The present study supports our contention that the guinea-pig pubic symphyseal relaxation at parturition resembles an inflammatory process. The data demonstrate 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. These findings show how these processes work closely together.
Relaxation of the pelvic joint during pregnancy has been recognized for many years in guinea-pigs . Previous reports have demonstrated the existence of an extensive collagenolytic process in association with interpubic relaxation [26, 8, 9]. Using morphometry for quantifying changes in the interpubic distance together with collagen remodeling in the symphyseal ligament, we have obtained results that are in agreement with previous reports. The pubic relaxation is temporarily associated with a marked reduction in the intensity of birefringence, reflecting a widespread reduction in density and orientation of collagen fiber bundles, together with an increase in interpubic length.
The current study indicates that the pubic symphyseal relaxation at parturition resembles an inflammatory process. Results showed that an infiltration of leukocytes into the pubic symphysis occurs in guinea-pig. Eosinophils reached their highest level immediately before parturition, whereas greater levels of neutrophilic and mononuclear infiltration were recorded one day after parturition. This infiltration pattern suggests that eosinophils could perform its major effect before parturition, whereas mononuclear and neutrophilic cells might play a role after parturition. Similarly, cervical dilation during parturition has been characterized as a widespread collagenolysis that follows a heavy polymorphonuclear leukocyte invasion of the uterine cervix [10, 11]. The cells that infiltrate human, guinea-pig and sheep cervical tissue at term are neutrophils [10, 21, 27–29] whereas eosinophils are responsible for the infiltration of the rat cervix [11, 22, 30]. In the pubic symphysis at term, eosinophils and mononuclear cells were the main infiltrating ones. Proinflammatory cytokines together with an increase in the vascular area have been described in the uterine cervix at term [15, 16, 31], supporting the inflammatory response. In this study an increased angiogenesis was observed in the pubic symphysis from mid-pregnancy (D49) to parturition.
The significance of leukocyte infiltration in these tissues at term has not been clearly defined. It was observed in the uterine cervix that the polymorphonuclear leukocytes degranulate and this event coincides with the widespread collagenolysis observed in the extracellular matrix [10, 11]. In addition, there is vast evidence that both collagenase and proteolytic enzymes do increase at term in humans and rats [32, 33]. Previous studies have shown that eosinophils and neutrophils contain collagenase and other enzymes capable of digesting extracellular matrix proteins [34–36]. Therefore, it has been suggested that polymorphonuclear leukocytes are involved, at least in part, in the loosening of the collagenous framework that follows the leukocyte invasion of the uterine cervix. For the pubic symphysis, our results are in agreement with this hypothesis, since collagen remodeling is positively correlated with eosinophils infiltration. Moreover, we have observed typical histological images of dissolution of the connective tissue matrix around eosinophilic leukocytes. Nevertheless, the hypothesis that eosinophils are involved in enzymatic degradation and depolymerization of the pubic symphysis collagen needs to be tested in future experiments. Another possible role for the leukocytes invasion of the interpubic ligament in late pregnancy might be the formation of intercommunicating channels through the extracellular matrix. These channels would allow a rapid diffusion of hormones or collagenase-activating factors that may act at term on pre-existing collagenases (such as collagen-bound collagenases), since it has been shown that the migration of neutrophils increases the permeability of certain tissues .
On the other hand, E2 treatment of intact or ovariectomized guinea-pigs caused a significant increase in interpubic length [38–40]. In addition, relaxin promoted an extensive dissolution and disorientation effect on symphyseal collagen when administered to estrogen-pretreated animals [5, 26, 39, 40]. In this study, E2 correlated with interpubic distance and eosinophils infiltration, suggesting that changes in the symphyseal stroma and eosinophils invasion are hormonally regulated processes. It would be interesting to asses whether relaxin effects on collagen remodeling of the interpubic joint is achieved through the promotion of eosinophilic invasion and degranulation.
Biological effects of hormones depend not only on serum levels but on receptors concentration and signaling pathways in target organs as well. Results of experimental and clinical studies with P4 and its antagonist indicate that parturition is composed of two major steps: a relatively long conditioning (preparatory) phase, followed by a short secondary phase (active labor) [2, 41]. In the cervix, the preparatory phase involves a change in the composition of the connective tissue and the invasion by inflammatory cells . The conditioning phase can be induced with antiprogestins in all species studied so far, including humans and other primates . Thus, it has been hypothesized that withdrawal of P4 inhibition is the major mechanism in the initiation of conditioning phase . In species without a fall in P4 serum levels prior to parturition (guinea-pigs, non-human primates and humans), it has been proposed that a functional withdrawal of P4 due to decreased activity in the target organs, e.g. at receptor or post-receptor level, represents the major mechanism in the initiation of this preparatory phase and therefore the onset of parturition . Recently, we have demonstrated that before parturition, diminished responsiveness of the cervix to P4 might be caused by a decrease in progesterone receptor (PR) levels and this may be the mechanism of functional P4 withdrawal in the uterine cervix at term . In this study results showed that, preparatory changes similar to those observed in the cervix (i.e.: collagen remodeling and leukocyte infiltration) also take place in pubic symphysis at term, simultaneously with high serum levels of P4. This parallelism between cervical ripening and symphyseal relaxation at term suggests that pubic symphyseal relaxation could be hormonally controlled by some mechanism of functional P4 withdrawal such as decreased expression of PR at the end of gestation. Our laboratory is carrying out experiments to clarify this issue.