Impact of parity on ewe vaginal mechanical properties relative to the nonhuman primate and rodent

Parity is the leading risk factor for the development of pelvic organ prolapse. To assess the impact of pregnancy and delivery on vaginal tissue, researchers commonly use nonhuman primate (NHP) and rodent models. The purpose of this study was to evaluate the ewe as an alternative model by investigating the impact of parity on the ewe vaginal mechanical properties and collagen structure. Mechanical properties of 15 nulliparous and parous ewe vaginas were determined via uniaxial tensile tests. Collagen content was determined by hydroxyproline assay and collagen fiber thickness was analyzed using picrosirius red staining. Outcome measures were compared using Independent samples t or Mann-Whitney U tests. ANOVA (Gabriel's pairwise post-hoc test) or the Welch Alternative for the F-ratio (Games Howell post-hoc test) was used to compare data with previously published NHP and rodent data. Vaginal tissue from the nulliparous ewe had a higher tangent modulus and tensile strength compared with the parous ewe (p < 0.025). The parous ewe vagina elongated 42 % more than the nulliparous ewe vagina (p = 0.015). No significant differences were observed in collagen structure among ewe vaginas. The tangent modulus of the nulliparous ewe vagina was not different from that of the NHP or rodent (p = 0.290). Additionally, the tangent moduli of the parous ewe and NHP vaginas did not differ (p = 0.773). Parity has a negative impact on the mechanical properties of the ewe vagina, as also observed in the NHP. The ewe may serve as an alternative model for studying parity and ultimately prolapse development.