Fig S.3.

Simulation to estimate the volume fraction of fibres using the area fraction derived from 2D microscopy images. A 3D voxel space was created in which randomly packed fibres at random orientation were added. At each increase in the fibre volume fraction, a virtual slice was taken from which an area fraction was estimated as the percentage of positive pixels after thresholding, just as in the histological images. This yielded a mapping between the area fraction and the volume fraction that is given by the curve on the right.