Tensile Mechanical Properties and Dynamic Collagen Fiber Re-Alignment of the Murine Cervix are Dramatically Altered Throughout Pregnancy

Abstract

The cervix is a unique organ able to dramatically change its shape and function by serving as a physical barrier for the growing fetus and then undergoing dramatic dilation allowing for delivery of a term infant. As a result, the cervix endures changing mechanical forces from the growing fetus. There is an emerging concept that the cervix may change or remodel "early" in many cases of spontaneous preterm birth (sPTB). However, the mechanical role of the cervix in both normal and preterm birth remains unclear. Therefore, the primary objective of this study was to determine the mechanical and structural responses of murine cervical tissue throughout a normal gestational time course. In this study, both tissue structural and material properties were determined via a quasi-static tensile load-to-failure test, while simultaneously obtaining dynamic collagen fiber re-alignment via cross-polarization imaging. This study demonstrated that the majority of the mechanical properties evaluated decreased at midgestation and not just at term, while collagen fiber re-alignment occurred earlier in the loading curve for cervices at term. This suggests that although structural changes in the cervix occur throughout gestation, the differences in material properties function in combination with collagen fiber re-alignment as mechanical precursors to regulate term gestation. This work lays a foundation for investigating cervical biomechanics and the role of the cervix in preterm birth.

Fig. 1

Cervical mechanical testing dissection. (a) Schematic of cervix anatomy. (b) NP and (c) E18.5 cervices before and (b′) and (c′) after dissection. During dissection the cervix was cut open on the side opposite to the bladder and then the bladder was removed. The cervix was then laid flat to expose the lumen, and then rotated. The ends were affixed between two pieces of sandpaper for gripping so that when the cervix was pulled in tension, this represented circumferential loading.

Fig. 2

(a) Custom testing setup to record collagen fiber re-alignment during the mechanical testing. (b) Close-up of the cervical tissue in grips during mechanical testing. (c) Schematic depicting typical force-displacement and stress-strain curves from a uniaxial test. Stiffness and maximum load were obtained from the force-displacement curve and Young's modulus and maximum stress were obtained from the stress-strain curve. Collagen fiber re-alignment was assessed at the beginning and end of the toe region, 45% of maximum load and 90% of maximum load. (d) Representative load-stretch plots for each gestational time point.

Fig. 3

Cross-sectional area of the cervix during gestation. (a) Cross-sectional area is shown for all gestational time points. The white bar on the left represents the NP and during gestation different time points are shown in increasing shades of grey. PP samples are represented on the far right with a checkered pattern. All of the data is presented as means with standard deviations, and significance noted at p < 0.05 with a line. Although no statistical differences were observed between the (b) NP and the (c) E18.5 samples, qualitative visual differences were observed.

Fig. 4

Cervix structural and material mechanical properties. (a) Stiffness, (b) Max Load, (c) Max Stress, and (d) Modulus for all samples. The white bar on the left represent the NP samples and during gestation different time points are shown in different shades of gray. PP samples are represented on the far right with a checkered pattern. All of the data is presented as means with standard deviations, and significance is noted at p < 0.05 with lines.

Fig. 5

Collagen fiber re-alignment through polarized light analysis of the cervix during mechanical testing. Representative plots of polarized light analyzed at toe, end of toe, 45% of maximum load, and 90% of maximum load. (a) NP, (b) E10.5, (c) E12.5, (d) E14.5, (e) E16.5, and (f) E18.5. Lines represent significance (p < 0.05) while dashed lines represent tends (p < 0.1).