Biomechanical Adaptations of the Uterus During Parturition and Birthing

Table of Contents

Abstract

Birthing is a fundamental process to the evolution of animal life on earth. The creation of new life pushes a species further in time, continuing millions of years of advancements. However, birthing itself is often dangerous, putting the mother and the fetus in a vulnerable position. Being the center of the female reproductive system in animals, the uterus, which includes the cervix, must naturally adapt to overcome the ever-present strain of birthing. From the conception of the embryo to its exit into the world, the uterus (mainly its counterpart, the cervix), experiences essential physical adjustments. This research paper examines slightly different but interconnected physical processes that the cervix undergoes to prepare for and to proceed with parturition. Through analyzing different studies focused on collagen in the cervix, it is possible to thoroughly understand the biomechanical networking of this protein and its participation in cervical softening and ripening. These two processes are characterized by structural and mechanical changes in the cervical tissue, which allow for an embryo’s smooth exit from the cervix. Physical tests conducted on animal cervical tissue throughout gestation provide insight into these mechanical changes. Furthermore, an analysis of the electrical activity leading to cervical contractions will be conducted to understand the active physical movements displayed in the cervix leading up to birthing. Lastly, a deeper look into cervical mucus, the cervical plug and their preventative functions as barriers will explore these physical designs engineered by nature. Finally, this research paper demonstrates the cervix’s mechanical accommodation to birthing and its increasingly noticeable demonstration leading up to parturition.

Introduction

Since birthing is an intricate and complex operation present in all animal life, a simple infection, logistical mishap or rupture can be deadly for both the fetus and the mother during the gestation period. Due to this, animals have evolved certain features and characteristics that aid them. Many of these characteristics can be found in the uterus and the cervix. The uterus is an organ found in most female animals in the lower abdomen used to house and nourish a fertilized egg (The Editors of Encyclopedia Britannica, 2021); the cervix is the narrow end of the uterus which connects the uterus to the vagina in many animals. The tissue that is present in the uterus is largely composed of collagen, a protein that is essential in adapting to the needs of parturition. This protein network evolves throughout the different stages in pregnancy and allows the uterus to prepare its tissue for a smooth delivery. Whether it be the process of the cervical tissue getting softer and more resilient or repetitive contractions that allow the fetus to safely exit, these physical adaptations all tend to manifest along with the gestation stage.

To protect the uterus, the cervix often creates mucus to act as a plug to protect the fetus from infection (The Editors of Encyclopedia Britannica, 2021). All of these mentioned physical accommodations are present in order to protect both the mother and the developing fetus inside the uterus. The mother and the developing embryo are both extremely vulnerable at this stage and require elements to guarantee absolute safety.

Collagen Networking

In order for a successful pregnancy to proceed, it is imperative for the cervix to adapt to the large number of mechanical forces being increasingly applied throughout gestation due to the growth of the fetus. Therefore, up until parturition, its role is to resist the load. However, there is a dramatic shift in cervical responsibility at term in which it must change from remaining closed (in order to maintain the fetus in the womb) to preparing for parturition through dilation (Barnum et al., 2017). The cervix, which is rich with connective tissue, must thus remodel itself during gestation in order to accommodate for the increased load and prepare for parturition. Within the cervical connective tissue of mice, as well as many other animals, the primary component is fibrillar collagen, a structural protein (Barnum et al., 2017). Although there are other constituents in the cervical extracellular matrix (ECM), fibrillar collagen is the primary contributor to cervical strength and tensile properties, through the degree and type of collagen crosslinks that exist in the cervical tissue (Yoshida et al., 2014). The mouse cervix goes through a dramatic and complicated remodeling process characterized by the changes in the ECM and cervical mechanical properties, which are directly related to each other.

The first phase of pregnancy during which tissue remodeling begins is called cervical softening, which actually lasts for the majority of the pregnancy until cervical ripening takes place in the final days. This phase is characterized by important structural and mechanical changes, most importantly a significant drop in tissue stiffness compared to a non-pregnant mouse’s tissue. The length of gestation in mice is approximately 19 to 21 days, and the cervical softening phase lasts until approximately day 17 or 18, when cervical ripening begins (Jayossi et al., 2018). In the early days of gestation, up until day 6, the stiffness of the mouse cervix does not decrease significantly. Rather, the rate at which stiffness declines is most pronounced between days 6 and 12, after which this rate becomes less drastic until day 15 (Jayossi et al., 2018). Thus, the rate at which cervical stiffness declines is not homogeneous throughout the cervical softening phase.

Throughout cervical softening, quantifiable changes in the microstructure of collagen fibers and in the cervical biomechanics are detectable, specifically in the increase in the percentage of soluble collagen (Mahendroo, 2012). Although the microstructure of the collagen changes, the collagen content or density remains constant. While the amount of collagen does not change throughout gestation, there is turnover throughout the process in which the mature cross-linked collagen is replaced by immature cross-linked collagen (Jayyosi et al., 2018). Through microscopic imaging of the collagen network in mouse cervixes, it can be seen that the fibers themselves undergo a significant change throughout gestation from straight fibers in a non-pregnant mouse to thicker, crimped fibers in pregnant tissues (Jayyosi et al., 2018).

Cross-links between collagen molecules are an essential part of providing strength to the collagen fibers and consequently providing strength to the cervical tissue as a whole. This means that in order for the tissue to weaken, it must remodel the collagen fibers in order to reduce the quality of the cross-links, since this directly reflects on the subsequent strength of the cervical tissue (Fig. 1) (Mahendroo, 2012). Thus, this degradation in collagen cross-linking contributes to cervical softening during mouse gestation in order to decrease the stiffness and strength of the cervix.

Fig. 1 Cervical collagen cross-linking, thrombospondin 2 (THBS2), and tenascin C (TNC) progressively decrease starting in early pregnancy, resulting in larger collagen fibers with reduced mechanical strength being formed. Synthesis of collagen is increased during pregnancy, suggesting increased collagen turnover as well. These early pregnancy changes contribute to the “progressive replacement of mature collagen with poorly cross-linked collagen,” which causes the gradual increase in the diameter size of cervical collagen fibril and fiber and subsequently the gradual decline in strength of the tissue, which is lowest at parturition (Mahendroo, 2012).

Cervical Softening and Ripening

While the complexity of uterine structure certainly contributesto the labor process, the muscular abilities of the uterus are crucial to a successful birth. During parturition, female animals experience uterine contractions, which encourage proper fetal delivery and protect the mother’s uterus to avoid postpartum hemorrhage and other complications (Rees, 2016). However, in order for these contractions to take place, the cervix has to prepare its tissue for delivery. This is the process of cervical softening and ripening. Towards the end of a pregnancy, the stiffness of the cervix decreases, and certain mechanical and structural changes are observed (Peralta et al., 2015). Partially due to collagen network remodeling and hormone presence, cervical softening and ripening in mice allow the tissue in the cervix to become tougher, be more resilient to deformation and be able to handle more mechanical load. (Jayyosi et al., 2018).

Tensile cyclic loading tests conducted by Jayyoci et al. (2018) demonstrated the mechanical properties of the mouse’s cervix. By applying stretch and stress forces on cervical tissue from mice at different stages of their pregnancy, there was an evident trend in their response. While early-stage mouse cervixes appeared to be stronger than late-stage pregnancy samples, the later pregnancy samples had a higher capacity to stretch and extend than early pregnancy tissues (Fig. 2).

Fig. 2 The reaction of a mouse’s cervix to an application of (A) stress and (B) stretch on samples from different stages of pregnancy. Study was conducted on 5 samples and the maximum force they can withstand before breaking is represented on the graphs (Jayyosi et al., 2018).

 Figure 2 describes the maximum stress and stretch that the researchers were able to apply on the cervical tissue without it breaking. Since the last three cervix types are treated with medication or infected with bacteria, only the NP (new pregnancy), d6, d12, d15 and d18 samples are significant in displaying the cervix’s decrease in stiffness and increase in elasticity as pregnancy progresses. This study shows the mechanical adaptation that the animal cervix experiences in order to prepare itself for childbirth.

Collagen Ultrastructure

Studies suggest that the evolution of mechanical properties of cervical tissue in pregnancy is related to cervix softening due to cervical ECM remodeling (Lee et al., 2021). Transmission electron micrographs (TEM) can be used to observe collagen organization through changes in collagen solubility, type, degree of cross-links, and matricellular protein composition.

 In the study conducted by Akins et al. (2011) electron micrographs of mouse cervical tissue at Days 6 and 18 were taken at a magnification of x4200 (Fig. 3). On Day 6 during early pregnancy in the mice, cellular components and electron-dense components of the ECM were observed to be dense and within close proximity of each other (Fig. 3A). In late gestation on Day 18, the collagen fibers were more dispersed and less associated with cellular components of the connective tissue (Fig. 3B).

Fig. 3 Throughout pregnancy in mice, the cervical extracellular matrix becomes dispersed as shown in electron micrographs taken at x4200 magnification on (A) Day 6 and (B) Day 18 of pregnancy. Bar = 1000 nm (Akins et al., 2011).

For further evaluation of changes in the collagen ultrastructure, the diameters of collagen fibrils of cervical tissue in the same mice were measured from non-pregnant metestrus and gestation Days 6, 12, 15, and 18 (Fig. 4A-E). From early to late pregnancy in mice, the mean diameter of collagen fibrils significantly increased along with a shift in the distribution toward a higher frequency of fibrils with larger diameters.

On Day 6 of pregnancy, the fibril diameter increased 12.1nm compared to non-pregnant metestrus. From Day 6 onwards to Day 18, there was an additional 20 nm increase (Fig. 4F-J). From the beginning of pregnancy to near the end, there has been a 32.1 nm total increase in collagen fibril diameter.

Fig. 4 Throughout pregnancy, collagen fibrils increase in size as shown in electron micrographs taken at x20 500 magnification of (A) non-pregnant metestrus, (B) Day 6, (C) Day 12, (D) Day 15, (E) Day 18. Bar = 1000 nm. The analysis of frequency of fibril diameter of (F) non-pregnant metestrus, (G) Day 6, (H) Day 12, (I) Day 15, (J) Day 18 is shown. n = 8260 – 13,030 fibrils (Akins et al., 2011).

In addition to using transmission electron microscopy, a different recent study by Zhang et al. (2012) used second harmonic generation (SHG) microscopy to observe dynamic changes in the collagen matrix architecture in cervical tissue sections during preterm birth and normal pregnancy in mice. SHG microscopy is an effective method for direct non-invasive imaging of collagen type I in biological tissues at submicrometric resolution.

Figure 5 shows representative images of cervical sections from nonpregnant mice and pregnant mice at Days 6, 12, 15, 18. In the cervix of nonpregnant and early pregnant (e.g., at Day 6 of gestation) mice, the collagen fibers were highly aligned, thin, and relatively straight. In the later stages of pregnancy, the collagen fibers in the cervix become more curved and are observed to be thicker in appearance. This observational assessment of the dynamic size of these collagen fibers in SHG images shows that characteristic fiber size increases progressively throughout the course of mice pregnancy, in conjunction with a transition from relatively straight fibers to a kinked or undulating, wavy appearance (Zhang et al., 2012). In general, shorter persistence length of a polymer is associated with reduced flexural rigidity. Therefore, the change in morphology of the fibrils is consistent with decreased tensile strength of the collagen matrix as pregnancy progresses toward term in mice. In exchange for the loss of strength in cervix tissue structure during late pregnancy, the tissues can stretch and extend due to their wavy shapes. This mechanical shift from strong and stiff cervical tissue structures to increased elasticity supports the previously demonstrated applications of cervical softening and ripening.

Fig. 5 SHG microscopy images of cervical tissues sections from (A) nonpregnant (NP) mice and pregnant mice at gestation (B) Day 6, (C) Day 12, (D) Day 15, (E) Day 18. Significant morphological changes in cervical collagen are evident throughout the course of pregnancy (Zhang et al., 2012).

The observed morphological changes in the dispersal of collagen fibers in mice cervical ECM using TEM supports the quantifiable changes in the collagen fibril that was determined by SHG microscopy. The observed increase in the diameter of collagen fibril and dispersal of these fibers studied using micrographs correlates with the cumulative biomechanical changes that occur in the cervix function throughout pregnancy and birthing.

Cervical Contractions

While relaxation of the cervical muscles allows the fetus to travel through some of the mother’s reproductive organs, the presence of cervical contractions is another method that physically induces the birthing process in animals. These intricate and repetitive reconstructions in the uterus’s muscular and mechanical abilities are primarily connected to the myometrium, a layer of muscular tissue present in the uterine wall (Malik et al., 2020). Myometrial cells have receptors for certain hormones, oxytocin for example, which induce cellular response. These cells are crucial to this process since they are responsible for the electric action potential that allows for these uterine contractions to occur. With an increase in Ca2+ through the myometrial cell’s ion channels, an imbalance of charge happens and a change in voltage is noticed. This activates actin and myosin filaments to express a mechanical force and contract the muscle tissue. Hormones like oxytocin can then increase polarization, which leads to a more pronounced action potential which then encourages stronger and longer contractions (Malik et al., 2018). Overall, the action potential produced by this mechanism and by hormone regulations directly correlate with these uterine contractions and the magnitudes of their strength (Fig.6).

Fig. 6 The plotting of action potentials and cellular Ca2+ concentrations with respect to time are compared to a Force v Time plot of the resulting contractions. These three show a direct correlation between cellular electric activity and mechanical activity in the uterine muscles (Malik et al., 2018).

 This electrical activity within the animal myometrium tends to be more pronounced and enhanced as the pregnancy progresses (Miller et al., 1989). This is demonstrated by both the speed and the frequency of these contractions. An analysis of electrical propagation was conducted by Miller et al. (1989). By placing electrodes on muscle layers along a rat’s uterus, researchers were able to record the speed and frequency of these electric signals which led to physical contractions. By testing on preterm and delivering rats, it was possible to describe the evolution and adaptation of the uterus in relation to the progress of gestation (Fig. 7).

Fig. 7 Plot of detected electrical signals (in mV) from myometrium in (A) preterm and (B) delivering rats with respect to time (in seconds). This plot describes frequency, speed, and strength of this electrical activity (Miller et al., 1989).

By analyzing Figure 7, we can see that there is quite a noticeable difference in the electrical signals detected in the preterm and delivering rat uterus. The delivering myometrium was electrically active or contracting at higher frequencies and would also happen at a greater speed than the preterm rat myometrium (Miller et al., 1989).

Since the goal of these uterine contractions is to push the fetus out, the direction of the electrical activity responsible for this muscular movement is also crucial. Studies done by Parkington et al. (1988)compared electrical activity in the longitudinal and circumferential directions. By placing electrodes in different conformations along the ewe’s myometrium, it was possible to measure the difference in electrical signals. The electromyography (EMG) activity was more pronounced and more frequent in the longitudinal direction than in the circumferential direction (Fig. 8). Animal uterine contractions are directed along the uterus and the velocities of the electrical propagation are enhanced in the longitudinal direction (Rabotti, 2014).

Fig. 8 Detection of electrical activity in the ewe’s myometrium. Plots a), b), and c) are detections of longitudinally placed electrodes and plots d) and e) are those in a circumferential conformation (Parkington et al., 1988).

Cervical Plug/Mucus

The cervix has many roles throughout an active pregnancy and is also very important in the process of birthing. The cervix serves two main purposes: facilitating the entrance and passage of sperm through dilation (or widening) of both openings of the cervix, and keeping the uterus sterile (EEB, 2021). The cervix’s physical properties, as well as its muscular movements throughout the birthing process, function to make the dangerous ordeal of pregnancy and birth much safer for the mother. The presence of the cervical plug in many mammals is important in order to block out any harmful elements, primarily by inhibiting microbial entry (Loux et al., 2013). In certain species such as sheep and cattle, the ewe and cow have many interlocking cervical rings which in addition to the cervical plug further inhibit any entry (Fig. 9) (Kershaw et al., 2005). 

Fig. 9 Three dissected ewe uteruses. Each image (a, b, c) shows a different sheep’s uterus, with each having a different complexity of folds and rings within the cervix. The arrows are the maximum penetration of a pipette ovine insemination pipette. The 2 most folded and asymmetrical cervices have the least penetration (Kershaw et al., 2005).

The mare’s cervical plug is extremely important, more so than in other similar species since it does not have the extra support from the cervical rings (Loux et al., 2013). The cervical plug is the only barrier blocking the passage of microbes into the uterus. In the case of the mare, it is composed of a multitude of proteins coming from both the innate and acquired immune system. Though these proteins all come from the immune system, many aid exclusively in managing the pregnancy, while others have roles in both protecting the immune system and regulating the pregnancy of the mare (Loux et al., 2013). The composition of the mucus plug gives it a reddish tint and makes it very viscoelastic (Fig. 10) (Loux et al., 2013).

Fig. 10 The cervical mucus plug of a mare. The plug consists of mucus with a reddish tint (Loux et al., 2013). It is described as very viscoelastic, allowing deformation due to external stresses but being able to quickly go back to its original position.

Fig. 10 The cervical mucus plug of a mare. The plug consists of mucus with a reddish tint (Loux et al., 2013). It is described as very viscoelastic, allowing deformation due to external stresses but being able to quickly go back to its original position.Viscoelastic materials are materials with both viscous and elastic properties. Viscosity describes a fluid that deforms slowly when an external force is applied, and elasticity describes a material being able to return to its original state after a deformation (Gould et al., 2019). Viscoelastic materials are able to absorb energy, and make good shock absorbers. However, the stiffness of these materials changes when they are deformed or when they experience a change in temperature (Gould et al., 2019).

In general, the mucus of the cervix is diluted during ovulation, and thicker before and after. During these times, the mucus does not stay in place and does not act as a plug (EEB, 2021). During pregnancy, however, the cervical plug forms, and the mucus uses its viscoelastic properties to seal up the cervix and prevent the penetration of microbes into the uterus (Bastholm et al., 2014). Since the plug must be sustained for extended periods of time in most mammals, the ability for it to deform and change under strain is necessary for it to hold its place and maintain the seal. However, its properties are very dependent on temperature and if temperature augments too much, it can lose its ability to stay in place, and thus be useless as a seal against foreign microbes.

Other than the immune functions and physical properties of the mucus, certain proteins within the mucus are also for helping prevent infection (EEB, 2021). These proteins are negatively charged (Loux et al., 2013). Due to this, negatively charged molecules are repelled and allowed to pass through, whereas anything positively charged is bound to the protein and thus cannot pass through (Becher et al., 2010). This acts as a final barrier for anything that should not be allowed into the uterus. These proteins play a major role in keeping the uterus safe, as well as keeping the form of the cervical plug, allowing it to seal and prevent anything from passing the cervix. The cervical mucus plug itself is the major contributor in the uterus, and thus fetus protection.

Other than the cervical plug, certain mammals, such as female sheep, have rings within the cervix that act as an additional barrier. The cervix of the ewe is filled with many twists and turns, as well as between 4-7 rings, often out of alignment with each other, once again making it harder for anything to pass through (Kershaw et al., 2005). In the ewes, the first few rings are in line with each other, meaning that their centers are in line going up. The later rings, however, can be in different arrangements not in line with the others. This random and asymmetrical nature of the rings means that bacteria must travel further and therefore have less of a chance to make it through (Fig. 9). Furthermore, at the level of the early rings, the cervix is very narrow compared to later. These rings and folds within the cervix are incredibly variable between ewes (Kershaw et al., 2005). All these characteristics of the cervix combine to make a maze-like structure which is very hard for any microbe to go through. The extended length of this structure and various folds plays a key role in the protection of a sheep fetus. Other than this, there is still the cervical mucus plug that acts as another defense to a list of many already present within the sheep, denying any entry of bacteria into the uterus as a sort of last resort.

Discussion

The studies, analyses and figures present in this research paper demonstrate how the animal cervix redesigns its physical properties in order to fulfill the specific needs of birthing. The cervical ripening and softening contribute greatly to this redesign. In certain mammals such as the mouse, the cervix must adapt and bear the ever-growing load during the pregnancy (Akins et al., 2011). It must do this until closer to parturition. The cervix in mice changes dramatically throughout their pregnancy, due to the collagen fibers present within it. These fibers go from being straight in non-pregnant mice to much thicker and compressed during the pregnancy (Zhang et al., 2012). The cross-links created by the collagen are weakened throughout the pregnancy in order to soften the cervix, thus decreasing strength and stiffness (Mahendroo, 2012). The stages of cervical softening and ripening are characterized by a distinct change in the cervix’s mechanical properties. This ripening allows the cervix to become able to handle more deformation and handle more mechanical load. Though early in the pregnancy, the cervix is stronger, it is less able to stretch and extend in contrast to near the end of the pregnancy (Jayyosi et al., 2018). Other than the cervix’s structural changes, contractions also play a role in the birthing process. An increase in certain ionic concentrations signal the contraction of filaments, and therefore overall uterine contractions. This activity is of course more pronounced later, closer to the actual birth. More electrical activity is present, and thus the contractions are faster and more frequent (Miller et al., 1989). These all aid in the birthing, but to keep the fetus safe within the uterus before this final step, the cervical plug is very important in sealing the cervix during pregnancy and therefore blocking any microbial entry with the help of its viscoelastic properties (Loux et al., 2013). Some animals such as the sheep and cow have an extra line of defense against infection known as cervical rings and folds which increase surface area, making it harder for bacteria to get through (Kershaw et al., 2005). Therefore, the intricate conditions needed for a safe birthing process are satisfied with these equally intricate physical and natural adaptations.

Conclusion

 The mechanisms and characteristics discussed in this research paper are constantly working together to aid in a safe and effective birthing process. Birthing is a vulnerable process and must be approached with complexity and precision. If needed adaptations do not occur properly, the birthing process can become completely compromised. Interestingly, a major cause of preterm birth in animals is an early transformation of these physical properties in the cervix. Since the fetus is meant to exit through the cervix, its early reduction in stiffness and an early onset of cervical contractions lead to preterm deliveries in animals, which are far more dangerous for the animal fetuses and their development (Mahendroo, 2012). Overall, the complexities of these adaptations pose interesting questions about the possibility of artificially bioengineered uteri becoming accessible for animals in the future (Hellström, 2020). Researchers and scientists will have to satisfy all the physical conditions required for birthing, which complement the chemical adaptations that are present leading up to and during parturition.

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