INTRODUCTION AND HYPOTHESIS: Animal models are useful for investigating the genesis of pelvic floor dysfunction and for developing novel therapies for its treatment. There is a need for an alternative large-animal model to the nonhuman primate. Therefore we studied the effects of the first vaginal delivery, ovariectomy and systemic hormonal replacement therapy (HRT) on the biomechanical and structural properties of the ovine vagina. METHODS: We examined the gross anatomical properties of nulliparous, primiparous, ovariectomized multiparous, and ovariectomized hormone-replaced multiparous sheep (six animals per group). We also harvested mid-vaginal and distal vaginal tissue to determine smooth muscle contractility and passive biomechanical properties, for morphometric assessment of the vaginal wall layers, to determine collagen and elastin content, and for immunostaining for α-smooth muscle actin and estrogen receptor-α. RESULTS: There were no regional differences in the nulliparous vagina. One year after the first vaginal delivery, stiffness and contractility of the distal vagina were decreased, whereas the elastin content increased. The mid-vagina of ovariectomized sheep was stiff, and its epithelium was thin and lacked glycogen. HRT decreased the stiffness of the mid-vagina by 45% but had no measurable effect on contractility or elastin content, and increased epithelial thickness and glycogen content. HRT also increased the epithelial thickness and glycogen content of the distal vagina. At this location, there were no changes in morphology or stiffness. CONCLUSION: In sheep, life events including delivery and ovariectomy affect the biomechanical properties of the vagina in a region-specific way. Vaginal delivery mainly affects the distal region by decreasing stiffness and contractility. HRT can reverse the increase in stiffness of the mid-vagina observed after surgical induction of menopause. These observations are in line with scanty biomechanical measurements in comparable clinical specimens.

Centre for Surgical Technologies Faculty of Medicine KU Leuven Leuven Belgium Department of Development and Regeneration Clinical Specialties Research Groups Faculty of Medicine KU Leuven Leuven Belgium

Centre for Surgical Technologies Faculty of Medicine KU Leuven Leuven Belgium Department of Development and Regeneration Clinical Specialties Research Groups Faculty of Medicine KU Leuven Leuven Belgium Institute for the Care of Mother and Child and 3rd Faculty of Medicine Charles University Prague Czech Republic

Centre for Surgical Technologies Faculty of Medicine KU Leuven Leuven Belgium Department of Development and Regeneration Clinical Specialties Research Groups Faculty of Medicine KU Leuven Leuven Belgium Pelvic Floor Unit Departments of Obstetrics and Gynecology and Urology University Hospitals KU Leuven Leuven Belgium

Department of Development and Regeneration Clinical Specialties Research Groups Faculty of Medicine KU Leuven Leuven Belgium Pelvic Floor Unit Departments of Obstetrics and Gynecology and Urology University Hospitals KU Leuven Leuven Belgium Institute for Women's Health University College London London UK Department of Development and Regeneration Group Biomedical Sciences KU Leuven Herestraat 49 box 7003 3000 Leuven Belgium

Department of Obstetrics and Gynaecology University of Liège Liège Belgium

Laboratory of Tumor and Development Biology GIGA Cancer Institute of Pathology University of Liège Liège Belgium

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