Triggering Labor at Term: The Role of the Fetal Membranes | Article

This chapter should be cited as follows:
Menon R, Glob. libr. women's med.,
ISSN: 1756-2228; DOI 10.3843/GLOWM.419003

The Continuous Textbook of Women’s Medicine Series – Obstetrics Module

Volume 19

Pregnancy shortening: etiology, prediction and prevention

Volume Editors: Professor Arri Coomarasamy, University of Birmingham, UK
Professor Gian Carlo Di Renzo, University of Perugia, Perugia, Italy
Professor Eduardo Fonseca, Federal University of Paraiba, Brazil

Chapter

Triggering Labor at Term: The Role of the Fetal Membranes

First published: February 2024

AUTHORS

Ramkumar Menon, MS, PhD

Division of Basic Science and Translational Research, Department of Obstetrics and Gynecology, The University of Texas Medical Branch at Galveston, Galveston, TX, USA

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INTRODUCTION

The 'Born Too Soon' report on global estimates of preterm birth (birth before 37 weeks' gestation) and actionable items to reduce the preterm birth rate has reported 152 million preterm births that also contributed to 15 million neonatal deaths around the world in the past decade.¹ The staggering number of adverse outcomes of pregnancies and mortality rates in the past decade after the publication of the first report suggest that our knowledge of mechanisms of term and preterm parturition has not advanced, or the knowledge has not yet translated into an actionable practice to make an impact in the field of obstetrics to reduce the risks of preterm births. The lack of progress in perinatal obstetrics can be attributed to the fact that knowledge of the developmental trajectories of normal pregnancy, maintenance of pregnancy, and initiation of parturition at term are still unclear to further understand the mechanisms that can contribute to the initiation of preterm birth. Pregnancy and parturition are two fascinating but perplexing phenomena to understand. Two independent biological and physiological systems co-exist, namely the fetus and the mother, to maintain pregnancy and aid fetal growth and development.² Parturition is a unique physiologic process that reverses all homeostatic states of pregnant uterine tissues in a synchronized way to ensure timely and normal delivery at term.³^,⁴^,⁵^,⁶^,⁷ Reducing PTB risk remains a challenge as this condition may arise with a feto-maternal medical indication for early delivery or could be spontaneous with known or unknown etiology.²^,⁷ Advances made in reproductive biology research has improved our knowledge on feto-maternal uterine organ system and their contribution to pregnancy and parturition at term and preterm.⁸^,⁹^,¹⁰^,¹¹

FETO-MATERNAL UNITS AND INITIATION OF TERM PARTURITION

The harmonious state that maintains pregnancy for almost 40 weeks is mediated between the mother and her fetus by various factors. They include, but are not limited to, endocrine, immune, complement and vascular biological systems, as well as mechanical and immune components.²^,⁶^,¹² Maternal organ systems, such as the cervix, myometrium, and decidua, and fetal organ systems, such as the fetal membranes (amniochorion) and placenta, synchronously contribute to pregnancy well-being and ensure protection from all exogenous and endogenous factors while maintaining homeostasis of various physiologic processes.¹³ During pregnancy multitudes of physiologic processes generate a well-balanced and regulated inflammation, primarily localized to specific feto-maternal organ systems' function, involving changes in oxygen tension generating reactive oxygen species radicals.¹³ Thus, a balanced inflammation and redox system supports feto-placental growth and maintains pregnancy.¹⁴ The gestational period is a ‘lifetime’ for fetal organs where they mature and develop to be ready for an independent life outside of the uterus without maternal support. The longevity of fetal tissues (placenta and fetal membranes) is the gestational period with no life outside of the uterus.⁶^,¹⁵ In contrast, maternal organ systems repair, remodel, and revert to a non-pregnant state for future reproductive purposes.¹³ This suggests that the fate of these organs is vastly different and their physiologic trajectory for ending their roles, leading to parturition is also different.

Towards term, both feto-maternal organ systems tend to transition from their quiescent status that maintains pregnancy to active status to facilitate labor and delivery. However, this transition is not the same in each feto-maternal tissue.¹⁶^,¹⁷^,¹⁸ Inflammation is the underlying mechanism to transition from a quiescent state to an active state or to remain active once that phase is achieved.¹³^,¹⁴ The initiators, activators, and effectors contributing to the inflammatory events transitioning them to a pro-parturition phenotype are different in each feto-maternal system.¹³ The maternal tissues and the placenta are well studied for their contributions during pregnancy and how they prepare for parturition at term. This chapter provides an overview of one of the most understudied intrauterine organs – the fetal membrane or the amniochorionic membranes – and its functional contributions for the maintenance of pregnancy and parturition.

STRUCTURE AND FUNCTIONS OF THE FETAL MEMBRANES

Human fetal membranes, the inner-most tissue layers that form the intrauterine cavity, are fetal in origin and serve as a barricade between the feto-placental and maternal compartments.¹⁹ Fetal membranes are constituted of the amnion (innermost layer of the intraamniotic cavity) and the chorion (fetal tissue connected to maternal decidua) and are connected by the collagen-rich extracellular matrix (ECM).¹⁹^,²⁰ ECM, which is made up of fibrous proteins embedded in a polysaccharide gel and various types of collagen, provides the architectural and structural framework of fetal membranes.²⁰^,²¹ The amnion is constantly bathed in amniotic fluid, signifying its importance as a primary responder to changes in the amniotic cavity. The chorion is in close proximity to maternal decidua and maintains the immune tolerance at the maternal-fetal interface.²²

The amnion and chorion, fetal tissues in origin, function as a single unit and can potentially be conceptualized as an organism that maintains its own homoeostatic balance.¹⁵ This organism plays a major role in maintaining pregnancy by providing multilevel protection to the growing fetus.²³ Fetal membranes accommodate constant challenges (immune, structural, mechanical, and endocrine) during pregnancy, continue to grow, and mechanistically as well as biochemically maintain elasticity to the stretches experienced during fetal growth. Despite the fact that membranes overlying the placenta and cervix face distinctly different environments and insults during pregnancy, they still maintain the homeostatic balance necessary to sustain fetal growth without interruption. This companionship between the fetus and membranes continues until term when the fetus reaches maturity, and the membranes reach longevity. As fetal membrane cells multiply and grow during pregnancy, they undergo a telomere-dependent cellular senescence, resulting in an aging phenotype indicative of its life expectancy. Histologic and biochemical changes associated with senescence and senescence-associated secretory phenotype (SASP), a unique inflammatory signature, have been documented in term fetal membranes at delivery.²⁴^,²⁵ This review describes the development and function of fetal membranes and proposes a novel mechanistic model, wherein aging but viable and inflamed fetal membrane cells, at term, may act to signal fetal maturity and their own dysfunctional status, in turn prompting parturition.

SENESCENCE OF THE FETAL MEMBRANES

Amniotic epithelium and chorionic trophoblast fuse to form the amniochorion membranes or fetal membranes by late first trimester or early second trimester of pregnancy. The two layers are connected to a collagen-rich extracellular matrix by a basement membrane made of type IV collagen. Fetal membranes remodel and grow throughout the pregnancy to accommodate the increasing volume (growing fetus and the amniotic fluid) during pregnancy. Remodeling involves cellular replication, shedding of aged cells and filling the gaps with new cells. Cellular replication can lead to replicative senescence, a mechanism of aging of cells. Fetal membranes develop microfractures, biologic changes in the membranes, during the remodeling process. Microfractures show the amnion epithelial monolayer with regions of cell shedding, localized degradation of the subepithelial basement membrane, fissures or tunnels that extend into the remodeled collagen matrix, and migrating or shed cells can be identified within the matrix tunnels. Examination of cells in the tunnels revealed them as senescent or transitioned, two biological cellular fates that can determine the functional and mechanical integrity of the membranes.

Fetal membrane cellular senescence is telomere (repetitive DNA structures that cap the ends of the chromosome) dependent process. Progressive decrease in the telomere length as gestation progresses is a marker of aging and this decline in fetal membrane telomere length coincides with the growth of the fetus. As the fetal maturation completes at term, intraamniotic oxidative stress and reactive oxygen radicals can build up in the amniotic fluid. These radicals have an affinity for telomere DNA fragments, cause oxidative damage and accelerate telomere shortening. Oxidative stress can also activate various cellular signaling pathways. One is stress signaling mediated by p38 mitogen activated kinase (MAPK). p38MAPK through its intermediary signaling molecules can stop cellular replication to aid aging of the fetal membranes. As mentioned above, aging is associated with a unique inflammatory signature called SASP that are primarily cytokines, chemokines, growth factors, matrix metalloproteinases, and receptors. Senescent cells can persist in a tissue environment and cause a localized inflammatory environment. Senescence can trigger a vicious cycle of events, inducing senescence of fresh cells and increase localized inflammation. Senescent-associated change can also cause damage to various cellular organelles, including the nucleus. These damages to the organelles also produce damage-associated molecular pattern markers (DAMPs) and they are also highly inflammatory molecules.

EPITHELIAL MESENCHYMAL TRANSITION OF AMNION AND CHORION CELLS

Amnion epithelial cells and chorion trophoblast cells express cytokeratin, proteins that form intermediate filaments to provide mechanical and structural support for the cell. However, cells under a specific environment can transition to become mesenchymal cells with the expression of vimentin, a cytoskeletal marker specific for mesenchymal cells. Amnion epithelial cells express both of these markers – a metastate – suggesting their ability to transition. Our recent work has shown that cyclic transition of amnion epithelial cells to mesenchymal cells (EMT) and its reversal [mesenchymal to epithelial cells (MET)] are a cyclic process, and it is required to seal the microfractures as well as the gaps created by epithelial cell shedding. This recycling is aided by progesterone, likely produced locally by the chorion trophoblast cells, and progesterone membrane receptor components (PGRMC) 2. Under oxidative stress at term, PGRMC2 levels are decreased, and it causes a functional progesterone withdrawal in the membranes. Lack of progesterone/PGRMC2 mediated signaling leads to the accumulation of mesenchymal cells in the extracellular matrix of the fetal membranes. This terminal state of mesenchymal cell accumulation is a highly vulnerable state for the fetal membranes. Mesenchymal cells are highly vulnerable to oxidative stress and inflammation and enhance the local inflammatory response.

Chorion trophoblast cells undergo senescence and associated inflammatory changes. However, chorion cells are not vulnerable to EMT. Cytokeratin-18 positive cells are seen at term and do not show vimentin positivity or metastate as seen in amnion epithelial cells. A recent study in our laboratory showed that amnion cells are vulnerable to infection, inflammatory mediators, and other pro EMT inducers. However, chorion is refractory to these factors and maintain their epithelioid morphology even under extreme exogenous environment. This suggests that chorion cells are capable of maintaining their structural integrity to protect the barrier functions with maternal decidua. A compromise to this architecture can cause decidual and maternal immune cell infiltration that can destabilize the membrane.

In summary, these two cellular level changes produce a localized inflammatory response that can enhance inflammatory load of the membrane. Additionally, these inflammatory mediators are capable of increasing the inflammation in other feto-maternal compartments.

PROPAGATION OF INFLAMMATION FROM FETAL MEMBRANES TO THE DECIDUA, MYOMETRIUM, AND CERVIX

Inflammatory mediators produced by cellular disturbances are not restricted to fetal membrane environment. Autocrine signaling can contribute to increasing localized inflammation, causing enhancement of senescence and EMT. This inflammation is not restricted to the fetal membranes but can be propagated to maternal decidua, myometrium, and the cervix. One of the mechanisms of propagation of the inflammatory mediators is via extracellular vesicles (EVs). EVs are small nanoparticles (ranging from 30 nm to >1000 nm). Based on the size of the particles, they can be generally classified as exosomes (30–200 nm), microvesicles (>150–1000 nm and apoptotic bodies >1000 nm). Heterogeneity exists among these particles regarding their size and other properties. EV biogenesis and release are unique for each of these particles and their primary purpose is thought to be the elimination of cellular metabolic waste. However, advancement in the field of EV biology has established a variety of roles for EVs, specifically as communication channels between cells and tissues, establishing them as paracrine signalers. EVs carry a variety of cellular metabolic byproducts, including DNA, RNA, proteins, lipids, and remnants of damaged organelles. EVs can be differentiated based on markers expressed by them besides the size of the particle. Therefore, EVs are considered as a 'snapshot' of the cellular function at the time of their release. The cargo content analysis of EVs can be used as biomarkers indicative of cellular functions.

EV studies, specifically exosomes, is well advanced in the field of perinatal biology. The role of EV as a communication mediator between feto-maternal tissues, biomarker indicative of pregnancy-associated risks and being proposed as drugs or drug delivery vehicles to treat fetal inflammatory response to reduce the risk of preterm birth. The role of exosomes as a communication mediator or paracrine signaler between the feto-maternal compartments will be discussed further in this chapter. Exosomes released from the senescent and EMT cells are very unique in their cargo content and contain SASP and DAMPs. Exosome based studies have identified and reported the following:

Exosomes are released from senescent fetal membrane cells, ~1200/cell/day based on an in vitro cell culture model. They do not differ in size, shape, and exosome markers like CD9, CD63, CD 81, ALIX, HSP and TSG 101.
Exosomes from senescent cells and non-senescent cells can be acquired by maternal uterine and cervical cells and can be localized in in vitro conditions. Senescent cell derived exosomes are capable of inducing an inflammatory response in maternal tissues capable of transitioning them from a quiescent status to an active status as observed during parturition.
Fetal exosomes (based on placental alkaline phosphatase expressing exosomes) can be seen in maternal circulation in the first trimester of pregnancy and the cargo profiles (miRNA and proteins) are different at different gestational ages and at term. This is an indication that fetal exosomes cross the feto-maternal barriers and reach the placenta.
Microvesicles from the fetal tissues also reach the maternal issues and cause inflammatory changes, however, the effect is not as pronounced as exosomes. This is likely due to the cargo contents.

To provide a physiologic validation of our in vitro models, and to determine if the in vitro models can be replicated in in vivo, we have conducted several studies using animal models. The summary of those studies is provided below:

Exosomes can traffic from the amniotic fluid through different layers of the fetal membranes and reach maternal side.
Exosomes isolated from maternal blood samples contain ~15–20% of fetal exosomes. The protein contents of exosomes change during pregnancy and shift from an antiinflammatory to proinflammatory cargo by day 18, a day before expected delivery. Experimental evidence shows that exosomes in maternal on day 18 can cause preterm birth if injected into animals on day 15. This effect is independent of luteolysis (expected mechanisms of progesterone withdrawal in mouse) suggesting that inflammatory cargo containing exosomes can bypass endocrine mediators parturition pathways and can independently cause labor-inducing changes.
Exosomes containing DAMPs (e.g., HMGB1, cell-free fetal DNA, heat-shock proteins) can traverse through feto-maternal tissues and cause inflammatory changes in the recipient maternal uterine cells. These changes can transition a quiescent myometrium and cervix into an active state of labor.
Exosomes from the mother can also reach the fetal side to cause some functional effects. When introduced, exosomal encoded antiinflammatory drugs were able to cross the feto-maternal barriers (fetal membranes and the placenta) and reduce inflammation, delay preterm parturition by minimizing the fetal inflammatory response, which is one of the mechanistic triggers. Thus, exosomes can also function as drugs or vehicles to carry drugs for use during pregnancy.

In summary, exosomes act as paracrine communication mediators between the fetus and the mother, carry fetal signals. At term, these exosomal fetal signals carrying inflammatory mediators from the senescent and transitioned cells are correlated with fetal growth and maturation. Thus, fetal exosomes at term are fetal signals of parturition.

In summary, feto interface tissues undergo changes resembling aging and produce aging associated inflammation. The inflammatory mediators transported across the feto-maternal uterine tissues increases overall inflammatory load in the uterine tissues and can trigger parturition.

PRACTICE RECOMMENDATIONS

Understanding the functional biology of human fetal membranes is essential in deciphering the mechanisms of normal parturition at term and preterm.
Extracellular vesicles (EVs) from fetal membranes, physiologically at term and pathologically at preterm, carry inflammatory cargo that can trigger labor-associated changes in the maternal uterine tissues.
These EVs are fetal signals of parturition and their cargo analysis (miRNA and proteins) can be developed as biomarkers (liquid biopsies) of fetal health during pregnancy.

CONFLICTS OF INTEREST

The author(s) of this chapter declare that they have no interests that conflict with the contents of the chapter.

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