It may occur via preexisting vascular redundancy (analogous to engineered redundancy), as in the circle of Willis in the brain, or it may occur via new branches formed between adjacent blood vessels (neovascularization), as in the eye after a retinal embolism or in the brain when an instance of arterial constriction occurs due to Moyamoya disease.
It can happen via preexisting vascular redundancy, similar to engineered redundancy, such as in the circle of Willis in the brain, or through new branches between adjacent blood vessels (neoformation), as seen in the eye after a retinal blockage or in the brain when arterial constriction due to Moyamoya disease occurs.