The tunica muscularis of human brain arteries: three-dimensional measurements of alignment of the smooth muscle mechanical axis, by polarized light and the universal stage.

The four-axis universal stage, with the polarizing microscope, was assessed for making precise three-dimensional measurements of alignment on muscle cells. The physiological dimensions of the arteries (obtained at autopsy) and the co-alignment of the myofilaments with the long axis of the muscle cells was assured by the fixation of the arteries at normal distending pressure. They were embedded in paraffin wax, either perpendicular to the cutting plane which permitted controlled realignment of the block later, or along with a rectangular block of previously fixed liver parenchyma (providing a cartesian reference). The test for accuracy showed that smooth muscle, when unstained, was measured to within +/- 5 degrees. The same tissue measured after staining by Gomori's silver impregnation, could be repeatedly measured to within 2.4 degrees. The optical axis, parallel to the morphological axis, was not altered by staining. Data from 11 tissue sections revealed that the tunica muscularis is a precisely organized structure, not only with an average alignment which is almost perfectly circumferential, but with a higher degree of alignment among the muscle fibres than has been previously reported. Smooth muscle tissue is abundant and perhaps highly ordered in many structures. The universal stage could be employed to reveal that order.