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

Abstract

Introduction and hypothesis 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. Methods 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. Results 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). Conclusions 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.

Vaginal wall weakness in parous ewes: a potential preclinical model of pelvic organ prolapse

“…It was not possible to use the Valsalva maneuver in ewes; it is therefore possible that the incidence of POP in ovine cohorts may have been higher. Only one study reported POP-Q in a small number of sheep (n = 15), but at post mortem-and consistent with our findings-there is greater anterior and posterior vaginal wall descent observed in the parous relative to nulliparous ewes [13].…”

“…A shorter perineal body and larger GH was found in the multiparous ewes, which could be explained by vaginal birth damage, and in one study, a larger GH and PB were strongly associated with prolapse [13,20,21]. A large GH would make vaginal access easier while operating and may be a useful measure for selecting ewes for vaginal surgery.…”

“…Advantages of the ovine model as a large animal preclinical model are that they are relatively inexpensive in many countries, available in large numbers, an established reproductive model for humans, have prolonged labor with relatively large fetuses, and develop POP related to pregnancy and vaginal birth. Ewes have an average lifespan of 10-12 years [11][12][13]. The ovine pelvis is also similar to human pelvic tissue anatomy, histology, size, and vaginal dimensions [9,10].…”

Ovine multiparity is associated with diminished vaginal muscularis, increased elastic fibres and vaginal wall weakness: implication for pelvic organ prolapse

“…Our study also supports a recent report comparing the mechanical properties of ewe vagina in non-human primates and rodent models where parity had a negative impact on mechanical integrity38. Though further comparative studies between these species are warranted, the similarities already identified suggests that ewes can be used for further investigations.…”

Female pelvic floor biomechanics

“…However, some of the greatest remaining challenges stem from the ethical issues surrounding procurement of human tissues, in significant quantities, that span the average 35-year time gap between maternal birth injury and the onset of symptoms for POP [11, 12]. Thus, researchers have begun to implement cross-disciplinary techniques to help bridge the gap.…”