Original Research
Obstetrics
A new paradigm for the role of smooth muscle cells in the human cervix

Presented in part as abstract no. 2067136, poster no. 581, at the annual meeting of the Society for Maternal-Fetal Medicine, San Diego, CA, Feb. 5-7, 2015.
https://doi.org/10.1016/j.ajog.2016.04.053Get rights and content

Background

Premature cervical remodeling resulting in spontaneous preterm birth may begin with premature failure or relaxation at the internal os (termed “funneling”). To date, we do not understand why the internal os fails or why funneling occurs in some cases of premature cervical remodeling. Although the human cervix is thought to be mostly collagen with minimal cellular content, cervical smooth muscle cells are present in the cervix and can cause cervical tissue contractility.

Objective

To understand why the internal os relaxes or why funneling occurs in some cases of premature cervical remodeling, we sought to evaluate cervical smooth muscle cell content and distribution throughout human cervix and correlate if cervical smooth muscle organization influences regional cervical tissue contractility.

Study Design

Using institutional review board–approved protocols, nonpregnant women <50 years old undergoing hysterectomy for benign indications were consented. Cervical tissue from the internal and external os were immunostained for smooth muscle cell markers (α-smooth muscle actin, smooth muscle protein 22 calponin) and contraction-associated proteins (connexin 43, cyclooxygenase-2, oxytocin receptor). To evaluate cervical smooth muscle cell morphology throughout the entire cervix, whole cervical slices were obtained from the internal os, midcervix, and external os and immunostained with smooth muscle actin. To correlate tissue structure with function, whole slices from the internal and external os were stimulated to contract with 1 μmol/L of oxytocin in organ baths. In separate samples, we tested if the cervix responds to a common tocolytic, nifedipine. Cervical slices from the internal os were treated with oxytocin alone or oxytocin + increasing doses of nifedipine to generate a dose response and half maximal inhibitory concentration. Student t test was used where appropriate.

Results

Cervical tissue was collected from 41 women. Immunohistochemistry showed cervical smooth muscle cells at the internal and external os expressed mature smooth muscle cell markers and contraction-associated proteins. The cervix exhibited a gradient of cervical smooth muscle cells. The area of the internal os contained 50-60% cervical smooth muscle cells that were circumferentially organized in the periphery of the stroma, which may resemble a sphincter-like pattern. The external os contained approximately 10% cervical smooth muscle cells that were randomly scattered in the tissue. In organ bath studies, oxytocin stimulated the internal os to contract with more than double the force of the external os (1341 ± 693 vs 523 ± 536 integrated grams × seconds, respectively, P = .009). Nifedipine significantly decreased cervical tissue muscle force compared to timed vehicle control (oxytocin alone) at doses of 10–5 mol/L (vehicle 47% ± 15% vs oxytocin + nifedipine 24% ± 16%, P = .007), 10–4 mol/L (vehicle 46% ± 16% vs oxytocin + nifedipine –4% ± 20%, P = .003), and 10–3 mol/L (vehicle 42% ± 14% vs oxytocin + nifedipine –15% ± 18%, P = .0006). The half maximal inhibitory concentration for nifedipine was 1.35 × 10–5 mol/L.

Conclusion

Our findings suggest a new paradigm for cervical tissue morphology–one that includes the possibility of a specialized sphincter at the internal os. This new paradigm introduces novel avenues to further investigate potential mechanisms of normal and premature cervical remodeling.

Introduction

Spontaneous preterm birth (sPTB) is a significant obstetric dilemma affecting approximately 10% of US pregnancies.1, 2 Etiologies vary, but sPTB must eventually involve premature remodeling and dilation of the cervix to allow for delivery of the premature fetus.3 Although the pathophysiology of premature cervical remodeling is not fully understood, sonographic findings and computational modeling suggest that in some cases, the process starts with dilation of the internal os (the top aspect of the cervix where the uterine arteries insert into the uterus), which is clinically termed “funneling”4, 5 (Figure 1). Clinicians also describe a “dynamic cervix” where the cervix appears shortened in the absence of uterine contractions, which can be seen if transvaginal ultrasound is performed for several minutes. A dynamic cervix has also been described as cervical shortening in response to fundal pressure. Despite these clinical findings, we still cannot explain why the internal os weakens first in some cases of premature cervical remodeling. Our overall goal is to study whether premature cervical failure at the level of the internal os is due to regional differences in cervical tissue morphology and function.

Since the 1940s, the cervix has been characterized as a mostly collagenous structure (90% collagen/extracellular matrix [ECM]) with minimal cellular content (10% smooth muscle cells [SMC]).6, 7, 8 However, these early studies suffered from technical limitations of the time and used immunohistochemical methods (Masson trichrome staining and subjective evaluation of SMC morphology)6, 7, 8 that do not specifically identify SMC. Decades later, investigators questioned why SMC exist in the cervix. Interestingly, Bryman et al9, 10, 11 found that adrenoreceptor agonists and oxytocin stimulate human tissue from the external os (lowermost aspect of the cervix closest to the vagina) to contract, suggesting SMC play a role in cervical tissue contractility.

Since the turn of this century, however, cervical SMC (CSMC) have been largely ignored since CSMC content was thought to be minimal. Instead, researchers focused on identifying alterations in the human cervical collagen network to explain premature cervical failure.12, 13, 14, 15, 16, 17 Since clinical observation demonstrates the area of the internal os can funnel first and/or is dynamic in some cases of premature cervical remodeling, we believe that the working paradigm of cervical tissue architecture needs to be reevaluated. Here, we use improved immunohistochemical techniques and functional studies to determine if regional differences in CSMC content and distribution influence cervical tissue function. The knowledge from this study will expand our understanding of cervical tissue characteristics that may contribute to normal and abnormal cervical function in pregnancy.

Section snippets

Tissue collection

This study was approved by the institutional review board at Columbia University Medical Center and Intermountain Healthcare. Nonpregnant, premenopausal women (<50 years old) undergoing a total hysterectomy for benign indications were consented. Women with an abnormal pap smear or prior cervical surgery were excluded. Demographic data (age, parity, obstetric history, menstrual phase, body mass index, and race) were collected.

Evaluation of CSMC at the internal and external os

Immediately following hysterectomy, whole transverse slices (3-mm

Evaluation of CSMC at the internal and external os

In some women with premature cervical failure, the internal os dilates first while the external os remains closed. To understand how cervical tissue structure influences its function, our initial goal was to understand general cervical tissue composition at the internal and external os using a Movat pentachrome stain, which allows for simultaneous visualization of collagen, mucin, and muscle. Whole transverse slices from the internal and external os were obtained from 13 nonpregnant women (5

Comment

The findings from this study suggest a revised paradigm of SMC organization and function in the human cervix (Figure 11). Unlike the prevailing paradigm that states the cervix is mainly collagen/ECM with minimal cellular content, our revised paradigm shows that the area of the internal os, the area that “fails” or “funnels” first in some cases of premature cervical remodeling, contains 50-60% CSMC and bundles of CSMC can be found circumferentially oriented around the periphery of the cervix.

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    Supported by a Society for Maternal-Fetal Medicine Foundation/American Association of Obstetricians and Gynecologists Foundation Scholarship. This foundation had no role in the study design, tissue collection, or analysis and interpretation of the data. Images were collected in the Confocal and Specialized Microscopy Shared Resource of the Herbert Irving Comprehensive Cancer Center at Columbia University, supported by National Institutes of Health grant no. P30 CA013696 (National Cancer Institute).

    The authors report no conflict of interest.

    Cite this article as: Vink JY, Qin S, Brock CO, et al. A new paradigm for the role of smooth muscle cells in the human cervix. Am J Obstet Gynecol 2016;215:478.e1-11.

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