Pathogenesis of the so-called cystic adventitial degeneration of peripheral blood vessels.

Eleven cases of cystic adventitial degeneration of peripheral blood vessels were examined by light and (in three instances) by electron microscopy. Eight cases were in arteries, three were detected in veins. In six cases a cellular lining of the cysts was observed. Ultrastructurally the lining cells resembled synovial mesothelial cells. In three cases a pedicle connection was detected between the adventitial cysts and an adjacent joint. In one case such a pedicle could be followed to its junction with the knee joint capsule and was also lined by synovial mesothelium. These findings indicate that the so-called cystic adventitial degeneration is not a primary dysplasia of the blood vessel wall but originates from ectopic tissue of a joint capsule or bursa.