micromachines Review Modelling the Human Placental Interface In Vitro—A Review Marta Cherubini, Scott Erickson and Kristina Haase * European Molecular Biology Laboratory (EMBL), 08003 Barcelona, Spain; [email protected] (M.C.); [email protected] (S.E.) * Correspondence: [email protected]; Tel.: +34-936-282-716 Abstract: Acting as the primary link between mother and fetus, the placenta is involved in regulating nutrient, oxygen, and waste exchange; thus, healthy placental development is crucial for a successful pregnancy. In line with the increasing demands of the fetus, the placenta evolves throughout pregnancy, making it a particularly difficult organ to study. Research into placental development and dysfunction poses a unique scientific challenge due to ethical constraints and the differences in morphology and function that exist between species. Recently, there have been increased efforts towards generating in vitro models of the human placenta. Advancements in the differentiation of human induced pluripotent stem cells (hiPSCs), microfluidics, and bioprinting have each contributed to the development of new models, which can be designed to closely match physiological in vivo conditions. By including relevant placental cell types and control over the microenvironment, these new in vitro models promise to reveal clues to the pathogenesis of placental dysfunction and facilitate drug testing across the maternal-fetal interface. In this minireview, we aim to highlight current in vitro placental models and their applications in the study of disease and discuss future avenues for these in vitro models. Keywords: placenta; maternal-fetal interface; trophoblast invasion; bioprinting; microfluidics; placenta-on-a-chip; in vitro models Citation: Cherubini, M.; Erickson, S.; Haase, K. Modelling the Human Placental Interface In Vitro—A Review. Micromachines 2021, 12, 884. 1. Introduction https://doi.org/10.3390/mi12080884 The human placenta is a crucial organ that supports fetal development throughout gestation. Placental growth and function are precisely regulated to ensure effective circula- Academic Editor: Aleksander Skardal tion of oxygen and nutrients, removal of waste, generation and release of metabolites, and protection against diseases, infections, and xenobiotic transfer to the fetus [1]. Considering Received: 27 June 2021 its vital role, it is essential to understand placental development and the causes of its Accepted: 24 July 2021 dysfunction. However, due to ethical concerns, our understanding of the placenta is largely Published: 27 July 2021 derived from explants at term or from unsuccessful pregnancies. Explants have provided many clues into pathological pregnancies, such as fetal growth restriction, pre-eclampsia, Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in and stillbirth at varied stages of disease [2–4]. However, explants begin to degenerate published maps and institutional affil- within hours after collection, making experimentation with human tissue challenging. iations. Efforts have been made to develop accurate animal models [5]; however, considerable differences between species make it difficult to develop a non-primate animal model that fully mimics human placentation [6,7]. Rodent models are useful for understanding spe- cific aspects of placentation, but many processes are difficult to assess in vivo. Ultimately, bioengineered in vitro models promise to bridge the gap between species and offer precise Copyright: © 2021 by the authors. control over the microenvironment to recapitulate specific aspects of human placentation Licensee MDPI, Basel, Switzerland. This article is an open access article in health and disease [8]. distributed under the terms and This minireview aims to: (1) discuss our current understanding of human placentation conditions of the Creative Commons and highlight areas that require further investigation; (2) discuss current in vitro placental Attribution (CC BY) license (https:// systems, ranging from 2D to 3D models; (3) explore recent applications of these models in creativecommons.org/licenses/by/ studying placental physiology and disorders; and (4) discuss key components to consider 4.0/). when developing or evaluating an in vitro model of the placenta. Micromachines 2021, 12, 884. https://doi.org/10.3390/mi12080884 https://www.mdpi.com/journal/micromachines Micromachines 2021, 12, x FOR PEER REVIEW 2 of 14 Micromachines 2021, 12, 884 2 of 14 models in studying placental physiology and disorders; and (4) discuss key components to consider when developing or evaluating an in vitro model of the placenta. 2. Development Development and and Functions of the Human Placenta The placenta, a fetal organ, forms shortly after fertilization and continues to change throughoutthroughout pregnancy pregnancy in in response to the metabolicmetabolic demandsdemands ofof thethe fetus.fetus. PlacentationPlacentation begins post-fertilization post-fertilization when when the the blastocyst blastocyst attaches attaches to to the the inner inner layer layer (endometrium) (endometrium) of theofthe uterus. uterus. The Theblastocyst blastocyst then begins then begins to inva tode invade the endometrium the endometrium with the with helpthe of its help outer of layerits outer of cells, layer termed of cells, trophoblasts. termed trophoblasts. This fetal trophoblastic This fetal trophoblastic layer is divided layer into is two divided cell types,into two an cellexternal types, multinucleated an external multinucleated syncytiotrophoblast syncytiotrophoblast layer (the invasive layer trophoblasts) (the invasive andtrophoblasts) an inner cytotrophoblast and an inner cytotrophoblast layer. About two layer. weeks About after two fertilization, weeks after the fertilization, external syn- the cytiotrophoblastexternal syncytiotrophoblast forms preliminary forms preliminary fluid-filled fluid-filledvilli structures villi structuresdirected outward, directed outward,towards thetowards decidual the deciduallayer of the layer mother’s of the mother’suterus (Fig uterusure 1a). (Figure Then,1a). the Then, cytotrophoblasts the cytotrophoblasts prolifer- ateproliferate and migrate and migrate through through the syncytiotrophoblasti the syncytiotrophoblasticc layer to layer form to the form primary the primary villi [9]. villi Soon, [9]. theseSoon, thesevilli expand villi expand and andbecome become vascularized vascularized with with fetal fetal placental placental vessels. vessels. Meanwhile, Meanwhile, trophoblaststrophoblasts remodel remodel the the maternal maternal spiral spiral arte arteriesries of of the decidua, which become dilated, allowing for for maternal maternal blood blood to tofill fillthe the intervillous intervillous space. space. As a result, As a result, there is there a large is surface a large areasurface for areathe exchange for the exchange of nutrients of nutrients traveling traveling from the from mother’s the mother’s circulation circulation into the intervil- into the intervillous spaces, through the trophoblast layers, and into the closed placental circulation lous spaces, through the trophoblast layers, and into the closed placental circulation of the of the villi, which nourishes the fetus via the umbilical cord [10,11]. By the second trimester, villi, which nourishes the fetus via the umbilical cord [10,11]. By the second trimester, the the main features of the mature placenta are formed (Figure1b). main features of the mature placenta are formed (Figure 1b). Figure 1. PlacentalPlacental development development timeline, trophoblast in invasion,vasion, and mature placental structure. ( a) Diagram Diagram of trophoblast trophoblast invasion around around day day 9, 9, wherein wherein the the syncytiotrophoblast syncytiotrophoblast laye layerr surrounding surrounding the embryoblast the embryoblast begins begins to invade to invade the endome- the en- trium. (b) Mature placental structure showing maternal and fetal vasculature. (c) Focus on exchange of nutrients between dometrium. (b) Mature placental structure showing maternal and fetal vasculature. (c) Focus on exchange of nutrients open maternal blood and closed fetal circulation across the two trophoblastic layers. between open maternal blood and closed fetal circulation across the two trophoblastic layers. Nutrient exchange exchange between between maternal maternal and and feta fetall blood blood is facilitated is facilitated largely largely by the by syn- the cytiotrophoblast,syncytiotrophoblast, which which is one is one continuous continuous multinucleated multinucleated layer layer of of cells cells (Figure (Figure 11c).c). TheThe ability of nutrients to cross this layer depends on transporter proteins and its thickness, which isis reducedreduced nearnear the the vascularized vascularized parts parts of of the the villi villi [12 ].[12]. Small Small hydrophobic hydrophobic molecules, mole- cules,such as such oxygen as oxygen and carbon and carbon dioxide, dioxide, can easily can diffuse easily acrossdiffuse plasma across membranes plasma membranes in response in responseto differences to differences in the concentration in the concentration gradient between gradient maternal between and maternal fetal blood, and which fetal blood, varies whichwith the varies maternal with the blood maternal supply, blood environment, supply, environment, and rate of blood and rate flow. of Partialblood flow. pressure Partial of oxygen in the maternal blood is considerably higher than in the fetal blood,