First delivery and ovariectomy affect biomechanical and structural properties of the vagina in the ovine model.

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.

Comparative Anatomy of the Ovine and Female Pelvis.

BACKGROUND: Pelvic organ prolapse affects half of vaginally parous women. Several animal models are used to study its pathophysiology and treatment. Sheep are interesting because they develop spontaneously prolapse with similar risk factors as women and can be used for vaginal surgery. This study describes ovine pelvis anatomy and compares it to women's pelvis to provide anatomical tools for translational researchers. METHODS: MRI, pelvic dissections, and histology were used for detailed macro- and microscopic analysis of relevant anatomical structures in 6 nulliparous ewes. RESULTS: Although sheep are quadrupeds, the gross and microscopic anatomies are similar to the female pelvis. Principal differences are the shape and its orientation, the absence of the sacrospinous ligament and the internal obturator. The levator ani (except for the puborectalis) and the coccygeus muscle are present, yet the latter is more developed - coinciding with the tail. The dimensions and morphology of the ovine vagina is comparable. The retropubic and the rectovaginal space are accessible transvaginally. There is a wide expression of estrogen receptors with low or absent immunoreactivity in the urethral epithelium, bladder, anus and internal anal sphincter. CONCLUSION: The ovine pelvic floor has many anatomical and ultrastructural similarities to the female pelvic floor.