Defining Structural and Architectural Changes in Mouse Pelvic Floor Muscles During Pregnancy

Objectives: Pelvic floor muscles are subjected to mechanical strain during parturition, which can contribute to pelvic floor muscle dysfunction and the subsequent development of debilitating pelvic floor disorders. Previous studies in a rat model demonstrate that pelvic floor muscles undergo structural adaptations so as to help withstand parturition-associated strains. Unfortunately, there are only a few mutant and inbred strains of rats with limited options for genetic manipulation compared to the mouse model; this limits the ability of rat model to fully define the mechanisms that underlie pelvic floor muscle dysfunction during birth. Addressing this gap, here our goal was to determine whether mouse pelvic floor muscles undergo similar structural and architectural adaptations during pregnancy and in response to vaginal distension, as those found in the rat. Ultimately, our goal is to establish a robust mouse model of birth injury, to be able to define mechanisms of pelvic floor disorders using transgenic mouse models.

Methods: The pelvic floor muscles: coccygeus, iliocaudalis, and pubocaudalis were harvested from 3-month-old C57/BL6 late-pregnant (E16.5) and non-pregnant animals who were, or were not, subjected to physiologic vaginal distension that replicates that experienced during vaginal birth (N = 4/group). Animals undergoing vaginal distension were anesthetized, a 6F transurethral catheter was inserted into the vagina and balloon was inflated with 0.3 ml and a 13 g weight attached to the end of the catheter for 1 hour to replicate the circumferential and downward strains associated with parturition; importantly, the vaginal distension volume was chosen so as to replicate neonatal mouse head parameters. Animals were then euthanized and whole pelvis was fixed in situ. Architectural parameters that determine muscle excursion and force-generation were measured; individual muscles fibers were dissected and muscle fiber length was measured with digital calipers and sarcomere length was measured by light microscopy.

Results: Pregnancy induced a significant increase in normalized muscle fiber length in all pelvic floor muscles by late gestation compared to non-pregnant animals (Fig 1A). Sarcomere length was not affected by pregnancy (Fig1B). Distension balloon inflation volume of 0.3 ml best approximated fetal head dimensions to mimic physiologic distension during parturition (Fig 2A). Balloon-mediated vaginal distention resulted in significantly longer sarcomere lengths in non-pregnant compared to pregnant animals, indicating protection against sarcomere hyperelongation in pelvic floor muscles of pregnant animals (Fig 2B).

Conclusions: This study demonstrates that similar to rat, pregnancy induces structural and architectural adaptations in mouse pelvic floor muscles, such that muscle fiber length increases through the adding of sarcomeres in series. These adaptations protect against sarcomere hyperelongation during vaginal distension that replicates neonatal mouse head parameters. Future research, will look to further validate the model of mouse simulated birth injury model for the advancement of pelvic floor disorders research using transgenic mouse models.