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.