Development of the cerebral vascular system

The cerebral circulatory system is formed from two systems that develop at different times: the vertebrobasilar and carotid systems. Around the 3rd month of gestation, they merge to form the Willis polygon, but in some people it remains anatomically open. By the time of fusion, the vertebrobasilar system is well developed and has many small branches. The carotid system has well-developed vessels that form the ventricular plexuses, as well as branches that supply the thalami and subcortical ganglia. The main large branches of the middle cerebral and anterior cerebral arteries pass through the brain as if in "transit", and small branches and capillaries begin to develop primarily in the cerebral cortex, leaving the white matter poorly vascularized, until the birth of the child. Probably, the expediency of such development of blood supply is explained by the fact that the periventricular zones can also be nourished by cerebrospinal fluid, and the growing neurons of the cortex need a constant influx of nutrients. The periventricular zones of adjacent circulation (parasagittal, in the area of the posterior horns of the lateral ventricles, etc.) are especially insufficiently supplied with blood.

The formation of siphons begins in the 8th month of antenatal life and ends after birth. The main purpose of siphons is to "break up" the instantaneous blood flow during systole and ensure its uniform flow regardless of the heart rhythm.

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