**2. Gestational cervical remodeling**

"Cervical remodeling" is the collective term for progressive cervical changes and recovery during pregnancy, labor, and postpartum, involving four overlapping phases: softening, ripening, dilation, and postpartum repair [9]. Clinicians use terms such as *softening, shortening, funneling, effacing,* and *dilating* interchangeably [10] for gestational cervical deformations, representing tissue intrinsic material changes with anatomical transformations. Cervical insufficiency (CI) determined a lot of comments. According to the American College of Obstetricians and Gynecologists [11], it is defined as "painless cervical dilation after the first trimester with subsequent expulsion of the pregnancy in the second trimester, typically before 24 weeks gestation, without contractions or labor, and in the absence of other clear pathology (as bleeding, infection, ruptured membranes)". It was recommended the terminology "premature cervical remodeling" to describe cervical softening, shortening, funneling, and IO dilation, and to retire the anachronistic "cervical insufficiency" [4].

*Abnormal Cervical Remodeling Early Depiction by Ultrasound Elastography: Potential… DOI: http://dx.doi.org/10.5772/intechopen.113314*

### **2.1 Cervix uteri microarchitecture**

Human cervical structure was established in the 1940s when Danforth D [12] described cervical stroma—a predominantly homogenous, hydrated, collagenous structure between exo and endocervix, which over time made the distinction to the muscular structure of the uterine corpus. Stroma consists of 85–90% extracellular matrix (ECM) that contains tropocollagen, aggregating to form collagen fiber, and bundles network surrounded by a viscous substance of proteoglycans (decorin, biglycan, asporin, osteomodulin and fibromodulin, lumican, and versican), glycosaminoglycans (GAG: sulfated-chondroitin/dermatan/heparan sulfate and non-sulfated: hyaluronan-hyaluronic acid (HA) GAGs), matricellular proteins (thrombospondin 2; tenascin C); adhesion molecules (fibronectin, integrins-α, β or α-β); specific aminoacids—phenylalanine; enzymes [hyaluronidase, matrix metalloproteinases—MMP: -2, -9; lysyl oxidase-like-1 (LOXl-1), involved in ECM transformations], according to their genes, and water [13]. Cells—about 10–15%—are represented by fibroblasts, smooth muscle, glandular, immune, and vascular cells. Fibroblasts synthesize ECM, rich in collagen fibrils (type I—70%, for tensile strength; type III, 30% for elastic properties) as tropocollagen molecules self-assembled into fibrils—a nanoscale rope-like, and elastin, as tropoelastin to form the elastin fibers complex network intertwined with collagen. Collagen bundles are organized in layers, which remodel differently because of independent molecular processes and cause softening and shortening [14, 15]. Collagen content—measured as hydroxyproline, is constant along pregnancy [9]; secondharmonic generation (SHG) microscopy presents collagen preferentially aligned around the canal [16]. The triple helix arrangement allows collagen to cross-link into complex 3-D networks of fibrils, fibers, and bundles that dictate both firmness and mechanical strength during non-pregnant/pregnant states [17]. From Aspden [18] early theory on three zones in cervix structure—**Figure 1A**, actually cervical stroma is considered heterogenic, and collagen network strength depends on the degree and type of collagen cross-links between each collagen microfibril [20, 21]. Recent studies show significant collagen cross-link heterogeneity in internal OS (IO) compared to external OS (EO) [22]; IO area is the most exposed to stretch, and it is resistant to tension associated with uterine content (fetus, placenta, amniotic fluid), cervical angle in pelvis [10], and fetal membranes properties, their stiffness sustaining IO closure,

#### **Figure 1.**

*A. The three zone in cervix early theory. B. New paradigm on SMCs at upper cervix half, circumferentially to IO, is like a specialized sphincter; it is not a homogenous collagenous structure, with minimal cellular content, as old paradigm. Adapted from Vink and Myers [19], free PMC article. HHS Public access.*

as 3D MRI revealed [23]. Elastin is a high thermal stable protein with an 80-year half-life and a linear, compliant mechanical response to tension, permitting cervix restoration after an applied stretch. Elastic fibers are present mostly in the subepithelial stroma, aligned at early pregnancy as collagen fibers, and undergo structural changes, becoming thicker, shorter, and losing directionality [24, 25]. Proteoglycans have key roles in controlling gradients and availability of potent growth factors, chemokines, cytokines, and morphogens, important in tissue's homeostasis, mechanical strength, development, and repair, by their covalent attachment to collagen fibers surface, contributing to fiber optimal formation [26].

From early studies on cervical structure and functions [27–29], immune—staining with smooth muscle cell markers (α-smooth muscle actin, smooth muscle protein 22 calponin) in non-pregnant women <50 years old cervices proved the presence of smooth muscle cells (HCSMCs) surrounding IO, about 50–60% of tissue, like a specialized sphincter, proved by positive contractile activity tests at oxytocin/nifedipine, similar to uterine SMCs. Bundles of cervical smooth muscle cells (CSMCs) are circumferentially oriented around endocervix canal periphery. Human cervical smooth muscle cells (HCSMCs)' density is gradually reduced from the mid cervix to EO (10%), where contraction tests showed less activity. Gap junctions are present in HCSMCs for direct communication with uterine corpus SMCs [4], a cervical active involvement in pregnancy progression, as Pajntar sustained for a long time [28]. Longitudinal muscle fibers are mostly located in cervix's inner part, while circular muscle fibers are mostly in the outer part. Longitudinal muscles can contract, helping cervical dilatation, while circumferential muscle fiber contraction impedes cervical dilatation, keeping the cervix closed." Dynamic cervix" nomination is used when a short cervix appears without uterine contractions or is correlated to uterine fundus pressure [30]. This discovery changed the paradigm on HCSMCs covering cervix first half part, and their role in IO normal closure, or relaxation-inducing "funneling"/"sphincter failure", the first sign of premature cervical remodeling [19, 30], and increases the duration of latent and active phases of labor [31], according to connections of SMCs from uterine cervix and corpus. Similar CSMCs were discovered in female rats during pregnancy, being considered as a continuous muscular layer from the uterine corpus (**Figure 1A** and **B**) [32].

Resident immune cells -neutrophils, eosinophils, and macrophages are present in cervix; their location and relative number change during pregnancy [33, 34]. Rat's cervices pathological exams [35] proved macrophages role in remodeling, mainly in ripening normal/abnormal timing. There are registered reduced cell nuclei density, decreased collagen content and structure, and a greater presence of macrophages per unit area in rats under progesterone receptor antagonist (onapristone/mifepristone) or after ovariectomy on day 17 postbreeding, similar to aspects before term.

#### **2.2 Timing of cervical remodeling**

Qualitative and quantitative cervical softness/stiffness and length dynamic changes previous to uterine contractions onset are harboring different mechanisms in term/PTB regarding cervical involvement in "closing"/"opening", to prevent/delay sPTB/to induce labor, were better understood using mathematical or animal models instead of cervical biopsies because women's risks [36], and confounders by variations in timing and collected samples region [37]. Shorter pregnancy duration and mice cervices resistance to break/stretch made mice prone to study [38]. Evidences provided similarity in women, and mice regarding dynamic gestational cervical

*Abnormal Cervical Remodeling Early Depiction by Ultrasound Elastography: Potential… DOI: http://dx.doi.org/10.5772/intechopen.113314*

remodeling [9, 37, 39], and simulations showed that changes are centered on stroma ECM content, which are associated to changes of SMCs surrounding IO [30], and all control gestational tissue's mechanical behavior.
