  The mechanical properties of biological tissues are strongly correlated to
the specific distribution of their collagen fibers. Monitoring the dynamic
reorganization of the collagen network during mechanical stretching is however
a technical challenge because it requires mapping orientation of collagen
fibers in a thick and deforming sample. In this work, a fast
polarization-resolved SHG microscope is implemented to map collagen orientation
during mechanical assays. This system is based on line-to-line switching of
polarization using an electro-optical modulator and works in epidetection
geometry. After proper calibration, it successfully highlights the collagen
dynamic alignment along the traction direction in ex vivo murine skin dermis.
This microstructure reorganization is quantified by the entropy of the collagen
orientation distribution as a function of the stretch ratio. It exhibits a
linear behavior, whose slope is measured with a good accuracy. This approach
can be generalized to probe a variety of dynamic processes in thick tissues.
