Endometrial Collagen in Woolly Monkey ( Lagothrix l. poeppiggi ) and the Uakari ( Cacajao calvus )

Introduction This study microscopically characterizes collagen changes and their relationship with endometrial architecture in two Neotropical primates. Methods Uterine fragments from six Lagothrix l. poeppigii and six Cacajao calvus in different reproductive phases were histologically processed using Picrosirius Red staining. Results In the functional layer, collagen was highest in the early proliferative phase (5.19% ± 2.53%; p < 0.001), whereas the basal layer had the lowest percentage (5.85% ± 4.69%; p < 0.01). Type I collagen predominated, while type III was less expressed. Type I collagen varied significantly between proliferative phases ( p < 0.001). The basal layer exhibited more fibers in the secretory phase (14.03% ± 8.50%; p < 0.001). Type III collagen peaked in the functional layer (0.41% ± 0.17%; p < 0.001) and basal layer (0.69% ± 0.46%; p < 0.05). Conclusion Picrosirius Red staining with polarized light effectively highlights collagen organization, aiding reproductive phase characterization in Neotropical primates.