Comparison of pelvic muscle architecture between humans and commonly used laboratory species

Abstract

Introduction and hypothesis Pelvic floor muscles (PFM) are deleteriously affected by vaginal birth, which contributes to the development of pelvic floor disorders. To mechanistically link these events, experiments using animal models are required, as access to human PFM tissue is challenging. In choosing an animal model, a comparative study of PFM design is necessary, since gross anatomy alone is insufficient to guide the selection. Methods Human PFM architecture was measured using micromechanical dissection and then compared with mouse (n=10), rat (n=10), and rabbit (n=10) using the Architectural Difference Index (ADI) (parameterizing a combined measure of sarcomere length-to-optimal-sarcomere ratio, fiber-to-muscle-length ratio, and fraction of total PFM mass and physiological cross-sectional area (PCSA) contributed by each muscle). Coccygeus (C), iliocaudalis (IC), and pubocaudalis (PC) were harvested and subjected to architectural measurements. Parameters within species were compared using repeated measures analysis of variance (ANOVA) with post hoc Tukey's tests. The scaling relationships of PFM across species were quantified using least-squares regression of log-10-transformed variables. Results Based on the ADI, rat was found to be the most similar to humans (ADI = 2.5), followed by mouse (ADI = 3.3). When animals' body mass was regressed against muscle mass, muscle length, fiber length, and PCSA scaling coefficients showed a negative allometric relationship or smaller increase than predicted by geometric scaling. Conclusion In terms of muscle design among commonly used laboratory animals, rat best approximates the human PFM, followed by mouse. Negative allometric scaling of PFM architectural parameters is likely due to the multifaceted function of these muscles.

Pregnancy-induced adaptations in the intrinsic structure of rat pelvic floor muscles

“…PFM architecture of 3-month-old Sprague-Dawley female virgin rats, which served as a control group, was previously reported. 26 Collagen content of the PFMs of virgin controls was determined using previously preserved tissue. Ten intermediate (4 weeks) and 10 late (12 weeks) postpartum animals were studied to determine whether PFMs returned to prepregnancy state.…”

“…Individual pelvic muscles were identified by tracking each muscle from its origin to insertion, weighed and divided into 3 regions, as previously described. 26 Tibialis anterior muscles were identified, harvested, and processed in parallel. The following architectural parameters were quantified: fiber length (L f ), a predictor of contractile velocity and muscle excursion; physiologic cross-sectional area (PCSA), proportional to the maximum force a muscle can generate isometrically; and sarcomere length (L s ), the “molecular machine” that determines a muscle’s active force (ie, the force a muscle produces when stimulated).…”

“…Significance level (α) was set to 5%. To obtain 80% power (1-β) to detect a 15% difference the sample size of 10/group was calculated utilizing the equation: n= (coefficient of variation) 2 /(ln [1-δ]) 2 using the experimental variability from our previous data 26 with variables n = sample size; δ = difference desired (15%). 35,36 All data were screened for normality and skewness to satisfy the assumptions of the parametric tests used.…”

“…24,25 Furthermore, we recently reported that with respect to major architectural parameters, rat PFM design is similar to human. 26 Rat has also been shown to be a reliable model for the study of pregnancy-related changes in cervix, vagina, and vaginal supportive structures. 14,27,28 In this study we quantified the structural adaptations that take place in the PFMs throughout pregnancy and determined the extent of recovery after vaginal delivery.…”

Pregnancy-induced adaptations in intramuscular extracellular matrix of rat pelvic floor muscles

“…Comparison of PFM structural parameters between human and rat demonstrated architectural similarity, suggesting that they have comparable functional design. 20 The rat model also has proven valuable in many studies of pregnancy-related changes in other pelvic structures. 17,21,22 Furthermore, we recently reported that adaptations during pregnancy occur in the myofibers of rat PFMs, which increase their length by adding sarcomeres in series.…”