Photomicrographs of sections showing central region of the guinea-pig pubic symphysis obtained at D21 of pregnancy (A, B) and at parturition (C, D). Sections were studied by the picrosirius-polarization method (a specific procedure for the detection of oriented collagenous structures in tissue sections). Densely packed bundles of interlaced collagen fibers are observed in the interpubic ligament with conventional illumination (A) or under polarizing optics (B). All brightly birefringent structures, which shine against a dark background, are of a collagenous nature. At parturition, irregularly disposed thin and thick collagenous fibers are observed in sections when studied under conventional illumination (C); polarization microscopy of the same field shows a high degree of collagen remodeling (D). Note the presence of high number of blood vessels at parturition (C). Scale bar = 25 μm.