Genetic background affects the biomechanical behavior of the postpartum mouse cervix

Abstract

Objective: We hypothesized that the genetic makeup has an impact on the functional behavior of the uterine cervix. Therefore, we compared the biomechanical properties of uterine cervix in postpartum in 2 strains of mice that differ in their underlying regenerative collagen remodeling characteristics: MRL/MpJ+/+ (MRL: high regenerative repair) and C57BL/6 (C57: low regenerative high fibrotic repair).

Study design: Cervical tensile proprieties were assessed on day 3, 15, and 60 postpartum in MRL (n = 14) and C57 (n = 13) mice (4-5 animals at each time point). Stress-strain curves were generated using Shimadzu EZ-test instrumentation. Cervical tissue was stretched by 0.42 mm/min until rupture. Parameters of viscoelasticity including slope (a measure of stiffness), yield point (YP; moment when tissue changes its proprieties from elastic to plastic), and break point (BP; measure of tissue strength) were recorded and analyzed blindly between strains. Data were normalized to the weight of the tissue and analyzed by 2-way analysis of variance. Histological and collagen birefringence evaluation of the uterine cervix (MRL: n = 4; C57: n = 4) was performed 5 days after delivery.

Results: At 3 and 15 days postpartum, cervices of MRL mice were significantly more compliant than those of C57 (P < .001). MRL mice displayed a significant increase in stiffness from day 3 to day 60 (slope, median +/- SEM: day 3: 3.1 +/- 0.5 vs day 15: 20.3 +/- 4.9 vs day 60: 33.1 +/- 3.5 N/mm per gram; P < .001). In contrast, the stiffness of C57 cervices reached maximum on day 15 (slope day 3: 14.1 +/- 4.3 vs day 15: 40.0 +/- 6.5 N/mm per gram; P = .02) and rested at a similar level on day 60 (day 60: 26.1 +/- 7.0 N/mm per gram; day 60 vs day 15: P = .937). More force was required to reach YP in C57 on day 3 (C57: 72.5 +/- 14.7 vs MRL: 19.9 +/- 1.6 N/g; P < .001) but not on either day 15 (C57: 156.1 +/- 27.5 vs MRL: 109.2 +/- 26.0 N/g; P = .120) or on day 60 (C57: 143.4 +/- 26.5 vs MRL: 164.5 +/- 18.7 N/g; P = .412). There was a significant decrease in BP in both strains on both day 15 and day 60 compared with day 3 postpartum (P = .856 for strain, P = .008 for day). MRL mice displayed significantly less cervical collagen birefringence compared with C57 control (P < .001) but increased proteoglycan staining and increased water content.

Conclusion: We provide evidence that genetic makeup may have an impact on cervical tissue remodeling and function. There are significant differences in postpartum cervical stiffness and compliance that vary with the regenerative collagen remodeling phenotype.

Figure 1. Load-displacement curves of the uterine cervix

Representative load-displacement curves are shown for C57Bl/6 (Panel A) and MRL/MpJ+/+ (Panel B) at 3 days postpartum. The mid-slope of the regression lines of the linear ascending part of the curve (elastic phase) (shown in red) is a measure of material stiffness. The yield point (YP: moment when tissue changes its proprieties from elastic to plastic) and break point (BP: moment when tissue is physically disrupted) are indicated with arrows.

Figure 2. Cervical weight post-delivery in MRL/MpJ+/+ and C57Bl/6 mice

Statistical analysis: 2-way ANOVA followed by post-hoc Student-Newman-Keuls tests. * P<0.05 vs C57Bl/6; # P<0.05 vs. day 3 post-delivery of the same strain.

Figure 3

Analysis of the load-displacement relationships of cervices from day 3, 15 and 60 post-delivery demonstrating differences in the slope (Panel A: measure of stiffness), yield point (Panel B: measure of plasticity) and break point (Panel C: measure of strength). Statistical analysis: 2-way ANOVA followed by post-hoc Student-Newman-Keuls tests. * P<0.05 vs C57Bl/6; # P<0.05 vs. day 3 post-delivery of the same strain.

Figure 4. Representative histological sections of the uterine cervix on day 5 post-delivery in MRL/MpJ+/+ and C57Bl/6 mice

Sections were stained with Sirius red and visualized in either polarized light (Panel A: MRL/MpJ+/+, Panel B: C57Bl/6) or white light. (Panel C: MRL/MpJ+/+, Panel D: C57Bl/6). Adjacent hematoxylin & eosin stained sections are shown in Panel E: (MRL/MpJ+/+) and Panel F (C57Bl/6). The squared areas are displayed at higher magnification in Panel G (MRL/MpJ+/+) and Panel H (C57Bl/6) after Alcian blue/neutral red staining to illustrate acid mucins stained in light blue. Arrows point to metachromatically stained mast cells. Lu: lumen of the endocervical canal. Endo: Endocervical epithelium.