum) which are responsible for controlling the smooth muscule tone of the vessel, and a network vessel capillaries called vasa vasorum, that nourish the vessel themselves. This outer layer blends with the surrounding connective tissue stroma of the organ in which it lies.

In the context of organs, the adventitia refers to the outer coat of loose connective tissue surrounding organs or parts of organs that lie outside closed body cavities. Thus, these organs are not lined with mesothelium, which forms serosa. This adventitia usually contains some amount of adipose tissue along with vessels and nerves that supply the organ. It surrounds several organs including organs such as the trachea, esophagus, anal canal, vagina and parts of the urinary bladder.