Poster 4: Histopathologic Evaluation of the Levator Ani Muscle in Squirrel Monkeys With/Without Pelvic Relaxation

Abstract

OBJECTIVES: The value of an animal model is determined by the extent that disease conditions mimic those in humans. The purposes of this study were to evaluate histologic signs of myogenic and neurogenic changes in the levator ani (LA) muscle in the squirrel monkey model of pelvic organ prolapse (POP) and to compare findings with previous studies in women. METHODS: The LA (pubocaudalis and iliocaudalis) muscles were collected from 9 squirrel monkeys aged 4–25 years (n=4 parous females with POP/anterior defect, n=4 nulliparous females without POP, n=one parous female without POP) and processed for histology and immunofluorescence. Serial sections were stained with hematoxylin and eosin, Masson trichrome, and elastin stains. Fiber typing was performed with the use of antifast (type II) and antislow (type I) skeletal myosin antibodies or ATPase histochemistry, and apoptotic nuclei were examined by the TUNEL (dUTP nick-end labeling) assay. Histologic signs for myogenic changes included fibrosis, variation in fiber diameter (defined as standard deviation ≥30% of mean minimum diameter), and centralization of nuclei, whereas signs for neurogenic changes included small angulated fibers and fiber type grouping. RESULTS: The pubocaudalis muscle had a greater percentage of type I fibers compared with the iliocaudalis (39% = pubocaudalis, 28% = iliocaudalis, P<0.001, paired t test). Heit et al reported 66% type I fibers in the pubococcygeus muscle of women. Percentages of type II and I fibers were not altered with POP/parity status or age. Compared with young (4 yr) nulliparous animals, myocyte diameter (in pubocaudalis and iliocaudalis) and LA wet weight were not decreased in animals with POP, but were decreased in an elderly (>20 yr) nulliparous animal. Myogenic changes, including endomysial connective tissue or fibrosis (P=0.048, Fisher exact test), variation in fiber diameter (P=0.018, Fisher exact test), and centralization of nuclei (P=0.013, t test) increased with age regardless of POP/parity status. These changes were more pronounced in the pubocaudalis than the iliocaudalis. Neurogenic changes indicative of denervation without reinnervation (small angulated fibers) and denervation with reinnervation (fiber grouping) were observed in one parous animal that did not have POP. Very few apoptotic myonuclei were detected in any animal. CONCLUSIONS: Myogenic and neurogenic changes similar to those seen in women occur in the squirrel monkey LA muscle and are associated with aging and/or parity. Myogenic changes are most likely a consequence of mechanical stress to this muscle induced by supporting the pelvic organs against gravity, as well as pregnancy and parturition. The neurogenic changes observed in one parous female provide histologic evidence that the squirrel monkey experiences LA denervation injury likely caused by vaginal delivery. Investigation of additional age- and parity-matched animals without POP will help differentiate the effects of POP, parity, and aging on the LA muscle.