'Medial defects' in the prenatal human cerebral arteries: an electron microscopic study.

Background and Purpose Fine structural studies were performed to investigate the histogenesis of human intracranial arteries. Special attention was paid to whether “medial defects” exist in these arteries. Methods Segments of the intracranial extracerebral arteries of normal human embryos (n=6) were examined with transmission electron microscopy. Results Focal defects of the medial smooth muscle cells were disclosed at every bifurcation of the developing arteries. This configuration persisted until the arteries obtained enough muscle coat. These areas, in which an absence of medial smooth muscle cells (ie, a medial defect) existed, were occupied by fibrous connective tissues of elastin and collagen. Conclusions The medial defect observed at the arterial bifurcation of the embryos seems to be a development process that accompanies human ontogenesis rather than a congenital anomaly, supporting a possible pathogenesis for intracranial saccular aneurysms.