Intracranial arteries have a unique structure when compared to extracranial vessels of similar size: see general histology of blood vessels entry.
Proximal larger arteries
The proximal arteries, arising from the internal carotid and vertebral arteries have differing distribution of elastic fibers compared to similar sized vessels elsewhere (this has been disputed by FT Merei; 1980). Although the tunica media and tunica adventitia are present they are only a third as thick as their extracranial counterparts, with the vast majority of elastic fibers located in a subendothelial elastic lamina. This fundamental difference accounts for the markedly different natural history of intracranial arterial dissections compared to their extracranial counterparts. When a tear breaches the aforementioned subendothelial elastic layer, then there is little tissue preventing extension into the subarachnoid space, thus accounting for the very high rate of subarachnoid hemorrhage.
Distal smaller arteries and arterioles
The branches that penetrate the brain are surrounded by a sheath of leptomeninges which prolongs the subarachnoid space, thus forming the Virchow-Robin spaces. This replaces the tunica adventitia which is absent in these vessels, and is in direct contact with the tunica media. The space terminates as the glia limitans (a subpial layer formed by end-feet of astrocytes) fuses with the basal lamina of the smallest arteriole.