Recent work supports the concept of an active sphincter in the uterine cervix

Nott et al. qualitatively and quantitatively investigate the architecture of the cervix in ex vivo nonpregnant human samples. Using diffusion tensor magnetic resonance imaging (MRI) and a fibre‐tracking algorithm, they identify an outer circular and inner longitudinal layer of fibres throughout the cervix. Quantitative measurements of diffusion, tract orientation, and volume confirm that the outer circumferential band of fibres is particularly dense and uniform in the proximal cervix, at the region of the internal os. They conclude that the purpose of this ‘occlusive structure’ at the internal os may be to resist intrauterine pressure forces from a developing pregnancy.

The finding of this ‘occlusive structure’ resonates with our recent study in which we used immunohistochemical analysis to explore the smooth muscle content and orientation in nonpregnant human cervical tissue. Similar to the fibres noted in Nott et al.'s study, we noted that the area of the internal os contains a significant amount of smooth muscle whose fibres are oriented circumferentially around the outer aspect of the cervix. We also noted longitudinal muscle fibres oriented parallel to the endocervical canal (Vink et al. Am J Obstet Gynecol 2016;215:478.e1–e11). Organ bath studies of cervix slices from the same patients demonstrated that tissue from the internal os contracts in response to oxytocin, supporting the argument that this circumferentially arranged band of muscle fibres may function as a sphincter to maintain cervical competence during pregnancy.

Such findings challenge the prevailing dogma, which is that the human cervix is an homogeneous collagenous structure with a minimal amount (<15%) of smooth muscle, and is a passive bystander in the parturition process. Interestingly, this ‘new’ paradigm of a sphincter at the internal os is not new. In 1931, Ivy et al. demonstrated that the macaque, a nonhuman primate that shares significant homology with humans, develops a functional sphincter at the internal os during pregnancy (Ivy et al. Am J Obstet Gynecol 1931;22:388–99). In the 1950s, Schild et al. demonstrated that the cervix contracts independently from the uterus in response to various contractile agents (Schild et al. Lancet 1951;1:250–3). In fact, perhaps a specialised sphincter in the cervix should be expected, given that neighbouring pelvic organs such as the bladder and rectum, whose function is similar to the uterus (i.e. to retain a product until the appropriate time for its release), possess sphincters.

In summary, the study of Nott et al. should compell us to discard the prevailing paradigm of cervical tissue structure and function, and closely investigate the role of this ‘occlusive sphincter’ in normal and abnormal pregnancy. Today we have at our disposal a growing array of tools to guide quantitative exploration of cervical structure and function in pregnancy, including noninvasive imaging techniques that can be used in vivo. It is an exciting time for our field of obstetrics, one in which a new paradigm may finally point us toward novel and effective therapies for prevention of poor pregnancy outcomes such as spontaneous preterm birth due to premature cervical failure.

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