Deportation of trophoblastic emboli to maternal lung: A source of cell-free DNA in maternal blood?

Cell dynamics in human villous trophoblast

BACKGROUND

Villous cytotrophoblast (vCTB) is a precursor cell population that supports the development of syncytiotrophoblast (vSTB), the high surface area barrier epithelium of the placental villus, and the primary interface between maternal and fetal tissue. In light of increasing evidence that the placenta can adapt to changing maternal environments or, under stress, can trigger maternal disease, we consider what properties of these cells empower them to exert a controlling influence on pregnancy progression and outcome.

OBJECTIVE AND RATIONALE

How are cytotrophoblast proliferation and differentiation regulated in the human placental villus to allow for the increasing demands of the fetal and environmental challenges and stresses that may arise during pregnancy?

SEARCH METHODS

PubMed was interrogated using relevant keywords and word roots combining trophoblast, villus/villous, syncytio/syncytium, placenta, stem, transcription factor (and the individual genes), signalling, apoptosis, autophagy (and the respective genes) from 1960 to the present. Since removal of trophoblast from its tissue environment is known to fundamentally change cell growth and differentiation kinetics, research that relied exclusively on cell culture has not been the main focus of this review, though it is mentioned where appropriate. Work on non-human placenta is not systematically covered, though mention is made where relevant hypotheses have emerged.

OUTCOMES

The synthesis of data from the literature has led to a new hypothesis for vCTB dynamics. We propose that a reversible transition can occur from a reserve population in G0 to a mitotically active state. Cells from the in-cycle population can then differentiate irreversibly to intermediate cells that leave the cycle and turn on genes that confer the capacity to fuse with the overlying vSTB as well as other functions associated with syncytial barrier and transport function. We speculate that alterations in the rate of entry to the cell cycle, or return of cells in the mitotic fraction to G0, can occur in response to environmental challenge. We also review evidence on the life cycle of trophoblast from the time that fusion occurs, and point to gaps in knowledge of how large quantities of fetal DNA arrive in maternal circulation. We critique historical methodology and make a case for research to re-address questions about trophoblast lifecycle and dynamics in normal pregnancy and the common diseases of pre-eclampsia and fetal growth restriction, where altered trophoblast kinetics have long been postulated.

WIDER IMPLICATIONS

The hypothesis requires experimental testing, moving research away from currently accepted methodology towards a new standard that includes representative cell and tissue sampling, assessment of cell cycle and differentiation parameters, and robust classification of cell subpopulations in villous trophoblast, with due attention to gestational age, maternal and fetal phenotype, disease and outcome.

The endogenous exposome of the pregnant mother: Placental extracellular vesicles and their effect on the maternal system

During pregnancy, there is an intense crosstalk between mother and placenta. During the entire time of pregnancy, the maternal system deals with a huge amount of foreign (fetal) material released from the placenta, which can be referred to as placental exposome. Besides the release of hormones and growth factors, the placenta releases a variety of extracellular vesicles into maternal blood. These vesicles contain specific molecules including proteins, lipids, DNA as well as miRNA, all of which may have specific sites and modes of action on maternal cells. During normal pregnancy, the fine-tuning of factors and vesicles helps maintaining a viable and healthy pregnancy. However, in pregnancy pathologies such as preeclampsia, quantity and quality of the placenta-derived vesicles are altered leading to a deleterious effect on the maternal vascular system. This review focuses on the different types of placenta-derived extracellular vesicles in pregnancy with special emphasis on the interplay between these placental vesicles and the maternal system. Additionally, it displays new techniques and ideas for the analysis of the placental exposome with placental extracellular vesicles as a key aspect.

[Deported syncytiotrophoblast and placental microparticles in the mother's body during normal pregnancy and preeclampsia (28 years later)]

Over the last 25 years, there has been new evidence for the need to systematize deported placental cells, by identifying 3 groups according to their size, blocking or passing the pulmonary capillaries. In group 1, deported syncytiotrophoblast is a viable multinucleated complexes 100 to 20 µm in diameter. Their common cytoplasm displays β-hCG immunoexpression. After apoptosis of these cells in the lung capillaries, placental bioproducts directly interact with endothelial cell receptors, by contributing to the gestational rearrangement of the woman's body. In Group 2, placental microparticles are necrotized parts of syncytiotrophoblast microvilli and organelles 1000 to 100 nm in size: they freely pass the lung capillaries, activate the production of proinflammatory cytokines by maternal macrophages, and cause a systemic inflammatory response. In excess they become triggers of extensive endotheliosis and vasospasm. In Group 3, placental nanoparticles (or exosomes) are the smallest structures 120 to 20 nm in size; their numbers in the blood are increased in preeclampsia. The given materials call for further investigation of deported placental cells by standardized studies.

Trophoblast deportation to the lungs of cotton rats (Sigmodon hispidus)

Cotton rats (Sigmodon hispidus) have been used to study a variety of infectious agents, particularly human respiratory viral pathogens. During the course of comprehensive pathologic evaluations of aging breeders from our breeding colony, 6 of 22 (27%) female cotton rats had histologic evidence, limited to the lungs, of embolized cells that were confirmed to be trophoblastic in origin by HSD3B1 immunoreactivity. When pulmonary trophoblast emboli were numerous, they usually were associated with additional histologic findings in the lungs, including pulmonary edema and hemorrhage, endothelial hypertrophy, fibrinoid vascular necrosis, and abundant alveolar macrophages containing fresh fibrin and hemolyzing erythrocytes. Of the 6 cotton rats with pulmonary trophoblast emboli, 5 (83%) were at 8 to 18 d of the 27-d gestation period, with the greatest number of emboli per lung present between days 10 through 14. The remaining cotton rat had a focal pulmonary trophoblast embolus and was not pregnant but had delivered a litter 3 mo previously. Three other cotton rats in either the early or late stages of gestation showed no histologic evidence of pulmonary trophoblast deportation. This report is the first to document pulmonary trophoblast emboli in cotton rats. This finding suggests that cotton rats may be an alternative animal model for the study of normal and aberrant trophoblast deportation in routine pregnancies and gestational pathologic conditions in women.