Pregnancy involves a gradual change in tissue consistency, where, as gestational age increases a drop in stiffness is noticeable. The extracellular matrix (ECM) of the cervix whose behavior is controlled by the collagen structure is reconfigured, due to hormonal and protein related factors that degraded it. There is an increase in cellular and water content. The collagen maintains a stable percentage, although the synthesis-solubility interaction causes a transformation in the network increasing its diameter and waviness, which destabilizes the mechanical integrity. Further understanding of the remodeling of the cervix becomes a key element in the progress against preterm birth. However, since the woman is in a very delicate state, the available information is scattered. The aim is to provide a framework where the variation of the histological properties is linked with the individual biomechanical evolution of the constituents; therefore a multiscale approach to the problem can be made. The results of different authors were reviewed to obtain values of each component depending on the gestational age, and then they were combined with the mechanical development of each variable obtained from the outputs of numerical simulations of another author. An additive function is proposed, in which the mechanical contribution of each constituent is added to a reference value established for a nonpregnant state. Results show how biochemistry models the mechanical behavior of the tissue through the histology and morphology of the ECM.
The anatomical function of the cervix is to serve as a gatekeeper during pregnancy, separating the fetus from the outside. Gestation is a delicate process where there is often lack of information. The possibility of obtaining samples of cervical tissue at any time is solely limited to scientific and at risk cases, which is preventing an adequate breakthrough in the characterization of the etiology of the cervix. This is now an urgent problem in the case of premature birth, the first cause of infant mortality in children under five years of age [
It is well known that from the moment a woman gets pregnant biological transformations occur, coming from the combination of the growing pressures exerted by the fetus and biochemical processes, all of which involve the remodeling of the cervical tissue. This implies changes in the morphology of the tissue to the point of altering mechanical properties [
The remodeling process is a progressive transformation of the appearance of the tissue that is translated into four phases which are superimposed as they occur until birth, namely, softening, ripening, dilation, and postpartum [
As can be inferred from the above, the morphology of the cervix during pregnancy is a complicated subject to study, where the greater information collected comes from the moments before and after delivery. Experimental data of the morphological and biochemical characteristics at the microscopic level is essential to monitor trends and to be able to program specific actions since the mechanical properties at the macroscopic level of the tissue are related to them [
Hence, the main objective of this work is to provide a first approach in a temporal prediction of the stiffness evolution of the cervix, where the contribution of each constituent will be highlighted. To get here, an histobiomechanical framework that materializes in a curve of stiffness versus gestational age is built. Besides, the relationships established between biochemistry and histology will be shown.
Histological transformations in the tissue are responsible for variations in consistency. As a starting point, the basis of this study is on the multiscale approach proposed by Peralta [
Since each person is different, whether by obstetrical or medical history, the values chosen to represent each component should be representative of the important variations recorded. Selection criteria are proposed based on existing information: (i) It must follow the behavior recorded by most studies. Some measures deviate much from the general trend and remain isolated as cases without validation by other sources. (ii) The biochemical analysis that has been used for measuring is crucial. First of all, the result should be in dry weight, because large variations are registered when working with wet weight [
Selected data are summarized in Table
Stroma evolution during gestation. Percentages refer to dry tissue, except for water and dry tissue, which are in relation to the total.
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Nonpregnant | Early | Late | Postpartum | Reference |
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Water | 78 | 75 | 83 | 76 | [ |
Dry tissue | 22 | 25 | 17 | 24 | - |
Collagen content | 71 | - | 78 | - | [ |
Collagen solubility | 17 | 75 | 76 | 88 | [ |
Elastin | 1.33 | - | 0.73 | - | [ |
GAGs | 1.6 | - | 2.6 | 1.2 | [ |
Cells | 8 | - | 15 | - | [ |
The chosen values are fitted to time evolution curves with the intent to relate the gestational age to absolute values (%) of each ECM component. Many different mathematical models for growth can be useful [
The following content describes the temporal evolution of each ECM component and its characteristics.
In terms of dry weight, the collagen content remains practically constant. This has not been measured by many researchers, as it is a rigorous process. It is fitted to a logistic curve (see (
where
It is expected that as collagen turnover increases, a synthesis-degradation process will be appreciated, affecting the morphology [
A crystallization-like system is proposed for synthesis representation. The formation of a new crystal starts with a nucleation process [
Equation (
As discussed above, the interaction solubility-synthesis is key to understand the loss of tensile strength, since the concentration of collagen does not seem to vary significantly and therefore could not be the cause of the changes. This is perceived as a nutritional intake, where the created collagen is destroyed, but when synthesis exceeds degradation, a saturation environment is reached. To represent this, a Morgan-Mercer-Flodin sigmoidal model is suggested (see (
where
Following the synthetic model created by Peralta et al. [
Pinheiro et al. [
Three distributions are conceived, ranging according to the phases of early, mid, and late pregnancy. Equation (
where
Taking into account the interaction solubility-synthesis and the values of the probability distribution, a similar evolution is anticipated. This is reflected in a logistical function (see (
The interstitial fluid controls the stiffness response of the tissue. It presents a similar development to hyaluronic acid (HA) that affects collagen organization [
In general terms, there is an increase in vascularization. In the nonpregnant state, there are approximately 8 to 10 % of fibroblast, epithelial, and smooth muscle cells. Near delivery there is an enormous increment of cells due to the proinflammatory state [
As a whole, they show a very rapid and abrupt evolution when it comes to labor, which can be divided into two phases and then fitted to an exponential model; see
This section summarizes the results of the work of Peralta et al. [
The effect of each constituent was studied independently, i.e., not affecting the behavior of the whole; when a content varied, reference values were set. The reference configuration for the model (see Table
Reference input values of the model. Source: [
Fiber diameter |
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Interfibrillar space | 1 |
Fiber waviness (sd) | 15 |
Fiber fraction | 20% |
Hydration | 80% |
GAGs | 1.5% |
Cells | 8% |
They proposed different models of analysis, and here the one that has the fibers arranged longitudinally and perpendicular to the propagation is adapted. The election is based on a recent study by [
Regarding the behavior of the components, several differences between this work and Table
The final results are fitted with polynomial functions in order to get an interpolation that collects all the values from the range. The most relevant properties of the model considered are fiber diameter, fiber waviness, hydration, and cells. Since interfibrillar space and diameter are linked characteristics, they have a joint contribution represented as a single variable. Although GAGs have been found to have no appreciable effect on stiffness, they are known to alter the mechanical structure of the tissue [
The last step is to combine these two previous results to create a curve that relates gestational age to cervix stiffness. Growth curves (gestational age dependent variables) are entered into the outcome of numerical simulations (mechanical development of each variable). Once implemented, each component will contribute to the consistency, either by adding to or subtracting from the reference value set for the nonpregnant state; therefore it works in relative terms. The influence on the reference value can be represented as an analogy of a Taylor series, as in (
First of all, the chosen data will be examined for each component; readers are referred to Table
The latest data recorded for the collagen content was made by Myers et al. [
Collagen content evolution during pregnancy, with
In the first few weeks the rate of solubility shows a very large increase, and then it stabilizes and stagnates near the end (see Figure
Collagen solubility evolution during pregnancy. Parameters:
The interaction solubility-synthesis configures the ratio of synthesis. There must be a balance to preserve the structural integrity, and so, synthesis is opposed to solubility action with similar values (see Figure
Collagen synthesis evolution during pregnancy. Parameters:
The data for the diameter and interfibrillar space were taken from the model of Peralta et al. [
Collagen diameter evolution during pregnancy.
In the case of waviness, the fibers increase its crimping in agreement with the synthesis of collagen. The range of the variance
The shape parameters of the distribution are computed according to the phase of pregnancy, where it is assumed that the mean (
Waviness probability density function. Parameters: for early (
Due to their reliance on collagen synthesis, a similar evolution is expected for waviness, starting with cautious crimping and then undertaking a remarkable increase, reaching a value that could not be higher. Equation (
Fiber waviness evolution during pregnancy.
Some authors have measured the water content in the cervix, to conclude that there is an increase of 5%. However, only a few were able to complete the data in all the stages of pregnancy. Anderson et al. [
Water content evolution during pregnancy. Parameters:
The cell proliferation is based on the protection and preparation for the last phase of pregnancy, where there are a number of chain reactions that affect the hormones responsible for pregnancy. Due to this, an exponential evolution (see (
Cells content evolution during pregnancy. Parameters: A
The last constituent studied is GAGs. They have a strong relationship with collagen morphology and water content. Therefore, they begin with gradual growth, to reach birth and fall exponentially (see Figure
GAGs content evolution during pregnancy. Parameters:
In the next point, the reference mechanical value of the cervix has been calculated. It comes from the conjunction of the profiles of the numerical model and the distribution of Von Mises. The module G for the perpendicular profile is
The response of the diameter and waviness of collagen, water, and cells, which are the elements considered for the final model, are adapted with polynomial fittings. Figure
Content of each component versus elasticity. Polynomials of degree three were used for diameter and hydration, degree two for waviness, and degree one for cells.
The combination of both results, introducing the temporal evolution in the mechanical contribution and subtracting the initial value given to each constituent, is presented in Figure
Content of each component versus elasticity. The polynomial degree for diameter and hydration is three, for waviness is two, and for cell is one.
The final model is displayed in Figure
Evolutive biomechanical model of the cervix.
Several authors have conducted feasibility studies regarding the use of shear wave elastography on pregnant women. They are based on the SSI technique [
Furthermore, this elastic transformation of the cervix can be assimilated with the remodeling that is experienced in the different phases of pregnancy.
Cervical softening is the first and longest phase that commonly starts in the first month. At this moment it begins a slowly increasing turnover of ECM components, which shows a decline in resistance, associated with unordered collagen fibers, elastic tension, increased water content, and proteoglycans [
Cervical ripening begins a few weeks before delivery and is characterized by rapid changes in the cervix, where integrity is being lost in order to prepare for the delivery. There is a marked increase in the synthesis of proteoglycans, collagen, and hyaluronic acid (HA), a hydrophilic glycosaminoglycan, which interacts with epithelium cells and proteoglycans as versican. HA is in a large molecular weight form that has been said to favor tissue hydration, resulting in a viscoelastic material, which unbalances the structure of the collagen network, dispersing and reducing its mechanical resistance due to increasing collagen solubility [
At term there must be a dilation of the cervix, which is preceded by biochemical changes caused by a new hormonal regulation, contractions of the myometrium, and the tension of the fetal protrusion, triggering the maximal loss of tensile strength. This process involves the infiltration of leukocytes, PGs, proteases, and collagenases into the ECM [
The postpartum phase, the uterine involution, is meant to recover the tensile strength of the tissue, to avoid environmental contamination and to prepare for ensuing pregnancies. An increase in leukocyte cells allows the renewal of tissue functionality, breaking down components causing weakness and disorganization of the ECM through the secretion of the enzyme protease. On the other side, there is an upregulation of synthesis of ECM constituents to return to a nonpregnant state [
This flow explains and validates decisions on both the content of components and their evolution.
This work has certain limitations. The resulting curve was achieved through values from a simulation, which does not necessarily have to be correct. With real values, the curve would be more reliable and optimized. The fit models have been chosen for convenience to simulate the behavior explained, so that some parameters are not clearly defined. The advantage of studying the components individually becomes a constraint as soft tissue remodeling is usually approached from the perspective of mixing theory. There is a mass balance where the constituents make a continuous exchange of percentages of properties. In the work of Myers et al. [
A framework of histobiomechanical properties of the cervical stroma is proposed. A literature review was carried out to explore the contributions of several authors, and then nonordinary developments have been identified. In the case of hydration there is a cyclical tendency, probably related to the feeding of the fetus and eventually when water breaks. GAGs decrease exponentially when delivery arrives. It may be thought that it is due to the increase in mucus plug that somewhat dissolves the polysaccharides of GAGs, allowing a hyperstretching of cervical tissue by removing most of its viscosity. The rest of the constituents follow an increasing trend, either logistically for collagen or exponentially for cells. This was combined with the outputs of a numerical model to build a function that relates gestational age to stiffness. The spatial medium generated is based on increasing the diameter and waviness of collagen fibers along with hydration and cellular content.
Hormones have been identified as the most active elements of pregnancy. The role of oxytocin, PGs, and progesterone as triggers of the whole process should always be kept in mind. More detailed monitoring of these variables can lead to improvements. The combination of cells and the proteolysis of enzyme plays a key role near childbirth. They degrade collagen while trying to maintain the integrity of an increasingly unstable structure. Thus biochemistry is able to control histology.
The viscous part of the model is irrelevant, being more sensitive to modifications in the microstructure than to the ECM content. This has shown that there is a drop in elasticity during pregnancy represented by a function with histological parameters. It can be seen that everything leads to collagen, which is why it has become the most studied component in this study. In a more general view, it is proposed to study the behavior of collagen in specimens in the absence of other constituents.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This research was supported by the Ministry of Education [DPI2017-83859-R, DPI2014-51870-R, DPI2010-17065, and UNGR15-CE-3664], Ministry of Health [DTS15/00093 and PI16/00339, PI-0107-2017, PIN-0030-2017], Junta de Andalucía [P11-CTS-8089], and the European Social Fund (youth employment initiative) projects.