The squirrel monkey as an animal model of pelvic relaxation: an evaluation of a large breeding colony

Female squirrel monkeys as models for research on women's pelvic floor disorders

Animal models enable research on biological phenomena with controlled interventions not possible or ethical in patients. Among species used as experimental models, squirrel monkeys (Saimiri genus) are phylogenetically related to humans and are relatively easily managed in captivity. Quadrupedal locomotion of squirrel monkeys resembles most other quadrupedal primates in that they utilize a diagonal sequence/diagonal couplets gait when walking on small branches. However, to assume a bipedal locomotion, the human pelvis has undergone evolutionary changes. Therefore, the pelvic bone morphology is not that similar between the female squirrel monkey and woman, but pelvic floor support structures and impacts of fetal size and malpresentation are similar. Thus, this review explores the pelvic floor support structural characteristics of female squirrel monkeys, especially in relation to childbirth to demonstrate similarities to humans.

Animal models for pelvic organ prolapse: systematic review

INTRODUCTION AND HYPOTHESIS

We aimed to summarize the knowledge on the pathogenesis of pelvic organ prolapse (POP) generated in animal models.

METHODS

We searched MEDLINE, Embase, Cochrane and the Web of Science to establish what animal models are used in the study of suggested risk factors for the development of POP, including pregnancy, labor, delivery, parity, aging and menopause. Lack of methodologic uniformity precluded meta-analysis; hence, results are presented as a narrative review.

RESULTS

A total of 7426 studies were identified, of which 51 were included in the analysis. Pregnancy has a measurable and consistent effect across species. In rats, simulated vaginal delivery induces structural changes in the pelvic floor, without complete recovery of the vaginal muscular layer and its microvasculature, though it does not induce POP. In sheep, first vaginal delivery has a measurable effect on vaginal compliance; measured effects of additional deliveries are inconsistent. Squirrel monkeys can develop POP. Denervation of their levator ani muscle facilitates this process in animals that delivered vaginally. The models used do not develop spontaneous menopause, so it is induced by ovariectomy. Effects of menopause depend on the age at ovariectomy and the interval to measurement. In several species menopause is associated with an increase in collagen content in the longer term. In rodents there were no measurable effects of age apart of elastin changes. We found no usable data for other species.

CONCLUSION

In several species there are measurable effects of pregnancy, delivery and iatrogenic menopause. Squirrel monkeys can develop spontaneous prolapse.

Pelvic and neonatal size correlations in light of evolutionary hypotheses

OBJECTIVE

This study aimed to analyze the correlations between maternal size, neonatal size, and gestational variables.

METHODS

Our sample comprises 131 mother-infant dyads. We investigated correlations between five neonatal traits (gestational age, birthweight, head, suboccipito-brematic, and abdominal girths), three maternal traits (height, BMI, and uterus height), and three pelvic variables (conjugate, inter-spinous diameters, and sub-pubic angle) using computed tomography pelvimetry.

RESULTS

We found that the five neonatal traits were significantly intercorrelated. BMI was not correlated with neonatal traits while maternal height was correlated with birthweight, suboccipito-brematic, and abdominal girth. In the multiple regression models, gestational age was correlated with birthweight, head, and abdominal girth. Among the neonatal and pelvimetry correlations, conjugate diameter was slightly correlated with suboccipito-bregmatic girth, but inter-spinous and sub-pubic angle were not correlated with neonatal traits. Uterus height predicted all neonatal variables, but it was not correlated with gestational age.

DISCUSSION

Our results suggest that fetal growth is shaped by maternal phenotype rather than external ecological factors. The association of the inlet size with suboccipito-bregmatic girth reflects the tight fit between the neonatal brain and the maternal pelvis dimensions, an adaptation that would reduce the risk of cephalo-pelvic disproportion, while the absence of tight fit at the midplane and outlet could be due to the effect of the pelvic relaxation. Uterus distention is not the only mechanism involved in the initiation of parturition. Birth and pregnancy are complex processes and we suggest that maternal-neonatal associations are the result of a combination of multiple obstetric tradeoffs.

Between Scylla and Charybdis: renegotiating resolution of the 'obstetric dilemma' in response to ecological change

Hominin evolution saw the emergence of two traits-bipedality and encephalization-that are fundamentally linked because the fetal head must pass through the maternal pelvis at birth, a scenario termed the 'obstetric dilemma'. While adaptive explanations for bipedality and large brains address adult phenotype, it is brain and pelvic growth that are subject to the obstetric dilemma. Many contemporary populations experience substantial maternal and perinatal morbidity/mortality from obstructed labour, yet there is increasing recognition that the obstetric dilemma is not fixed and is affected by ecological change. Ecological trends may affect growth of the pelvis and offspring brain to different extents, while the two traits also differ by a generation in the timing of their exposure. Two key questions arise: how can the fit between the maternal pelvis and the offspring brain be 'renegotiated' as the environment changes, and what nutritional signals regulate this process? I argue that the potential for maternal size to change across generations precludes birthweight being under strong genetic influence. Instead, fetal growth tracks maternal phenotype, which buffers short-term ecological perturbations. Nevertheless, rapid changes in nutritional supply between generations can generate antagonistic influences on maternal and offspring traits, increasing the risk of obstructed labour.

Randomized trial of cesarean vs vaginal delivery for effects on the pelvic floor in squirrel monkeys

BACKGROUND

Vaginal delivery is a risk factor in pelvic floor disorders. We previously described changes in the pelvic floor associated with pregnancy and parturition in the squirrel monkey, a species with a humanlike pattern of spontaneous age- and parity-associated pelvic organ prolapse.

OBJECTIVE

The potential to prevent or diminish these changes with scheduled cesarean delivery (CD) has not been evaluated. In a randomized, controlled trial, we compared female squirrel monkeys undergoing spontaneous vaginal delivery with those undergoing scheduled primary CD for pelvic floor muscle volumes, muscle contrast changes, and dynamic effects on bladder neck position.

STUDY DESIGN

Levator ani, obturator internus, and coccygeus (COC) muscle volumes and contrast uptake were assessed by magnetic resonance imaging in 20 nulliparous females examined prior to pregnancy, a few days after delivery, and 3 months postpartum. The position of bladder neck relative to bony reference line also was assessed with abdominal pressure using dynamic magnetic resonance imaging.

RESULTS

Baseline measurements of 10 females randomly assigned to scheduled primary CD were not different from those of 10 females assigned to spontaneous vaginal delivery. Levator ani and obturator internus muscle volumes did not differ between groups, while volumes were reduced (P < .05) in the observation immediately after pregnancy. The COC muscles increased (P < .05) immediately after delivery for females in the spontaneous vaginal delivery group, but not for females in the scheduled CD group. Position of the bladder neck descended (P < .05) by 3 months postpartum in both groups.

CONCLUSION

Scheduled CD diminishes changes in COC muscle volume and contrast reported to be associated with spontaneous vaginal delivery in squirrel monkeys. However, pelvic support of the bladder was not protected by this intervention suggesting that effects of pregnancy and delivery are not uniformly prevented by this procedure.

Lack of association between pelvic outlet diameter and pelvic organ prolapse in squirrel monkeys

INTRODUCTION AND HYPOTHESIS

The aim was to test the hypothesis that the pelvic outlet diameter (POD) is associated with pelvic organ prolapse (POP) in squirrel monkeys.

METHODS

Magnetic resonance images (MRI) were obtained from 55 females with and without POP. Commercial software was used by two observers to measure the POD. POD, age, parity, and weight for the two groups were compared using Student's t test. Associations of age, parity, and body weight with POD were evaluated with linear regression analysis. Multiple regression analysis examined the relationships of age, parity, and POD with POP. Receiver operating curve methods were used to set thresholds.

RESULTS

Observers concurred on measurements (Cronbach's alpha = 0.96 with lower 95% confidence limit of 0.94). POD was not related to parity (p = 0.10) or weight (p = 0.053), but was inversely related to age (p = 0.011). Animals with POP did not differ from those without POP with regard to age (p = 0.10), weight (p = 0.17), or POD (p = 0.99). The groups differed with regard to parity (p = 0.007) and multiple regression methods demonstrated that only parity had a significant relationship with POP (p = 0.002). Parity greater than 2 had 64% specificity and 89% sensitivity for POP.

CONCLUSIONS

POD size does not contribute to POP in squirrel monkeys. This study confirms that among variables of age, body weight, POD, and parity, only parity appears as a risk factor for POP in squirrel monkeys.

Animal models of female pelvic organ prolapse: lessons learned

Pelvic organ prolapse is a vaginal protrusion of female pelvic organs. It has high prevalence worldwide and represents a great burden to the economy. The pathophysiology of pelvic organ prolapse is multifactorial and includes genetic predisposition, aberrant connective tissue, obesity, advancing age, vaginal delivery and other risk factors. Owing to the long course prior to patients becoming symptomatic and ethical questions surrounding human studies, animal models are necessary and useful. These models can mimic different human characteristics - histological, anatomical or hormonal, but none present all of the characteristics at the same time. Major animal models include knockout mice, rats, sheep, rabbits and nonhuman primates. In this article we discuss different animal models and their utility for investigating the natural progression of pelvic organ prolapse pathophysiology and novel treatment approaches.

Alterations in pelvic floor muscles and pelvic organ support by pregnancy and vaginal delivery in squirrel monkeys

INTRODUCTION AND HYPOTHESIS

The objective of this study was to measure the effects of pregnancy and parturition on pelvic floor muscles and pelvic organ support.

METHODS

Levator ani, obturator internus, and coccygeus (COC) muscle volumes and contrast uptake were assessed by MRI of seven females prior to pregnancy, 3 days, and 4 months postpartum. Bladder neck and cervix position were measured dynamically with abdominal squeezing.

RESULTS

The sides of three paired muscles were similar (p > 0.66). COC volumes were greater (p < 0.004) after parturition than before pregnancy or after recovery. COC contrast uptake increased (p < 0.02) immediately after delivery. Bladder neck position both in the relaxed state and abdominal pressure descended (p < 0.04) after delivery and descended further (p < 0.001) after recovery. Cervical position in the relaxed state before delivery was higher (p < 0.001) than postpartum but was unchanged (p = 0.50) with abdominal pressure relative to delivery.

CONCLUSION

In squirrel monkeys, coccygeus muscles demonstrate the greatest change related to parturition, and parturition-related bladder neck descent seems permanent.

Postpartum stress urinary incontinence: lessons from animal models

Postpartum stress urinary incontinence (SUI) is associated with chronic SUI in later life, which is 240% more likely to occur in women who deliver vaginally than those who did not. The etiology of SUI is multifactoral and has been associated with defects in both neuromuscular and structural components of continence. Specifically, clinical studies have demonstrated that pudendal nerve damage occurs during vaginal delivery, supporting the concept that neuromuscular damage to the continence mechanism can result in postpartum SUI. Urethral hypermobility and the loss of pelvic floor support, such as that involved in pelvic organ prolapse, have also been associated with SUI. Animal models provide an opportunity to investigate these injuries, individually and in combination, enabling researchers to gain further insight into their relative contributions to the development of SUI and the effectiveness of potential therapies for it. This article discusses the use of animal models of postpartum SUI in addition to the broad insights into treatment efficacy they provide.

Effects of bilateral levator ani nerve injury on pelvic support in the female squirrel monkey

OBJECTIVE

To determine whether experimental denervation of the levator ani (LA) and its subsequent atrophy contribute to the development of pelvic organ prolapse in the squirrel monkey.

STUDY DESIGN

Thirty-seven female monkeys were evaluated including 7 that underwent bilateral LA neurectomy (bLAN), 17 nulliparous monkeys without prolapse, 7 parous monkeys without prolapse, and 6 parous monkeys with prolapse. Magnetic resonance imaging was used to calculate LA muscle volumes and obtain measurements of the position of bladder and cervix. Repeat observations in bLAN females occurred at different times in relation to parturition.

RESULTS

LA volumes were reduced in bLAN monkeys (P = .02). Bladder (P = .03) and cervix (P = .04) positions varied between groups, with nulliparous females having the most cephalad positions and females with prolapse having the most caudal positions. Bladder descent was observed in a subset of 4 bLAN females that experienced vaginal parturition (P = .04) and correlated with external findings of vaginal prolapse.

CONCLUSION

Bilateral transection of the LA nerve results in atrophy of denervated LA muscles but not a loss of pelvic support in nulliparous monkeys, suggesting that connective tissue components compensate for weakened pelvic floor muscles. LA denervation may accelerate the onset of vaginal prolapse subsequent to parturition.

Levator ani muscle and connective tissue changes associated with pelvic organ prolapse, parity, and aging in the squirrel monkey: a histologic study

OBJECTIVE

This study was undertaken to evaluate histologically the levator ani muscle and paravaginal attachments in squirrel monkeys with and without pelvic organ prolapse.

STUDY DESIGN

Serial sections from 19 females were processed with routine histology stains. Fiber typing was performed with antifast (type II) and antislow (type I) skeletal myosin antibodies, and apoptotic nuclei were examined by dUTP nick-end labeling (TUNEL).

RESULTS

Gross disruption of the levator ani muscle and its innervation was not observed in animals with or without visible support defects. Myogenic changes occurred more frequently in the pubocaudalis than iliocaudalis muscles, and a significant association was found with aging (P < .05, Fisher exact test) but not with pelvic organ prolapse or parity. Neurogenic changes were observed in 3 of 13 multiparous monkeys. Myocyte diameter increased in animals with pelvic organ prolapse compared with age-, weight-, and parity-matched animals without pelvic organ prolapse (P = .005) and correlated with levator ani muscle wet weight (R = 0.76; P = .0006). In the paravaginal attachments, the numbers of fibroblasts and apoptotic nuclei were not different between animals with and without pelvic organ prolapse, but parity was associated with increased apoptosis (P = .025).

CONCLUSION

Vaginal prolapse in the squirrel monkey does not result from atrophy or gross disruption of the levator ani muscle or its innervation. As in women, myogenic changes are a common finding in the levator ani muscle and increase with aging, whereas denervation with subsequent reinnervation occurs in some multiparous monkeys.

The biology behind fascial defects and the use of implants in pelvic organ prolapse repair

Implant materials are increasingly being used in an effort to reduce recurrence after prolapse repair with native tissues. Surgeons should be aware of the biology behind both the disease as well as the host response to various implants. We will discuss insights into the biology behind hernia and abdominal fascial defects. Those lessons from "herniology" will, wherever possible, be applied to pelvic organ prolapse (POP) problems. Then we will deal with available animal models, for both the underlying disease and surgical repair. Then we will go over the features of implants and describe how the host responds to implantation. Methodology of such experiments will be briefly explained for the clinician not involved in experimentation. As we discuss the different materials available on the market, we will summarize some results of recent experiments by our group.

Magnetic resonance imaging of the levator ani in the squirrel monkey: a comparison of muscle volume between a cohort with pelvic organ prolapse and matched normals

OBJECTIVE

Magnetic resonance imaging was used to test whether squirrel monkeys with pelvic organ prolapse have reduced pelvic muscle volumes, compared with matched normals.

STUDY DESIGN

Levator ani and obturator internus volumes obtained from T1-weighted axial scans of matched groups were measured. Muscle volumes and weights were compared for animals necropsied after magnetic resonance imaging.

RESULTS

Two observers concurred on measures of levator ani and obturator internus (Kendal tau > or = 0.60 with P < .003). Levator ani volume was related to mass (R2 = 0.62, P = .0009). Animals with pelvic organ prolapse did not differ (P = .67, Wilks multivariate test) from those without pelvic organ prolapse in age, parity, and weight. Levator ani differed between groups (pelvic organ prolapse = 520 mm3 versus normals = 392 mm3, P = .015) and not sides (P = .80). The obturator internus did not differ between groups (P = .29) or sides (P = .72).

CONCLUSION

Magnetic resonance imaging demonstrates that levator ani volumes in parous squirrel monkeys with pelvic organ prolapse were not reduced, suggesting that prolapse is not related to pelvic muscle size reduction in this species.

Evaluation of the aged female baboon as a model of pelvic organ prolapse and pelvic reconstructive surgery

OBJECTIVE

This study examines the baboon as an animal model of pelvic organ prolapse (POP) by describing the pelvic floor anatomy and adapting human clinical assessment tools.

STUDY DESIGN

The pelvic anatomy of an adult female baboon was observed at necropsy, and comparisons were made to the human and squirrel monkey. The pelvic organ prolapse quantification (POP-Q) system was used to assess vaginal support in 12 living adult baboons, including 6 young, reproductive-age, nulligravid females (4.8 +/- 0.5 years) and 6 older, multiparous females (23.0 +/- 0.5 years).

RESULTS

The female baboon pelvic anatomy was found to have similar architecture to the human and squirrel monkey female. Six multiparous females with mean parity of 5 (range 2-8) showed no evidence of POP or differences in POP-Q measurements from 6 nulliparous females.

CONCLUSION

The POP-Q system can be used to assess female baboon vaginal support. In a sample of baboons, pelvic support loss consistent with POP was not identified. As the pelvic anatomy is similar to the human female, the baboon may prove useful for evaluating surgical materials and for modeling pelvic floor reconstructive surgeries.

Immunohistochemical evidence for the interaction between levator ani and pudendal motor neurons in the coordination of pelvic floor and visceral activity in the squirrel monkey

OBJECTIVE

The purpose of this study was to characterize the spinal distribution of afferent and efferent pathways that innervate the levator ani (LA) muscle in the female squirrel monkey.

STUDY DESIGN

Cholera toxin B (CTB) was injected unilaterally into the LA muscle of 5 monkeys to identify primary sensory neurons in the dorsal root ganglia (DRG) and motor neurons in the spinal cord that contribute fibers to the LA nerve. Fluoro-Gold (FG) was injected into the external anal sphincter of 2 of these animals to label pudendal motor neurons (1 of these animals underwent unilateral LA neurectomy before CTB injection). Spinal cord and DRG were processed for immunofluorescence 3 to 7 days after injections.

RESULTS

Retrograde transport of CTB from the LA muscle labeled primary afferent neurons in the ipsilateral DRG, their central projections, and motor neurons in the medial portion of the ipsilateral ventral horn of the spinal cord (L7-S2 segments). Injection of FG into the external anal sphincter labeled cells in Onuf's nucleus, primarily in L7. Importantly, CTB-labeled LA motor neurons were virtually absent in Onuf's nucleus, where all pudendal motor neurons are located. CTB-labeled processes were observed within Onuf's nucleus, adjacent to FG-labeled pudendal motor neurons, and appeared to derive from dendrites of LA motor neurons that project into Onuf's nucleus.

CONCLUSION

The LA muscle has a distinct innervation with very little or no contribution from the pudendal nerve. The intriguing labeling of LA neural elements within a nucleus that innervates the external urethral and anal sphincters (involved in pelvic visceral control) may represent a neuroanatomic substrate for physiologic integration of spinal and supraspinal inputs for the coordination of pelvic floor and visceral activity.

A rat model to study the structural properties of the vagina and its supportive tissues

OBJECTIVE

The purpose of this study was to investigate the rat as a model for evaluating the structural properties of the vagina and its supportive tissues.

STUDY DESIGN

The in situ relationships between the vagina and its supportive tissues were studied grossly (n = 10), and in serial cross sections (n = 4). For biomechanical testing (n = 10), the lumbar spine was fixed to a testing machine while the distal vagina was gripped with a soft-tissue clamp mounted to a load-cell on the crosshead of the machine. The vagina was systematically pulled through the pelvis until failure of the supportive tissues occurred. Parameters describing the structural properties of the tissues (linear stiffness, ultimate load, ultimate elongation, and energy absorbed to failure) were determined from the resulting load-elongation curve. A coefficient of correlation (R) was used to determine experimental reproducibility.

RESULTS

The rat vagina is supported by structures analogous to those in humans. The parameters describing the structural properties of these tissues were highly correlated among specimens (R = 0.95). The mechanism of failure was similar for all specimens.

CONCLUSION

We have developed a mechanical testing protocol in which the structural properties of the vagina-supportive tissue complex can be measured. This will be a powerful tool for testing the impact of life events on vaginal support.

Basic science and translational research in female pelvic floor disorders: proceedings of an NIH-sponsored meeting

AIMS

To report the findings of a multidisciplinary group of scientists focusing on issues in basic science and translational research related to female pelvic floor disorders, and to produce recommendations for a research agenda for investigators studying female pelvic floor disorders.

METHODS

A National Institutes of Health (NIH)-sponsored meeting was held on November 14-15, 2002, bringing together scientists in diverse fields including obstetrics, gynecology, urogynecology, urology, gastroenterology, biomechanical engineering, neuroscience, endocrinology, and molecular biology. Recent and ongoing studies were presented and discussed, key gaps in knowledge were identified, and recommendations were made for research that would have the highest impact in making advances in the field of female pelvic floor disorders.

RESULTS

The meeting included presentations and discussion on the use of animal models to better understand physiology and pathophysiology; neuromuscular injury (such as at childbirth) as a possible pathogenetic factor and mechanisms for recovery of function after injury; the use of biomechanical concepts and imaging to better understand the relationship between structure and function; and molecular and biochemical mechanisms that may underlie the development of female pelvic floor disorders.

CONCLUSIONS

While the findings of current research will help elucidate the pathophysiologic pathways leading to the development of female pelvic floor disorders, much more research is needed for full understanding that will result in better care for patients through specific rather than empiric therapy, and lead to the potential for prevention on primary and secondary levels.

Innervation of the levator ani muscles in the female squirrel monkey

OBJECTIVE

The purpose of this study was to characterize the innervation of the levator ani muscles in the female squirrel monkey and to investigate its usefulness as an animal model of pelvic organ prolapse.

STUDY DESIGN

Eleven nulliparous female squirrel monkeys with no pelvic organ prolapse were used in this study. Detailed pelvic dissections were conducted (n = 3), and the Koelle stain for acetylcholinesterase was used to identify the motor endplate zone in the levator ani muscles (n = 2). Unilateral levator ani (n = 4) and pudendal (n = 2) neurectomies were performed; changes in levator ani muscle mass and myocyte diameter were examined 14 days after neurectomy. Nerve biopsy specimens from each animal were processed for microscopy.

RESULTS

The levator ani nerve originated from the S2 spinal root and entered the pelvic cavity adjacent to the pelvic nerve between the flexor caudalis brevis and iliocaudalis muscles. The levator ani nerve then projected caudally and bifurcated to penetrate the iliocaudalis and pubocaudalis. A single motor endplate zone in each muscle correlated with the point of levator ani nerve penetration. The pudendal nerve originated from the S1-S2 spinal roots to innervate the urethral and anal sphincters, clitoris, and perineum, but not the iliocaudalis or pubocaudalis. Significant atrophy and myocyte shrinkage occurred in the iliocaudalis and pubocaudalis ipsilateral to the levator ani nerve transection (P <.05). Pudendal neurectomy produced no levator ani muscle changes.

CONCLUSION

Intrapelvic skeletal muscles in the female squirrel monkey are similar to humans and have distinct innervation with no contribution from the pudendal nerve. The squirrel monkey is likely to be a useful model of pelvic organ prolapse and warrants further study.

Innervation of the female levator ani muscles

OBJECTIVE

The objective of this study was to characterize the innervation of the human female levator ani muscles.

STUDY DESIGN

Detailed dissections of the peripheral innervation of the iliococcygeal, pubococcygeal, puborectal, and coccygeal muscles were performed in 12 fresh-frozen female cadavers (aged, 32-100 years) with the use of transabdominal, gluteal, and perineal approaches. Both the pudendal nerve and the sacral nerve roots that enter the pelvis from the cephalic side were followed from their origin at the sacral foramina to their termination. Pelvic floor innervation was described with reference to fixed bony landmarks, particularly the coccyx, the ischial spine and the inferior pubis. Photographs were taken, and nerve biopsies were performed to confirm the gross findings histologically. Biopsy specimens were stained with Masson's trichrome.

RESULTS

In each dissection, a nerve originated from the S3 to S5 foramina (S4 alone, 30%; from S3 and S4, 40%; from S4 and S5, 30%), crossed the superior surface of the coccygeal muscle (3.0 +/- 1.4 cm medial to the ischial spine [range, 1.0-4.2 cm]), traveled on the superior surface of the iliococcygeal muscle innervating it at its approximate midpoint, and continued on to innervate both the pubococcygeal and puborectal muscles at their approximate midpoint. The pudendal nerve originated from the S2 to S4 foramina, exited the pelvis through the greater sciatic foramen, traversed Alcock's canal, and branched to innervate the external anal sphincter, the external urethral sphincter, the perineal musculature, the clitoris, and the skin. Despite specific attempts to locate pudendal branches to the levator ani, none could be demonstrated. Nerve biopsy specimens that were obtained at gross dissection were confirmed histologically.

CONCLUSION

Gross dissections suggest that the female levator ani muscle is not innervated by the pudendal nerve but rather by innervation that originates the sacral nerve roots (S3-S5) that travels on the superior surface of the pelvic floor (levator ani nerve). Because definitive studies (eg, nerve transection or neurotracer studies) cannot be performed in humans, further studies that will use appropriate animal models are necessary to confirm and extend our findings.

The rhesus macaque as an animal model for pelvic organ prolapse

OBJECTIVE

The purpose of this study was to characterize the pelvic floor of the rhesus macaque as an experimental model for human pelvic organ prolapse and to initiate an evaluation of the effects of estradiol and progesterone on the rhesus paravaginal attachment.

STUDY DESIGN

Histologic specimens were prepared from the paravaginal attachment of 13 oophorectomized rhesus macaques. Three animals were treated with estradiol; 6 animals were treated with estradiol and progesterone, and 4 animals were untreated (hormone deprived). Immunocytochemistry was used to localize steroid receptors in the paravaginal attachment.

RESULTS

Spontaneous pelvic organ prolapse was observed in rhesus macaques. The paravaginal attachment is comprised of dense collagen and elastic fibers that infiltrate the levator ani muscle. The fibroblasts of this attachment are estrogen and progesterone receptor positive, and the receptors are hormone responsive.

CONCLUSION

The rhesus macaque has pelvic floor anatomy that is similar to women and makes an excellent experimental model for the study of prolapse. The rhesus paravaginal attachment is ligamentous and hormone sensitive. Its fibroblast activity may be modified by estrogen treatment in a manner similar to that reported in human pelvic connective tissue. The connective tissue of the paravaginal attachment interdigitates with the levator ani muscle cells, which suggests that this muscle plays a critical role in pelvic floor support.