STEREOLOGICAL ANALYSIS OF THE LAYERED COLLAGEN OF HUMAN INTRACRANIAL ANEURYSMS

Intracranial saccular aneurysms are balloon-like distensions of the ar terial wall: they increase in size gradually, a few to the point of bl eeding or catastrophic rupture, Collagen is the primary structural com ponent of the aneurysmal wall, and because only a small fraction of an eurysms fail, the collagen fabric must effectively reorganize in order to maintain mechanical integrity as an aneurysm changes size. Data we re obtained from four human aneurysms, fixed at 100 mmHg of distending pressure with 10% buffered formalin, and sectioned completely through at 4 mu m thickness. Each set of measurements included groups of data taken layer by layer from a radial corridor across the aneurysm wall. Each three-dimensional orientation measurement, for which we used a Z eiss polarizing microscope with a universal stage attachment, is defin ed by an azimuth and elevation angle relative to the section plane. We compared the interdependence of these measured angles with a mathemat ical model based on fibres following great circle trajectories, and re lated the measured azimuth and elevation angles to the relative depth of the section into the aneurysm. Data were plotted on Lambert equal-a rea projections, along with the theoretical relation between azimuth a nd elevation, that included wall thickness and depth of sectioning. Th e graphical relationship between measured azimuth and elevation for co llagen fibres across the layered fabric of the aneurysmal wall is cons istent with the theoretical great circle trajectories for collagen fib re alignment, Analysis was based on statistics for spherical data to g ive values for the mean orientation and the circular standard deviatio ns (CSD) about that mean, The results indicate that any given region o n the aneurysm wall is made up of many, very thin sublayers, most of w hich have a relatively coherent organization (mean CSD 8 degrees). The se measurements agree well with the mathematical model and. when consi dered collectively, the layers provide a balanced distribution for bea ring the biaxial tensile stress of the wall.