Berry aneurysm - defect lies in -
Berry's aneurysm is a brain aneurysm caused due to a defect in media. Berry aneurysms arise at the point of weakness in the arterial media, especially branch points in the circle of Willis.
There is a congenital defect in tunica media of cerebral vessels which leads to aneurysm formation.
Degeneration of internal elastic lamina
Degeneration of media/muscle cell layer
Deposition of mucoid material in media
Low grade inflammation of vessel wall