The histological microstructure and in vitro mechanical properties of pregnant and postmenopausal ewe perineal body.

Objective: The mechanical properties and microstructure of the perineal body are important for the improvement of numerical models of pelvic organs. We determined the mechanical parameters and volume fractions of the ewe perineal body as an animal model. Methods: The 39 specimens of 13 pregnant swifter ewes delivering by cesarean section (aged 2 years, weight 61.2 +/- 6.2 kg (mean +/- standard deviation) and 24 specimens of 8 postmenopausal swifter ewes 150 days after surgical ovariectomy (aged 7 years, 58.6 +/- 4.6 kg)) were loaded uniaxially to determine Young's moduli of elasticity in the small (E-0) and large (E-1) deformation regions, and ultimate stresses and strains. The 63 adjacent tissue samples were processed histologically to assess volume fractions of smooth and skeletal muscle, adipose cells, elastin, and type I collagen using a stereological point testing grid. We compared the structural and mechanical differences along the ewe perineal body, and between pregnant and postmenopausal groups. Results: The pregnant/postmenopausal perineal body was composed of smooth muscle (12/14%; median), skeletal muscle (12/16%), collagen (10/23%), elastin (8/7%), and adipose cells (6/6%). The E-0 was 37/11 kPa (median), E-1 was 0.97/1.04 MPa, ultimate stress was 0.55/0.59MPa, and ultimate strain was 0.90/0.87 for pregnant/postmenopausal perineal body. The perineal body showed a structural and mechanical stability across the sites. The pregnant ewes had a higher amount of skeletal muscle, higher E-0, and a less amount of collagen when compared with postmenopausal ewes. Conclusions: The data can be used as input for models simulating vaginal delivery, pelvic floor prolapsed, or dysfunction.