Cervical ripening: biochemical, molecular, and clinical considerations

The effect of bilateral pelvic neurectomy on cervical ripening in pregnant rats

AIMS

To determine the effect of bilateral pelvic neurectomy (BPN) on cervical ripening in pregnant rats by measuring cervical extensibility and changes in collagen cross-linkages.

METHODS

Timed-pregnant rats were randomly laparotomized on days 9 or 10 of gestation and the pelvic nerves were exposed and either bilaterally transected, or left intact in sham control animals. The rats were sacrificed on day 18 and the uterine cervices obtained. Cervical ripening was assessed by cervical resistance-to-stretch, light-induced autofluorescence (LIF) of cross-linked collagen, and collagen changes analyzed by picrosirius polarization microscopy.

RESULTS

Measurements of extensibility and collagen cross-linkages indicated that after BPN the cervix was significantly more ripened than the cervix from sham control animals.

CONCLUSION

BPN stimulates cervical ripening instead of inhibiting this event as previously proposed. Further studies in this area could be critical for developing treatments for dystocia, preterm labor, and cervical insufficiency.

The association between intrauterine inflammation and spontaneous vaginal delivery at term: a cross-sectional study

Background

Different factors contribute to the onset of labor at term. In animal models onset of labor is characterized by an inflammatory response. The role of intrauterine inflammation, although implicated in preterm birth, is not yet established in human term labor. We hypothesized that intrauterine inflammation at term is associated with spontaneous onset of labor.

Methods/Results

In two large urban hospitals in the Netherlands, a cross-sectional study of spontaneous onset term vaginal deliveries and elective caesarean sections (CS), without signs of labor, was carried out. Placentas and amniotic fluid samples were collected during labor and/or at delivery. Histological signs of placenta inflammation were determined. Amniotic fluid proinflammatory cytokine concentrations were measured using ELISA. A total of 375 women were included. In term vaginal deliveries, more signs of intrauterine inflammation were found than in elective CS: the prevalence of chorioamnionitis was higher (18 vs 4%, p = 0.02) and amniotic fluid concentration of IL-6 was higher (3.1 vs 0.37 ng/mL, p<0.001). Similar results were obtained for IL-8 (10.93 vs 0.96 ng/mL, p<0.001) and percentage of detectable TNF-α (50 vs 4%, p<0.001).

Conclusions

This large cross-sectional study shows that spontaneous term delivery is characterized by histopathological signs of placenta inflammation and increased amniotic fluid proinflammatory cytokines.

Are animal models relevant to key aspects of human parturition?

Preterm birth remains the most serious complication of pregnancy and is associated with increased rates of infant death or permanent neurodevelopmental disability. Our understanding of the regulation of parturition remains inadequate. The scientific literature, largely derived from rodent animal models, suggests two major mechanisms regulating the timing of parturition: the withdrawal of the steroid hormone progesterone and a proinflammatory response by the immune system. However, available evidence strongly suggests that parturition in the human has significantly different regulators and mediators from those in most of the animal models. Our objectives are to critically review the data and concepts that have arisen from use of animal models for parturition and to rationalize the use of a new model. Many animal models have contributed to advances in our understanding of the regulation of parturition. However, we suggest that those animals dependent on progesterone withdrawal to initiate parturition clearly have a limitation to their translation to the human. In such models, a linear sequence of events (e.g., luteolysis, progesterone withdrawal, uterine activation, parturition) gives rise to the concept of a "trigger" mechanism. Conversely, we propose that human parturition may arise from the concomitant maturation of several systems in parallel. We have termed this novel concept "modular accumulation of physiological systems" (MAPS). We also emphasize the urgency to determine the precise role of the immune system in the process of parturition in situations other than intrauterine infection. Finally, we accentuate the need to develop a nonprimate animal model whose physiology is more relevant to human parturition. We suggest that the guinea pig displays several key physiological characteristics of gestation that more closely resemble human pregnancy than do currently favored animal models. We conclude that the application of novel concepts and new models are required to advance translational research in parturition.

Cervical ripening and insufficiency: from biochemical and molecular studies to in vivo clinical examination

To understand cervical ripening and especially the pathophysiology of cervical insufficiency, it is important to know the cervical composition: the cervix is dominated by fibrous connective tissue, consisting predominantly of Type I collagen (70%). Despite many studies of the cervix, we still rely upon relatively crude methods for clinical evaluation of the cervix. If the amount of cervical collagen plays a role in cervical insufficiency and in success of or length of induction of labor, then measurements of cervical collagen may provide an objective means of establishing the diagnosis or prognosis. We have established and reported a non-invasive means, called Collascope, to measure collagen cross-linking using light-induced fluorescence (LIF), and which is specifically designed to assess cervical ripening, and functions by measuring the natural fluorescence of non-soluble collagen in the cervix. Studies conducted in animals and humans in a variety of settings indicate that cervical function can be successfully monitored using the Collascope during pregnancy: LIF correlates negatively with gestational age and positively with time-to-delivery interval, and is predictive of delivery within 24h. Additionally LIF is significantly lower in women with cervical insufficiency. We suggest that the Collascope might be useful to better define management in cases of spontaneous preterm or induced term cervical ripening. From our studies and others, it is clear that in forecasting (pre-)term cervical ripening, the capability of the technologies and bioassays that have been generally accepted into clinical practice are limited. Any devices shown to be superior to the clinically accepted tests currently used should be quite useful for clinicians. The Collascope offers an objective measurement of both the function and state of the cervix, by directly measuring collagen cross-linking using LIF.

Pregnancy increases excitability of mechanosensitive afferents innervating the uterine cervix

BACKGROUND

Labor pain derives primarily from stimulation of afferents innervating the uterine cervix and lower uterine segment. The authors have previously shown that the excitability of these afferents is regulated by sex hormones and test in this study whether pregnancy also alters their excitability.

METHODS

After animal care committee approval, Sprague-Dawley rats (nonpregnant, pregnant days 17 and 21) were anesthetized, and two metal rods were placed through the cervix for distension. The right hypogastric nerve was dissected and carefully teased until recording from a single unit was obtained. Spontaneous activity and the response to a graded distension (20-80 g) were recorded for off-line analysis.

RESULTS

A total of 151 fiber units were recorded. Pregnancy was associated with an increase in spontaneous nerve activity in the absence of a mechanical stimulus (median of 0.98 and 1.56 Hz from pregnant days 17 and 21, respectively, compared with 0.45 Hz in nonpregnant; P < 0.01). The proportion of fibers responding to the weakest stimulus (20 g) was significantly greater in pregnant than in nonpregnant animals. The response to graded distension differed significantly among groups, with day 21 > day 17 > nonpregnant.

CONCLUSIONS

Afferents that innervate the uterine cervix sprout into this tissue during late pregnancy, and estrogen increases excitability of these mechanosensitive afferents. Here, the authors show that excitability also increases during pregnancy. These data suggest that, close to the onset of labor, there is an increased input to the spinal cord from cervical distension and an increased depolarization of afferent terminals in the cervix, effects that could influence pain and the progress of labor.

Preventing cervical ripening: the primary mechanism by which progestational agents prevent preterm birth?

OBJECTIVE

Recent clinical trials suggest that progestational agents may prevent preterm birth, specifically in women with short cervices. These studies sought to assess novel pathways by which progestational agents (PAs) may modify signal transduction pathways that are involved in cervical ripening.

STUDY DESIGN

A microarray analysis was performed on pregnant mouse cervix that was exposed to a MPA. Appropriate microarray and cluster analyses were performed. Target genes of interest were investigated in both PA- and inflammation-exposed cervices by quantitative polymerase chain reaction and immunohistochemistry.

RESULTS

Microarray analysis identified both the previously recognized and novel pathways that are involved in cervical ripening. PAs differentially regulate expression of claudin-2, hyaluronan synthase 2, and lipocalin 2. Claudin expression is significantly decreased by inflammation, which is prevented by PAs.

CONCLUSION

PAs significantly modulate gene expression in the cervix in the presence and absence of inflammation. The regulation of these pathways, specifically claudin proteins, may be a critical mechanism by which PAs prevent preterm birth, especially in women with premature cervical shortening.

Nitric oxide in the human uterine cervix: endogenous ripening factor

The human uterine cervix can produce nitric oxide (NO), a free radical with an ultra-short half-life. The release of NO changes during pregnancy and is increased in early nonviable pregnancies compared to normal uncomplicated pregnancies. This review concentrates on the role of NO release in cervical ripening in pregnant women. Also some suggestions on future aspects are discussed.