As the vertebral artery enters the skull, its adventitia and media undergo a significant reduction in thickness, associated with gross diminution or total loss of elastic fibers in these two layers of the artery wall. This transformation is most marked in the last 0.5 cm extradurally, but is not complete until about 0.5 cm beyond the point of dural perforation.
Wilkinson IMS. The Vertebral ArteryExtracranial and Intracranial Structure. Arch Neurol. 1972;27(5):392–396. doi:10.1001/archneur.1972.00490170024004