Layered structure of saccular aneurysms assessed by collagen birefringence

Cerebral aneurysms are composed principally of collagen, a birefringent protein which is responsible for withstanding the forces of blood pressure. The known correlation between collagen birefringence and its mechanics provides the basis for using polarizing microscopy to evaluate the strength of collagen, layer by layer across the aneurysmal wall. In order to obtain better quantitative measurements, several birefringent enhancement stains were investigated. We concluded that sirius red F3B, at a concentration of 0.05% in saturated picric acid, is an excellent stain to enable measurement of both birefringence and directional organization on the same tissue sections. Six aneurysms from autopsy, fixed at 120 mmHg, and one surgical specimen were cut at 4 microns to provide sets of tangential sections. The polarizing optics emphasizes the multi-layered structure of the aneurysmal wall with the mean fiber alignments distinguishing one layer from another. Birefringence measurements showed that the outer third of the wall had mainly higher strength collagen, although not as high as nearby artery adventitia. The inner layers of the aneurysms had intermediate values, similar to the artery media and subendothelium. Our results are consistent with a model of aneurysmal enlargement that requires the reorganization of higher strength outer fibers while new collagen is added to the inner layers.