Introduction: Vascular smooth muscle cells (SMCs) play a pivotal role in regulating vascular tone in arteries, and are therefore an essential part of constitutive models of the arterial wall. Since an SMC exerts its contractile force mainly along its longitudinal axis, SMC orientation is a major determinant of arterial biomechanical behaviour. To our knowledge, all current constitutive models assume SMC orientation to be perfectly circumferential. Moreover, the mechanical effect of any dispersion in SMC orientation is neglected. In this study, we developed a method to quantify in 3D the orientation of SMCs in the intact arterial wall.

Methods: SMC nuclei of six excised and mounted murine carotid arteries were imaged using two-photon laser scanning microscopy. 3D image processing was used to identify individual nuclei and their orientations. Subsequently, SMC locations and orientations were converted to cylindrical and spherical coordinate systems, respectively. We found SMCs to be arranged in distinct inner and outer layers. For each layer, SMC orientations were described by a Bingham distribution, a statistical distribution to describe axis orientations in 3D.

Results and Conclusion: Distributions show a statistically significant (p<0.05) helical angular component in both layers (inner: Θ_h=2.7±2.7°, outer: Θ_h=5.1±2.7°, mean±SD), and considerable dispersion. We conclude that vascular SMC orientation 1) can be quantified in 3D, 2) has a distinct helical component, and 3) shows considerable dispersion. The acquired distribution data are essential to improve constitutive modelling of the arterial wall, by quantitatively describing physiological SMC orientation and dispersion.