Circulation of the cephalic part of the embryo begins early in stage 10 (approx. 28 days), while the rostral neuropore is still open. It is the prosencephalon which is irrigated first by the internal carotid arteries, resulting from dorsal aortas and more particularly from the 3rd aortic arch.
The rhombencephalon and the mesencephalon are irrigated a little later by the basilar trunk (A. basilaris), resulting from the confluence of the anterior spinal artery and the vertebral arteries which arise from the subclavian arteries and whose embryonic origin diverges.
In fact, the right subclavian artery derives for its proximal part from the 4th aortic arch and continues with the 6th intersegmental artery. The right primitive dorsal aorta obliterates in its caudal part. The left subclavian artery, on the other hand, derives from the 6th intersegmentary artery which arises directly from the aorta.
Circulus arteriosus cerebri (Willis)
The cerebral arteries arise from two different vascular systems, one anterior and one posterior. The former is based on the Aa. carotideae internae (stage 9), the second starts from the Aa. vertebrales (stage 15). Via various anastomoses, these four arteries form the circulus arteriosus cerebri (Willis) (stage 19), a hexagonal vascular ring on the ventral part of the diencephalon and mesencephalon. The main vessels supplying the brain go from this ring. This circulus arteriosus cerebri reaches its definitive shape at the beginning of the 8th week (stages 22-23). This supply of the brain is subject to autoregulation and is therefore independent of peripheral blood pressure fluctuations. Although the brain only accounts for around 2% of the body's mass, it takes up around 15-20% of the cardiac output and thus the oxygen supply.
The anatomical situation is simplified as follows:
- The internal carotid artery supplies about 80% of the brain. It represents one branch of the common carotid artery and enters the cranial cavity through the carotid canal of the temporal bone. It crosses the sinus cavernosus, passes the dura mater and discharges a main vessel, the ophthalmic artery and four end arteries: the anterior cerebral artery, the middle cerebral artery, the posterior communicating artery and the A. choroidea anterior.
- The vertebral artery arises from the subclavian artery at the root of the neck. It passes through the transverse holes of the cervical vertebrae and enters the skull through the foramen magnum. After crossing the dura mater, it approaches the lateral aspect of the medulla oblongata. The two vertebral arteries then approach each other to merge near the border between the bulb and the protuberance. This is how the basilar trunk (A. basilaris) is formed. The latter rises to the ventral surface of the protuberance and divides into two terminal branches, the right and left posterior cerebral arteries (stage 17), a little above the ponto-mesencephalic sulcus.
- The anterior side of the circle of Willis is formed by the single anterior communicating artery and the two anterior cerebral arteries. Laterally, it is formed by the two posterior communicating arteries. Back it is finally completed by the posterior cerebral arteries.
In the embryo, the Willis polygon receives blood richer in oxygen than the rest of the body. This is because most of the blood from the placenta via the inferior vena cava passes from the right atrium to the left atrium through the foramen ovale (formerly called Botal's hole). The carotid arteries emanate from the aorta before the mouth of the ductus arteriosus and therefore before the influx of mixed blood from the right ventricle
It should be noted that the cerebral circulation is a terminal circulation, which explains the seriousness of the vascular lesions at the cerebral level. Indeed, the cortical arteries (superficial) and the central arteries (deep) are of the terminal type, that is to say without functional anastomoses with the neighboring arteries. In the event of arterial or arteriolar occlusion, there will be no replacement possible (except in rare individual cases).
Arterial vascular territories
The detail of the arterial vascularization exceeds the objective of this module. We will therefore schematically approach the distribution of the main cortical and central vascular territories according to the embryonic structures described in this module.
The venous brain system
Unlike its development in other organs, the cerebral venous circulation is very different from the arterial circulation. The venous system arises in all the capillary fields in the vicinity of the arterial endings and forms a deep venous system and a superficial or cortical venous system.
Venous blood from both systems is brought to incomprehensible vessels, large in diameter and free of valves located in the duplication of the dura. The whole of the sinuses (Sinus durae matris) is finally drained towards the confluens sinuum, the cinus transversus and the Vv. jugulares internae which bring blood back to the heart.