Early fetal development of the human vertebral artery especially at and above the occipitovertebral junction.

Little is known about the chronological changes that occur in the topographical anatomy of the fetal vertebral artery (VA), especially at and above the occipitovertebral junction. We histologically examined paraffin-embedded horizontal, sagittal and frontal sections of the heads of 25 human embryos and fetuses of crown-rump length 20-110 mm, corresponding to 6-15 weeks of gestation. At 6 weeks, the VA ran anterosuperiorly through a large intracranial subdural space filled with loose mesenchymal tissue. This intracranial course was distant from the brain stem, suggesting that the developing brain did not "guide" the VA. Before 8-9 weeks, the VA appeared to take an almost straight upward course at the occipitovertebral junction. Later, however, the atlanto-occipital joint growing along the mediolateral axis caused the VA to curve at the junction area. In specimens before 10 weeks, the terminal of the VA, or the origin of the basilar artery, was on the anterior side of the inferior olive and near the jugular foramen. The fetal posterior inferior cerebellar artery originated at the same caudal site distant from the primitive cerebellum. Later, the terminal or origin moved rostrally to the level of the pons. These findings indicate that the basic branching pattern of the VA-basilar artery is independent of the developing brain, whereas the arterial courses are secondarily "corrected" by the growing brain and atlanto-occipital joint.