The arterial pattern at the base of arhinencephalic and holoprosencephalic brains.

The mechanisms by which the anatomical variations of the circle of Willis develop is considered to be related to haemodynamic factors, i.e. the differential growth of the various parts of the brain will continuously change the haemodynamic demands and consequently the flow patterns in the cerebral arteries. It is therefore to be expected that, if a selected part of the brain does not develop, the change in the haemodynamic demand will affect the development of some cerebral arteries. Consequently the arteries at the base of 2 arhinencephalic and 8 holoprosencephalic brains were studied in conjunction with the brain malformations. The defects of holoprosencephaly are believed to arise from a failure of the prosencephalon to separate fully into the telencephalon and diencephalon and become manifest at the time that the prosencephalon normally starts to separate into the hemispheres, i.e. 28-34 d p.c. Arhinencephalic brains are fully diverticulated. There is only a partial or complete agenesis of the olfactory tracts and bulbs. The defect causing arhinencephaly starts at 43 d p.c. In the arhinencephalic brains no particular vascular abnormalities were found. However, at the base of the holoprosencephalic brains no complete circle of Willis was present; the anterior part was lacking and was replaced by anterior branches which emerged unilaterally or bilaterally from the internal carotid artery. The choroidal arteries were of very large calibre and ran to the highly vascularised wall of the dorsal cyst which is usually present in holoprosencephalic brains. In contrast to the anterior part, the posterior arterial pattern was almost identical to the posterior part of the circle of Willis of normal brains. The basic vascular patterns found in the holoprosencephalic brains displayed the features of Padget's developmental stages 2 and 3 of the cerebral vasculature, i.e. the pattern that has normally developed within 28-40 d p.c. The further modification of this pattern could largely be understood from the functional demand imposed on the circulation by the enlarged anterior choroidal arteries. Because the development of the anterior part of the circle of Willis precedes the developmental derangement causing arhinencephaly, a complete circle was found in these brains.