*2.2.2 Premature relaxation of cervical smooth muscles cells promoting preterm/ premature cervical failure*

One offers special attention on HCSMCs presence as a circumferential sphincter at IO level, since the new paradigm on their role in cervical closure in normal pregnancy [30]. A smooth muscle sphincter at IO, is considered one of the most important anatomic structures for pregnancy's normal progression, when it is contracted [10], and its relaxation is associated with the onset of labor, as it was supposed [59]. From Leppert [60] early studies, one knows that in early pregnancy, fibroblasts and CSMCs proliferation is at the highest level and decreases progressively, whereas apoptosis increases progressively in later pregnancy, parallel to water content in interstitial tissue, inducing mechanical imbalance [51]. Cervical contractile function contributes to cervix stiffness and closure maintenance until the latter part of the third trimester. During the ripening and dilation phases, the HCSMCs are not just a passive responder to uterine contractions; they independently initiate their own contractions, and biochemically their stretch increases MMPs [61], and proinflammatory cytokine secretions [62], representing new mechanisms on premature CR, and possible failure. Vink et al. [63] proved in pregnant women cervices with premature cervical failure (PCF) that CSMCs defect of contractility, and reduced IO sphincter tonus appear when CSMCs are exposed to a soft ECM. It is suggested that a soft ECM may lead to decreased CSMCs contractility and IO sphincter laxity, so ECM rigidity modulates HCSMCs contractility.

HCSMCs are physiological modulated by hormones (estrogens, progesterone, and oxytocin—increases intracellular calcium levels), local paracrine signals (inflammatory chemokines and cytokines), extracellular vesicles (exosomes and ectosomes), and pharmacological agents used for cervical ripening and labor induction. Vink et al. [64] demonstrated that E2 increases HCSMCs contractility by rising actin contractility, and calcium levels—less than oxytocin, and favors pregnancy progression, and

#### **Figure 3.**

*A: funneling of IO, with shape of T, Y, V, or the most dangerous of U, associated to short cervix (between white markers) B: complete cervical effacement (authors proper archive: [65]).*

P4 reduces HCSMCs contractility, by oxytocin-induced contractility alterations, and intracellular calcium flux; P4 appears favorable for IO "funneling"/dilation.

Since many years of conventional/standard sonography, starting from 16th weeks gestation, may show shortness and funneling, **Figure 3**. Funneling is considered more predictive than shortness to PTB risk, and it imposes a cervical stitch [64, 66]. Sludge association with short cervix (<25 mm) increases sPTB risks [67].
