Adventitia

Overview
Adventitia is the outermost connective tissue covering of any organ, vessel, or other structure.

For example, the connective tissue that surrounds an artery is called the tunica adventitia because it is considered extraneous to the artery.

To some degree, its role is complimentary to that of the serosa, which also provides a layer of tissue surrounding an organ. In the abdomen, whether an organ is covered in adventitia or serosa depends upon whether it is peritoneal or retroperitoneal:
 * peritoneal organs are covered in serosa (a layer of mesothelium, the visceral peritoneum)
 * retroperitoneal organs are covered in adventitia (loose connective tissue)

In the gastrointestinal tract, the muscularis externa is bounded in most cases by serosa. However, at (the thoracic esophagus, ascending colon, descending colon and the rectum), the muscularis externa is instead bounded by adventitia. (The muscularis externa of the duodenum is bounded by both tissue types.)

The connective tissue of the gallbladder is covered by adventitia where the gallbladder bounds the liver, but by serosa for the rest of its surface.