Engineering the Human Placenta: Advances in Modeling Placental Function

Abstract

The placenta is a highly specialized organ that mediates nutrient exchange, hormone production, and immune regulation at the maternal-fetal interface. Its dynamic architecture and function throughout gestation present significant challenges in studying placental biology and associated obstetric complications. Emerging developments in bioengineering offer innovative approaches to create physiologically relevant placental models that overcome the constraints of traditional in vitro and in vivo systems. This review highlights progress in microfluidic and organ-on-a-chip systems, spheroid and organoid models, bioengineered scaffolds, and in silico systems. We discuss how these platforms enable the investigation of trophoblast behavior and transport mechanisms and can even mimic the maternal-fetal barrier. By integrating principles of tissue engineering and materials science, advanced in vitro models hold immense promise for advancing our understanding of placental physiology, ultimately informing therapeutic strategies in maternal-fetal medicine.

1. Introduction

Obstetric complications, arising from fetal or maternal health issues, pose a significant threat to both mother and child. Alarmingly, approximately 15% of pregnancies worldwide are impacted by potentially life-threatening complications.^1,2 Still, the field of obstetrics has witnessed a striking lack of pharmaceutical advancements, resulting in a shortage of therapeutic interventions available during pregnancy.³ Developing effective and non-toxic treatments is complicated by the intricate nature of the maternal-fetal interface. Addressing these challenges requires elucidating the complex placental architecture and the nuanced dynamics of the maternal-fetal barrier, which will be critical for advancing innovative therapies.

The placenta, a fetal organ that develops throughout gestation, is integral in the maternal-fetal interface. Throughout gestation, it facilitates blood, oxygen, and nutrient transport to the fetus, with the maternal-fetal barrier primarily composed of the syncytium, extravillous trophoblast, terminal villi, fetal endothelium, and decidua.^4,5 These structures evolve throughout gestation as the placenta matures to support the growing fetus, therefore increasing the difficulty of modeling the placenta. This dynamic nature of placental development further complicates the design and implementation of safe and effective treatments for obstetric complications.

Animal models are commonly used to study organ systems and evaluate therapeutic efficacy. However, they remain insufficient for investigating the human placenta, because it is distinct amongst murine models in structure and development.^4,6 The main distinctions lie in the villous architecture of the human placenta and in the mechanisms governing trophoblast invasion, vascular remodeling, and nutrient exchange, which collectively influence therapeutic pathways and cellular behavior in health and disease. As such, investigating placental development and therapeutic delivery during gestation requires a biomimetic in vitro placental model to safely evaluate placental structure, function, and therapeutic effect. Ex vivo perfusion and explant models, while advantageous for studying native cell behavior, pose limitations such as short experimental viability, dependence on fresh placentae, and donor variability, limiting their use for therapeutic assessment. Identifying and optimizing physiologically relevant models is essential to advance maternal and fetal therapeutic developments.

In this review, we explore in vitro and in silico placental models focused on studying the human placenta and mimicking the maternal-fetal barrier. We highlight engineering-based approaches including microfluidic platforms, spheroids and organoids, bioprinting, perfusion and explant systems, and computational models. Together, these methods offer the potential to capture key features of placental structure and function while also providing tools to evaluate therapeutic safety and efficacy. Advancing placental modeling strategies is essential to address the limitations of animal models and enable clinically relevant insights into maternal and fetal health.

2. Placental Physiology and Trophoblast Cell Types

The placenta is a dynamic fetal organ that embeds into the uterine wall, establishing the maternal-fetal connection through invasion of the decidua, the endometrial layer of the uterus.⁷ At the beginning of formation, it consists of an outer trophoblast (TB) layer, known as the trophectoderm, and an inner embryoblast (EB) layer.⁸ The TBs, or placental progenitor cells, differentiate into two distinct cell types: cytotrophoblasts (CTBs) and the syncytium. Continued CTB proliferation leads to fusion, also known as syncytialization, forming the multinucleated syncytium and initiating the formation of early villous structures within the placenta.⁹ The syncytium is also referred to as the syncytiotrophoblast (STB) layer. However, in this paper, we use 'STB' specifically to describe fused, differentiated CTBs that remain as discrete clusters rather than forming a continuous multinucleated layer as it would appear in vivo. The syncytium is the outermost layer of the placenta and is in contact with maternal blood (Figure 1).

Figure 1:

Placental structural development throughout pregnancy displaying increased fetal vasculature and a decrease in the barrier diffusion distance from 5μm to 5μm for (A) First, (B) Second, and (C) Third trimester placenta. (D) Second trimester placenta and progressively zoomed-in views of the maternal-fetal barrier with labeled cells. (CTB: Cytotrophoblasts; STB: Syncytium; EVT: Extravillous trophoblast)

Formation of the placental villus structures begins as early as days 12–15 of gestation. At this point, CTB proliferation extends the syncytium closer to maternal blood, developing the primary villous structure. As CTB growth continues, they form a layer around the extraembryonic mesoderm, which fills the inner region of the villi, creating secondary villi.¹⁰ The extraembryonic mesoderm then gives rise to fetal arteries and veins within the villi,¹¹ and these capillary-like structures ultimately form the tertiary villous structures which connect to the umbilical cord (Figure 1A). To complete creating the nutrient exchange pathway, the syncytium invades the decidua, forming pockets called lacunae that are filled with maternal blood, establishing the hemochorial nature of the human placenta (Figure 1D).¹² With continued placental invasion by extravillous trophoblasts (EVTs), differentiated from CTBs, the lacunae increase in size, and villi produce increasing branches to accommodate the increasing need for nutrient exchange (Figure 1A–C). ^13–16 This structure composes the maternal-fetal barrier (MFb), the interface where nutrients from maternal circulation are actively transported or diffuse into the villous structures to allow uptake by fetal veins, and fetal waste products are released from the fetal arteries into maternal circulation. To facilitate necessary metabolic and endocrine exchange, the placental diffusion barrier decreases from approximately 50 μm at 8 weeks of gestation to 5 μm at 37 weeks, driven by increased villous branching and reduced thickness of the syncytial layer (Figure 1A–C).^17,18

Given the complex remodeling of villous structures and the continuous adaptations of the syncytial layer, in vitro models provide a necessary platform to study placental development and function under controlled conditions. However, maintaining placental cells at specific gestational ages in vitro presents challenges as their phenotype changes rapidly when cultured ex vivo. To address this, researchers have adapted various placental cell lines that preserve key functional TB characteristics (Table 1). Cells commonly used for in vitro placental models include BeWo, JEG-3, JAR, and HTR8/SVneo cells. BeWo, JEG-3, and JAR cells are derived from choriocarcinomas and exhibit TB invasion phenotypes, similar to EVTs.^19,20 BeWo cells can model syncytialization, whereas JAR and JEG-3 cells model TB invasion.^21,22 In contrast, HTR8/SVneo cells that are immortalized from first-trimester chorionic villi explants, provide a more biologically representative model of TB adhesion, invasion, migration, and proliferation.^23,24

Table 1.

Trophoblast cell types and common modeling uses.

Cell Type	Source	Key Properties	Common Uses in Placental Models	Ref
BeWo	Choriocarcinoma cell line	Can undergo syncytialization and expresses TB markers.	Modeling syncytialization; studying placental hormone production; drug transport studies	19–22
JEG-3	Choriocarcinoma cell line	Invasive phenotype and produces placental hormones.	Studying TB invasion; tumor-like TB behavior; placental transport	19–22
JAR	Choriocarcinoma cell line	Invasive TB phenotype and lacks the capacity for syncytialization.	Modeling TB invasion and migration; TB signaling studies	19–21
HTR8/SVneo	Immortalized first-trimester EVT from chorionic villi explants	Models TB adhesion, invasion, migration, and proliferation. Physiologically representative of early pregnancy TB behavior.	Early placental development studies; implantation and invasion models; maternal–fetal interface research	23,24
Primary Human TB	Isolated from human placental villous tissue	Represents in vivo TB phenotypes and include CTBs.	Studying healthy placental physiology, hormone secretion, and transporter activity	26–29

Although these cell lines facilitate placental research, additional signaling molecules are often required to replicate in vivo syncytialization, a crucial component of studying the MFb. Forskolin, a biochemical inducer, is most commonly used, but biomechanical cues such as equibiaxial compressive stress have also been shown to promote syncytialization.^25–27 For shorter term in vitro studies, primary human TBs are also used to better mimic and modulate placental cell properties.^28–31 The cells are obtained from third trimester placental tissue, providing another model for studying placental development and function. Collectively, these cells provide a foundation for developing advanced, biomimetic platforms to study the human placenta, specifically the syncytium and TB behavior.

3. Modeling Placental Function and Barrier Characteristics

Traditional in vitro and in vivo models provide valuable insights but remain limited in their ability to reproduce the structural and functional complexity of the human placenta. Most animal models differ markedly from humans in terms of trophoblast arrangement, barrier composition, and transport mechanisms, restricting their relevance for human-specific studies.^32–34 For example, in rodents, three distinct trophoblast layers separate maternal blood from fetal circulation, whereas in human term placenta only the syncytium forms this barrier.³³ These species-specific differences highlight the need for engineered models that more accurately recapitulate human placental physiology. To overcome these challenges, researchers have increasingly turned to engineering-based strategies to model the MFb (Table 2). Techniques such as microfluidics, bioprinting, and organoid cultures enable precise control of placental architecture, cellular interactions, and microenvironmental cues, providing physiologically relevant platforms for studying placental development and function.

Table 2:

Placental modeling methods and their considerations.

Model	Description	Applications	Advantages	Limitations	Cells	Ref
Microfluidic	Microchannels with precisely manipulated flow and cell adhesion properties.	- Transport studies - Barrier function - Drug delivery - Toxicity evaluation	- Dynamic environment - Supports co-cultures - Integration of barriers and ECM components	- Cleanroom fabrication	BeWo, JEG-3, HPVECs, HUVECs, Primary TBs	34–49
Spheroid	3D aggregated cells.	- TB invasion - MFb modeling	- Increased cell-cell communication	- Decreased nutrient diffusion	BeWo, JEG-3, primary TBs	37, 43, 52–56
Organoid	Self-organizing 3D cell co-culture system.	- Placental disease modeling - Drug testing	- Captures structure and function - Complex cell interactions	- Hard to control cell-cell interactions within the co-culture	TB stem cells, primary CTB	27–29, 58–60
Bioprinting	3D printed scaffolds integrated with cells.	- EVT migration - Invasion of uterine tissue - Barrier reconstruction	- Customizable geometry - Organized multicellular integration - Reproducible	- Bioink material properties range	TBs, endothelial cells, stromal cells	67–70
In Silico	Computational models analyzing tissue derived data	- Image analysis - Outcome prediction	- Non invasive - Scalability and efficiency	- Requires robust datasets - Limited early gestation data	N/A (analyzes existing biological datasets)	71–74

3.1. Microfluidic Platforms

Microfluidic platforms have been widely explored for various organ-on-a-chip models. These systems are particularly appealing due to their ability to support cellular co-cultures, integrate tunable polymer material barriers, and simulate dynamic flow conditions without disrupting the system. The inclusion of flow is particularly valuable, as it enables the application of controlled shear stress to cells, facilitating the creation of biomimetic models.

Semi-permeable polymeric barriers can be added to these modeling systems to maintain the separation between cells while culturing, while also allowing fluid flow amongst the cells at later stages (Figure 2). This technique permits the addition of an extracellular matrix (ECM)-mimicking layer amongst cells in a microfluidic chip. For instance, Lee et. al. and Blundell et. al. employed this system to allow isolated cell seeding, and later, facilitated cross-talk between channels.^35–37 The porous nature of the membrane allowed cellular attachment at the surface but prevented attachment within the pores. This attachment type prevented channel blockage and permitted flow across channels after the cells had adhered. The group first studied BeWo and human primary placental villous endothelial cells (HPVECs) using a fibronectin polycarbonate membrane, demonstrating a successful model with the uptake of glucose across the MFb.³⁶ In a later study, the group evaluated cell-based limitations of the model by using JEG-3 and human umbilical vein endothelial cells (HUVECs), yielding similar results.^35,36,38

Figure 2:

Schematic depicting placenta modeling platforms (In silico, Spheroids and Organoids, Microfluidic Platforms, Bioprinting). In silico modeling utilizes physiological data and placental structure captured from placental tissue to gather data on transport mechanisms and shear stress forces on the villus structures. Spheroids and organoids are formed using one or multiple cell types, respectively, using spinner flasks, round-bottom well plates, or a hanging drop technique. Microfluidic platforms are designed integrating cell layers to model cell layers found in vivo. Bioprinting techniques that can be employed with multiple bioprinter models are used to incorporate cells in hydrogel platforms.

Limitations in commercially available placental cells, particularly their biological relevance and ability to recapitulate in vivo-like characteristics, are difficult to overcome. However, the use of microfluidic devices helps mitigate these limitations with controlled flow and chemical stimulus, using forskolin to induce syncytialization in TBs.^39–42 Blundell et al. validated the biological relevance of their model by successfully replicating breast cancer protein mediated drug transport across the MFb, which is shown to be abundant in the placenta and limits transport.^37,43 This research on placenta-on-a-chip development highlights the ability to use microfluidic platforms to model drug retention and permeation through natural pathways. Similar microfluidic platforms have also been used to evaluate the embryotoxicity of microparticles using isolated embryoid bodies, common in toxicity studies, to represent the fetal component of the placenta as it interfaces with maternal blood.^44,45 Furthermore, the model facilitates further study of diseased placenta models such as the preeclampsia-associated hypoxia.⁴⁶

Microfluidic systems can also be used to apply physical stimuli to cells cultured in channels. Abostait et al. utilized this capability to investigate the effects of dynamic flow on TB syncytialization, microvilli formation, and the uptake of liposomes using a single-channel microfluidic system focused on modeling the syncytium.⁴¹ The group used a flow rate at 22.89∞L/min to produce biologically relevant shear stress of 0.025 dyne/cm² which has been reported as approximately between 0.001–0.1dyn/ cm².^47–49 Interestingly, the group observed an increase in syncytialization under dynamic flow conditions using the same concentration of forskolin. Additionally, control samples with no forskolin treatment in the dynamic flow systems also exhibited an increase in syncytialization compared to static control and treatment groups. These results suggest fluid shear stress enhances syncytialization, potentially representing an alternative or complementary technique to induce fusion. The study also demonstrated significantly greater uptake of liposomes in STBs under dynamic conditions. This might suggest that shear stress-induced syncytialization is more representative of biological conditions and better suited to mimic in vivo drug delivery conditions.

Microvilli formation monitored in microfluidic studies is an essential component of the placental structure as microvilli form on the apical surface of the syncytium to increase surface area and promote nutrient transport.^4,50,51 Although in previously discussed studies an increase in microvilli formation was observed with an increase in syncytialization, cell fusion might not be the key contributing factor. Fluid shear stress also impacts microvilli formation through the TRPV6 Ca²⁺ channel.⁴² No chemical stimulants, such as forskolin, were used in the study by Miura et al.; nonetheless, they observed an increase in microvilli with increased fluid shear stress. Moreover, when the TPRV6 channel was silenced, there was a significant decrease in Ca²⁺ influx and, as a result, a decrease in microvilli formation. However, it is unclear if the BeWo cells used here were fused to form multinucleated STBs which would present a more representative platform to monitor microvilli formation. These findings highlight the importance of considering both mechanical and biochemical cues when designing in vitro placental models.

To build upon existing placenta models for improved drug delivery testing in pregnant women, Schuller et al. modified these microfluidic platforms by integrating interdigitated impedance biosensors within the porous membrane representing the MFb.⁵² The biosensors measure change in the current that occurs when particles flow through the sensor.⁵³ The biosensors were employed to assess placental barrier integrity in the presence of materials commonly used to fabricate nanoparticles (silicon dioxide, titanium dioxide, and zinc oxide), minimizing the need for destructive techniques such as immunolabeling that require cell fixation and permeabilization. The nanoparticles evaluated in this study, consistent with typical drug delivery particles, carry no net charge. Therefore, if the particles are able impede the current through the system, the particle would have degraded and would result in a cytotoxic effect on cells at the other end of the sensor.⁵⁴ Importantly, this system provides continuous, real-time measurements of barrier function and nanoparticle effects, offering a more dynamic assessment compared to endpoint methods such as immunolabeling, TEER, or MTT assays.^52,53

Notably, only one TB cell type was used in these studies to model the maternal and fetal components of the placenta. In vivo, there are multiple cells interacting to create and support the MFb, so the system does not fully replicate the barriers a molecule would experience in vivo. Furthermore, while microfluidics can support a placenta model, they can be difficult to employ outside of academic engineering environments due to the need for a cleanroom in the fabrication process.

3.2. Spheroid and Organoid Models

Spheroids and organoids are another common method used to create placental models with TB and STBs. Spheroids are 3D cell aggregates often composed of one cell type, while organoids form a more complex system involving primary or stem cells to form an organ model through differentiation.³⁸ Transcriptomic analysis comparing EVT spheroid and 2D culture models show significant differences in gene expression implicated in the epithelial-mesenchymal transition, cell-cell contact, tight junction formation, invasion, and angiogenesis.⁵⁵ The upregulation of genes associated with these processes in the spheroid model, compared to 2D cultures, reflects in vivo EVT characteristics and suggests that 3D culture methods can generate more biologically relevant in vitro placental models. 3D culture techniques allow cells to interact with ECM material from cells or scaffolds rather than synthetic polymers commonly used in 2D cell culture, like tissue culture polystyrene, to create a more in vivo-like environment.⁵⁶

There are several spheroid fabrication techniques: hanging drop, spinner flasks, and culture on non-adherent surfaces (Figure 2).^56–60 Each of these methods relies on cells attaching to one another rather than a surface. The hanging drop method is when cells in media are dispensed on a petri dish lid that is then inverted before incubation.⁵⁸ In this method, as the cells hang from the lid in a droplet, they adhere to one another to form a spheroid. This method was employed by Boos et al. to form embryonic bodies on a microfluidic platform used to evaluate toxicity across the MFb, proposing a unique combination of two modeling techniques.⁴⁴ Spinner flasks are used to maintain cells in suspension and agitate them during incubation to form cellular aggregates.⁶⁰ This method of spheroid formation is less reliable, producing spheroids of varying sizes and shapes. Therefore, it is not commonly used for generating placental spheroids. Culturing on non-adherent surfaces, such as untreated polystyrene, is often the preferred technique, enabling high throughput with the use of 96-well round-bottom plates.^57,61 Additionally, this method of fabrication produces spheroids with more consistent size and shape compared to the hanging drop method where the droplet size is easily variable. Microfabricated non-adherent surfaces have been employed to generate spheroids of defined sizes.⁵⁹

Specific nutrients are often used to maintain the proliferative capacity of cells and regulate cell differentiation in spheroids. For instance, decorin, a proteoglycan found in the ECM of connective tissues, can be used to regulate cell renewal and differentiation. Specifically, decorin has been used to regulate HTR8/SVneo and primary TBs during spheroid formation using the 96-well round bottom plate.⁵⁷ The addition of decorin during spheroid formation prevented differentiation to STBs and EVTs, helping maintain a TB population. This could be beneficial in spheroid formation to expand the TB population before promoting differentiation. Specifically, culturing the cells as spheroids but preventing differentiation allowed TB proliferation where in 2D settings the cells differentiate instead of proliferating.⁵⁷

Similar to spheroids, organoids are generated in low-adhesion environments, though they also contain multiple cell types. Primary TBs isolated from first trimester placental tissue are commonly used for organoid fabrication.^28,29 The cells are encouraged to form organoids during cell culture by utilizing growth factors and Matrigel, a solubilized basement membrane matrix, to promote the intended differentiation and provide the physical support required. Sheridan et al. documented the organoid fabrication procedure including necessary media components and growth factors leading to successful TB organoid formation and maintenance for over a year.³⁰

While Matrigel is used to provide structural support for cells, since it is naturally derived there can be significant batch to batch variability leading to inconsistent results. To observe the benefits of a more tunable scaffold, Slaby et al. developed a synthetic polyethylene glycol (PEG)-based hydrogel.⁶² This system demonstrated significant improvement in TB viability, proliferation, and differentiation compared to Matrigel systems. The main contributors to the increased success of this system were the tunable degradation and cell adhesion rates.

Additionally, ECM-derived ligands (RGD peptide sequences that serve as cell adhesion motifs) were covalently conjugated to the PEG hydrogel backbone at defined concentrations to promote and modulate cell adhesion.⁶² While Slaby et al. used the scaffold to regulate spheroid growth in JEG-3, JAR, and BeWo cells, its application may be even more impactful for organoid formation using primary TBs, which require a well-defined environment to sustain proliferation. Following this approach, a PEG-based hydrogel was designed for use in organoid formation to model endometrial epithelial and stromal cell interactions.⁶³ The hydrogel supported the co-culture by mimicking the in vivo endometrial environment. As such, it would likely prove beneficial as a scaffold for TBs as well, specifically EVTs and STBs since they interact with the uterine wall.

With the continued development of organoids, there is an increased ability to model the placental barrier and even specific TBs. Differentiation of primary TBs produces the multiple cells present throughout the placenta; however, it is the organization of these differentiated cells that allows organoids to be representative models of the placenta. Hori et al. created an organoid model with a nearly complete outer layer of syncytium using TB stem cells.³¹ A key component of this model was the multiple culture media they treated cells with to promote proliferation and differentiation as the cells organize. Due to the order of the differentiated TBs allowing the STB on the outer-most layer, this model can be used to observe therapeutic transport across the MFb and barrier integrity in disease conditions. Additionally, organoid models can be used to observe specific cell structures within the placenta. Dietrich et al. established JEG-3 organoids by removing a WNT pathway activator resulting in EVT differentiation.⁶⁴ However, when they treated the organoid with nutrients used in stem cell spheroids, it formed organoids similar to CTB organoids with a lower proliferative rate.²⁸ This cell-specific model presents the ability to study the structure of EVTs, and if a similar technique was applied to BeWo cells, it might allow the specific visualization of STB formation without the use of forskolin.

A common result of spheroid and organoid models is fusion of TBs in the core of the model rather than on the surface of the cells. While Hori et. al. were able to overcome this effect and observe a nearly complete surface syncytium, it is not a commonly observed result. Furthermore, the process of controlled cell proliferation and differentiation to allow this result required many different nutrients and would make this system of MFb modeling either expensive or low throughput. As a result, it can be difficult to observe MFb properties in spheroids and organoid models but they can be useful for observing how compact cell interactions impact transcriptomics, proteomics, and cell morphology.

3.3. Bioprinted Models

Bioprinting has emerged as a transformative approach to address limitations in mimicking the complex 3D structure and cellular interactions of the placenta found in vivo. By combining biomaterials, microfabrication, and precise cell placement, placental microarchitecture and physiology can be recapitulated. Recent tissue engineering strategies in combination with bioprinting, particularly those leveraging hydrogel scaffolds and biomimetic substrates, offer promising alternatives for replicating the structural and functional characteristics of the maternal-fetal interface.

Bioprinting utilizes 3D printing with bioink, a composite of living cells and a polymer material (Figure 2). For placental modeling, the use of bioprinting allows cell layering to observe cell migration and mimic the in vivo proliferative and fusion properties of the cells. Bioprinting has also been used to fabricate a barrier model of the placenta focusing on recreating the syncytium.⁶⁵ In this model, Kreuder et. al. recreated the barrier cell layers found in the MFb using methacrylated gelatin (GelMA) as the base biomaterial, focusing on the cell layers rather than their individual structures. This allowed the group to include fibroblasts within the inner portion of the barrier structure, separating fetal and maternal circulation. While an innovative approach, placental fibroblasts are largely found in the stroma surrounding villus structures rather than on the exterior of the syncytium.⁶⁶ Therefore, this may have created an additional barrier between fetal and maternal circulation that is not common in vivo. Regardless, with bioprinting Kreuder et al. were able to create cell layers that maintained more than 85% cell viability for up to 28 days.

The polymer-integrated cell layers fabricated through bioprinting are commonly used in placental engineering to model and study the migration and invasion behavior of EVT cells. By using epidermal growth factor (EGF) as a chemoattractant, Ding et al. induced TB invasion that could be representative of EVT behavior as they invade the decidua during gestation.⁶⁷ They developed two significant models: a multi-ring and multi-strip model. In both cases, a layer of cell-free GelMA was used as barrier from between EGF and the cells. Additionally, in the multi-strip model, cell-free GelMA was added to both ends of the cell layer allowing a clear differentiation between cell migration and invasion. Importantly, although the group was able to recapitulate TB cell behavior, the stiffness of GelMA chosen was not representative of decidual tissue. These models can also be used to observe cell behavior in disease states such as preeclampsia by bioprinting structures implicated in the disease, specifically maternal spiral arteries.⁶⁸ While Kuo et. al. had success in observing TB migration, the stiffness of the GelMA used in the study was not effective for Ding et. al. This may be attributed to the different geometries the groups created and may be a consideration for future research using bioprinting.

By incorporating ECM scaffolds, physiologically relevant cell types, and modular designs, bioprinted systems are beginning to replicate the dynamic function of the placenta. As these approaches mature, they hold potential to complement other tissue engineering based modeling techniques such as microfluidic platforms. The integration could allow bioprinted systems to have increased biological significance by fabricating in vivo-like structures and including in vivo-like mechanical stimuli.

3.4. In Silico Models

Developments in in silico models across research areas have led to investigations of computational modeling to better understand placental development and inform treatments for various diseases. However, observing the key placental functions is difficult during pregnancy due to the low resolution of pregnancy-safe imaging, and can only be studied in animal models with limited relevance due to the species-specific nature of the placenta. In silico modeling using machine learning and computational techniques allows analysis of placental development as it impacts function using data collected from tissue samples across gestation (Figure 2).^69–71

First, the structure and cellular organization in biological samples is analyzed to create the assembly for modeling by using software like 3DSlicer or CMISS (Continuum Mechanics, Image analysis, Signal processing and System Identification) to identify capillaries and terminal villi.^70,72–75 With this model, researchers can conduct computational simulations using software such as COMSOL to determine how different geometries within the placenta impact oxygenation, blood flow, and transport kinetics, thereby informing the fabrication of in vitro models and the development of treatments.⁶⁹ These simulations can be used to investigate mechanisms driving placental disease states such as vascular structure and shear stress progression during gestation.⁷⁶

Although in silico modeling is beneficial for replicating systems with specific geometries and allows inclusion of many replicates, there are limitations to this method. The systems are designed based on known placental properties, but this knowledge remains incomplete. For instance, in these models, the TB epithelium, basement membrane, connective tissues, and capillary endothelium were modeled as one feature due to the lack of information about their material properties.⁷⁰ Nonetheless, in silico modeling allows study of the placental function and transport mechanisms without continuous tissue harvesting, which is difficult to achieve with early term placentae. Additionally, these models can guide the design of in vitro systems to improve physiological relevance.

4. Conclusion

Engineering-based strategies have significantly advanced our ability to model the placenta, providing tools that range from 2D culture systems to 3D organ-on-a-chip platforms. While 2D models remain valuable for their accessibility, they are insufficient in recapitulating the biomechanical complexity of the maternal-fetal interface. Ex vivo models, although closer to the native tissue, are limited by variability and accessibility, underscoring the need for reproducible engineered systems. In contrast, 3D models offer more physiologically relevant environments, capturing dynamic features such as shear stress, nutrient exchange, and cellular interactions. Additionally, in silico models quantify placental architecture using harvested tissue, improving predictive modeling and informing 3D in vitro systems. Ultimately, the convergence of the various modeling strategies to engineer a more representative human placental model has the potential to accelerate the development of novel therapeutic interventions and improve maternal and fetal health outcomes.

Abbreviations:

TB

trophoblast

EB

embryoblast

CTB

cytotrophoblast

STB

syncytiotrophoblast

MFb

Maternal-fetal barrier

EVT

extravillous trophoblast

ECM

extracellular matrix

HPVEC

human primary placental villous endothelial cell

HUVEC

human umbilical vein endothelial cell

PEG

polyethylene glycol

GelMA

methacrylated gelatin

EGF

epidermal growth factor