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Comparative Study
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Guinea-pig Interpubic Joint (Symphysis Pubica) Relaxation at Parturition: Underlying Cellular Processes That Resemble an Inflammatory Response

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Comparative Study

Guinea-pig Interpubic Joint (Symphysis Pubica) Relaxation at Parturition: Underlying Cellular Processes That Resemble an Inflammatory Response

Horacio A Rodríguez et al. Reprod Biol Endocrinol.

Abstract

Background: At term, cervical ripening in coordination with uterine contractions becomes a prerequisite for a normal vaginal delivery. Currently, cervical ripening is considered to occur independently from uterine contractions. Many evidences suggest that cervical ripening resembles an inflammatory process. Comparatively little attention has been paid to the increased flexibility of the pelvic symphysis that occurs in many species to enable safe delivery. The aim of this study was to investigate whether the guinea-pig interpubic joint relaxation process observed during late pregnancy and parturition resembles an inflammatory process.

Methods: Samples of pubic symphysis were taken from pregnant guinea-pigs sacrificed along gestation, parturition and postpartum. Serial sections of paraffin-embedded tissues were used to measure the interpubic distance on digitalized images, stained with Giemsa to quantify leukocyte infiltration and to describe the vascular area changes, or studied by the picrosirius-polarization method to evaluate collagen remodeling. P4 and E2 serum levels were measured by a sequential immunometric assay.

Results: Data showed that the pubic relaxation is associated with an increase in collagen remodeling. In addition, a positive correlation between E2 serum levels and the increase in the interpubic distance was found. On the other hand, a leukocyte infiltration in the interpubic tissue around parturition was described, with the presence of almost all inflammatory cells types. At the same time, histological images show an increase in vascular area (angiogenesis). Eosinophils reached their highest level immediately before parturition; whereas for the neutrophilic and mononuclear infiltration higher values were recorded one day after parturition. Correlation analysis showed that eosinophils and mononuclear cells were positively correlated with E2 levels, but only eosinophilic infiltration was associated with collagen remodeling. Additionally, we observed typical histological images of dissolution of the connective tissue matrix around eosinophils.

Conclusion: The present study shows that a timely regulated influx of infiltrating leukocytes is associated with an extensive collagen remodeling process that allows the pubic separation for a normal delivery in guinea-pig. Thus, the findings in this study support the hypothesis that the guinea-pig pubic symphyseal relaxation at parturition resembles an inflammatory process.

Figures

Figure 1
Figure 1
Guinea-pig interpubic symphysis distance (A) and collagen remodeling in the interpubic ligament (B) were simultaneously measured in nonpregnant animals (NP), along pregnancy (P), immediately after parturition (iAP) and at postpartum (PP). Collagen remodeling was measured by the picrosirius-polarization method as changes in the intensity of birefringence. Note that the lowest intensity of birefringence evidences the highest collagen remodeling and coincides with the maximum value of ligament length. Bars represent the mean symphysis distance (± SEM) of five animals/group.
Figure 2
Figure 2
Photomicrographs of sections showing central region of the guinea-pig pubic symphysis obtained at D21 of pregnancy (A, B) and at parturition (C, D). Sections were studied by the picrosirius-polarization method (a specific procedure for the detection of oriented collagenous structures in tissue sections). Densely packed bundles of interlaced collagen fibers are observed in the interpubic ligament with conventional illumination (A) or under polarizing optics (B). All brightly birefringent structures, which shine against a dark background, are of a collagenous nature. At parturition, irregularly disposed thin and thick collagenous fibers are observed in sections when studied under conventional illumination (C); polarization microscopy of the same field shows a high degree of collagen remodeling (D). Note the presence of high number of blood vessels at parturition (C). Scale bar = 25 μm.
Figure 3
Figure 3
Serum levels of P4 and E2 in nonpregnant (NP), along pregnancy (P), immediately after parturition (iAP), and at day 1 of postpartum (PP). Sequential immunometric assay was performed according to Material and Methods. Means with different letters differ significantly (p < 0.05).
Figure 4
Figure 4
Temporal changes in leukocytes infiltration in the guinea-pig interpubic joint along pregnancy (P), immediately after parturition (iAP), and postpartum (PP). Bars represent mean values of volume density (area fraction occupied by leukocytes; Vv, ± SD) of five animals/group. Highest Vv values of eosinophils were recorded before parturition (D63), whereas neutrophils and mononuclear cells reached their highest values one day after parturition (D1). Infiltration was absent in nonpregnant (NP) tissue samples. Means with different letters differ significantly (p < 0.05).
Figure 5
Figure 5
Photomicrograph of a section from the interpubic ligament obtained at parturition. Picrosirius-haematoxylin staining allowed the identification of eosinophils and collagen fibers. The empty halos (arrow) show collagen degradation surrounding eosinophils. This feature is shown at higher magnification (insert). Low magnification scale bar = 20 μm, inset scale bar = 5 μm.

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