Development of an in vitro human placenta model by the cultivation of human trophoblasts in a fiber-based bioreactor system

Beyond 2D: Novel biomaterial approaches for modeling the placenta

This review considers fully three-dimensional biomaterial environments of varying complexity as these pertain to research on the placenta. The developments in placental cell sources are first considered, along with the corresponding maternal cells with which the trophoblast interact. We consider biomaterial sources, including hybrid and composite biomaterials. Properties and characterization of biomaterials are discussed in the context of material design for specific placental applications. The development of increasingly complicated three-dimensional structures includes examples of advanced fabrication methods such as microfluidic device fabrication and 3D bioprinting, as utilized in a placenta context. The review finishes with a discussion of the potential for in vitro, three-dimensional placenta research to address health disparities and sexual dimorphism, especially in light of the exciting recent changes in the regulatory environment for in vitro devices.

Human Maternal-Fetal Interface Cellular Models to Assess Antiviral Drug Toxicity during Pregnancy

Pregnancy is a period of elevated risk for viral disease severity, resulting in serious health consequences for both the mother and the fetus; yet antiviral drugs lack comprehensive safety and efficacy data for use among pregnant women. In fact, pregnant women are systematically excluded from therapeutic clinical trials to prevent potential fetal harm. Current FDA-recommended reproductive toxicity assessments are studied using small animals which often do not accurately predict the human toxicological profiles of drug candidates. Here, we review the potential of human maternal-fetal interface cellular models in reproductive toxicity assessment of antiviral drugs. We specifically focus on the 2- and 3-dimensional maternal placental models of different gestational stages and those of fetal embryogenesis and organ development. Screening of drug candidates in physiologically relevant human maternal-fetal cellular models will be beneficial to prioritize selection of safe antiviral therapeutics for clinical trials in pregnant women.

Modeling of Artificial 3D Human Placenta

The placenta is the main organ that allows the fertilized oocyte to develop and mature. It allows the fetus to grow in the prenatal period by transferring oxygen and nutrients between the mother and the fetus. It acts as a basic endocrine organ which creates the physiological changes related to pregnancy and birth in the mother. Removal of wastes and carbon dioxide from the fetus is also achieved by the placenta. It prevents the rejection of the fetus and protects the fetus from harmful effects. Research on the human placenta focuses on understanding the placental structure and function to illuminate the complex structure of this important organ with technological advances. The structure and function of the placental barrier have been investigated with in vitro studies in 2D/3D, and various results have been published comparatively. In this review, we introduce the nature of the placenta with its 3D composition which has been called niche. Different cell types and placental structures are presented. We describe the systems and approaches used in the creation of current 3D placenta, placental transfer models as 3D placental barriers, and micro-engineered 3D placenta on-a-chip to explore complicated placental responses to nanoparticle exposure.

Bioengineering Approaches for Placental Research

Research into the human placenta's complex functioning is complicated by a lack of suitable physiological in vivo models. Two complementary approaches have emerged recently to address these gaps in understanding, computational in silico techniques, including multi-scale modeling of placental blood flow and oxygen transport, and cellular in vitro approaches, including organoids, tissue engineering, and organ-on-a-chip models. Following a brief introduction to the placenta's structure and function and its influence on the substantial clinical problem of preterm birth, these different bioengineering approaches are reviewed. The cellular techniques allow for investigation of early first-trimester implantation and placental development, including critical biological processes such as trophoblast invasion and trophoblast fusion, that are otherwise very difficult to study. Similarly, computational models of the placenta and the pregnant pelvis at later-term gestation allow for investigations relevant to complications that occur when the placenta has fully developed. To fully understand clinical conditions associated with the placenta, including those with roots in early processes but that only manifest clinically at full-term, a holistic approach to the study of this fascinating, temporary but critical organ is required.

Microphysiological systems of the placental barrier

Methods to evaluate maternal-fetal transport across the placental barrier have generally involved clinical observations after-the-fact, ex vivo perfused placenta studies, or in vitro Transwell assays. Given the ethical and technical limitations in these approaches, and the drive to understand fetal development through the lens of transport-induced injury, such as with the examples of thalidomide and Zika Virus, efforts to develop novel approaches to study these phenomena have expanded in recent years. Notably, within the past 10 years, placental barrier models have been developed using hydrogel, bioreactor, organ-on-a-chip, and bioprinting approaches. In this review, we discuss the biology of the placental barrier and endeavors to recapitulate this barrier in vitro using these approaches. We also provide analysis of current limitations to drug discovery in this context, and end with a future outlook.

Placenta-on-a-Chip: In Vitro Study of Caffeine Transport across Placental Barrier Using Liquid Chromatography Mass Spectrometry

Due to the particular structure and functionality of the placenta, most current human placenta drug testing methods are limited to animal models, conventional cell testing, and cohort/controlled testing. Previous studies have produced inconsistent results due to physiological differences between humans and animals and limited availability of human and/or animal models for controlled testing. To overcome these challenges, a placenta-on-a-chip system is developed for studying the exchange of substances to and from the placenta. Caffeine transport across the placental barrier is studied because caffeine is a xenobiotic widely consumed on a daily basis. Since a fetus does not carry the enzymes that inactivate caffeine, when it crosses a placental barrier, high caffeine intake may harm the fetus, so it is important to quantify the rate of caffeine transport across the placenta. In this study, a caffeine concentration of 0.25 mg mL-1 is introduced into the maternal channel, and the resulting changes are observed over a span of 7.5 h. A steady caffeine concentration of 0.1513 mg mL-1 is reached on the maternal side after 6.5 h, and a 0.0033 mg mL-1 concentration on the fetal side is achieved after 5 h.

Discovery of HeLa Cell Contamination in HES Cells: Call for Cell Line Authentication in Reproductive Biology Research

Continuous cell lines are used frequently in reproductive biology research to study problems in early pregnancy events and parturition. It has been recognized for 50 years that many mammalian cell lines contain inter- or intraspecies contaminations with other cells. However, most investigators do not routinely test their culture systems for cross-contamination. The most frequent contributor to cross-contamination of cell lines is the HeLa cell isolated from an aggressive cervical adenocarcinoma. We report on the discovery of HeLa cell contamination of the human endometrial epithelial cell line HES isolated in our laboratory. Short tandem repeat analysis of 9 unique genetic loci demonstrated molecular identity between HES and HeLa cells. In addition, we verified that WISH cells, isolated originally from human amnion epithelium, were also contaminated with HeLa cells. Inasmuch as our laboratory did not culture HeLa cells at the time of HES cell derivations, the source of contamination was the WISH cell line. These data highlight the need for continued diligence in authenticating cell lines used in reproductive biology research.

Use of perfusion bioreactors and large animal models for long bone tissue engineering

Tissue engineering and regenerative medicine (TERM) strategies for generation of new bone tissue includes the combined use of autologous or heterologous mesenchymal stem cells (MSC) and three-dimensional (3D) scaffold materials serving as structural support for the cells, that develop into tissue-like substitutes under appropriate in vitro culture conditions. This approach is very important due to the limitations and risks associated with autologous, as well as allogenic bone grafiting procedures currently used. However, the cultivation of osteoprogenitor cells in 3D scaffolds presents several challenges, such as the efficient transport of nutrient and oxygen and removal of waste products from the cells in the interior of the scaffold. In this context, perfusion bioreactor systems are key components for bone TERM, as many recent studies have shown that such systems can provide dynamic environments with enhanced diffusion of nutrients and therefore, perfusion can be used to generate grafts of clinically relevant sizes and shapes. Nevertheless, to determine whether a developed tissue-like substitute conforms to the requirements of biocompatibility, mechanical stability and safety, it must undergo rigorous testing both in vitro and in vivo. Results from in vitro studies can be difficult to extrapolate to the in vivo situation, and for this reason, the use of animal models is often an essential step in the testing of orthopedic implants before clinical use in humans. This review provides an overview of the concepts, advantages, and challenges associated with different types of perfusion bioreactor systems, particularly focusing on systems that may enable the generation of critical size tissue engineered constructs. Furthermore, this review discusses some of the most frequently used animal models, such as sheep and goats, to study the in vivo functionality of bone implant materials, in critical size defects.

Mass Transport in a Microchannel Bioreactor With a Porous Wall

A two-dimensional flow model has been developed to simulate mass transport in a microchannel bioreactor with a porous wall. A two-domain approach, based on the finite volume method, was implemented. For the fluid part, the governing equation used was the Navier–Stokes equation; for the porous medium region, the generalized Darcy–Brinkman–Forchheimer extended model was used. For the porous-fluid interface, a stress jump condition was enforced with a continuity of normal stress, and the mass interfacial conditions were continuities of mass and mass flux. Two parameters were defined to characterize the mass transports in the fluid and porous regions. The porous Damkohler number is the ratio of consumption to diffusion of the substrates in the porous medium. The fluid Damkohler number is the ratio of the substrate consumption in the porous medium to the substrate convection in the fluid region. The concentration results were found to be well correlated by the use of a reaction-convection distance parameter, which incorporated the effects of axial distance, substrate consumption, and convection. The reactor efficiency reduced with reaction-convection distance parameter because of reduced reaction (or flux), and smaller local effectiveness factor due to the lower concentration in Michaelis–Menten type reactions. The reactor was more effective, and hence, more efficient with the smaller porous Damkohler number. The generalized results could find applications for the design of bioreactors with a porous wall.

Airlift-driven fibrous-bed bioreactor for continuous production of glucoamylase using immobilized recombinant yeast cells

Continuous production of a fungal glucoamylase by immobilized recombinant Saccharomyces cerevisiae strain C468 containing plasmid pGAC9. Yeast cells were immobilized on hydrophilic cotton cloth in an inverse internal loop airlift-driven bioreactor. Free-cell culture in the airlift and stirred tank bioreactors confirmed the plasmid instability of the recombinant yeast. Enhanced glucoamylase productivity and plasmid stability were observed both in the free and immobilized cell cultures in the airlift bioreactor system. The glucoamylase level of the free-cell culture in the airlift bioreactor was approximately 20% higher than that in the in stirred tank bioreactor due to high cell density (cell dry weight/volume of bioreactor) and fraction of the plasmid-carrying cells. A potentially high glucoamylase activity of 161U/L and a corresponding volumetric productivity of 3.5U/Lh were achieved when a cell density of approximately 85g/L (or 12.3g/g fiber) was attained in the fibrous-bed immobilized cell bioreactor system. The stable glucoamylase production was achieved after five generations, at which time a fraction of approximately 62% of the plasmid-carrying cells was realized in the immobilized cell system. Plasmid stability was increased for the immobilized cells during continuous culture at the operating dilution rate. The volumetric and specific productivities and fraction of plasmid-carrying cells in the immobilized cell system were higher than in the free-cell counterpart, however. This was in part due to the high viability (approximately 80%) in the immobilized cell system and the selective immobilization of the plasmid-carrying cells in the fibrous bed, and perhaps increased plasmid copy number.

Amniotic fluid and placental stem cells

The amniotic fluid and the placenta are unique sources of different populations of stem cells--mesenchymal, hematopoietic, trophoblastic--and, possibly, of more primitive stem cells. Although much of the amniotic cavity/fluid and the placenta share a common embryonic origin, the specific origins of the stem cells found in these two compartments remain to be determined. Accordingly, it is not yet known whether all or part of these two stem-cell subsets are actually the same. The multilineage potential of the different stem cell populations from these two sources has begun to be described but still much remains to be learned. Thus, it is not surprising that clinical applications related to the use of these cells have yet to be reported. Nevertheless, fertile experimental work from many different groups has introduced a number of promising novel therapeutic concepts utilizing these cells, such as in tissue engineering, cell transplantation, and gene therapy.

Perfusion bioreactor system for human mesenchymal stem cell tissue engineering: Dynamic cell seeding and construct development

Human mesenchymal stem cells (hMSCs) have great potential for therapeutic applications. A bioreactor system that supports long-term hMSCs growth and three-dimensional (3-D) tissue formation is an important technology for hMSC tissue engineering. A 3-D perfusion bioreactor system was designed using non-woven poly (ethylene terepthalate) (PET) fibrous matrices as scaffolds. The main features of the perfusion bioreactor system are its modular design and integrated seeding operation. Modular design of the bioreactor system allows the growth of multiple engineered tissue constructs and provides flexibility in harvesting the constructs at different time points. In this study, four chambers with three matrices in each were utilized for hMSC construct development. The dynamic depth filtration seeding operation is incorporated in the system by perfusing cell suspensions perpendicularly through the PET matrices, achieving a maximum seeding efficiency of 68%, and the operation effectively reduced the complexity of operation and the risk of contamination. Statistical analyses suggest that the cells are uniformly distributed in the matrices. After seeding, long-term construct cultivation was conducted by perfusing the media around the constructs from both sides of the matrices. Compared to the static cultures, a significantly higher cell density of 4.22 x 10(7) cell/mL was reached over a 40-day culture period. Cellular constructs at different positions in the flow chamber have statistically identical cell densities over the culture period. After expansion, the cells in the construct maintained the potential to differentiate into osteoblastic and adipogenic lineages at high cell density. The perfusion bioreactor system is amenable to multiple tissue engineered construct production, uniform tissue development, and yet is simple to operate and can be scaled up for potential clinical use. The results also demonstrate that the multi-lineage differentiation potential of hMSCs are preserved even after extensive expansion, thus indicating the potential of hMSCs for functional tissue construct development. The system has important applications in stem cell tissue engineering.

Culturing and differentiation of murine embryonic stem cells in a three-dimensional fibrous matrix

Embryonic stem (ES) cells have indefinite self-renewal ability and pluripotency, and can provide a novel cell source for tissue engineering applications. In this study, a murine CCE ES cell line was used to derive hematopoietic cells in a 3-D fibrous matrix. The 3-D matrix was found to maintain the phenotypes of undifferentiated ES cells as indicated by alkaline phosphatase (ALP) activity and stage specific embryonic antigen-1 (SSEA-1) expression. In hematopoietic differentiation, cells from 3-D culture exhibited similar cell cycle distribution and SSEA-1 expression to those in the initial cell population. The Oct-4 expression was significantly down-regulated, which indicated the occurrence of differentiation, although the level was slightly higher than that in Petri dish culture. The expression of c-kit, cell surface marker for hematopoietic progenitor, was higher in the 3-D culture, suggesting a better-directed hematopoietic differentiation. Cells in the 3-D matrix tended to form large aggregates associated with fibers. For large-scale processes, a perfusion bioreactor can be used for both maintenance and differentiation cultures. As compared to the static culture, a higher growth rate and final cell density were resulted from the perfusion bioreactor due to better control of the reactor environment. At the same time, the differentiation capacity of ES cells was preserved in the perfusion culture. The ES cell culture in the fibrous matrix thus can be used as a 3-D model system to study effects of extracellular environment and associated physico-chemical parameters on ES cell maintenance and differentiation.

A fibrous-bed bioreactor for continuous production of developmental endothelial locus-1 by osteosarcoma cells

Genetically engineered human osteosarcoma cells containing developmental endothelial locus-1 (del-1) gene were studied for production of Del-1, a protein that has the properties of an extracellular matrix protein and can regulate vascular morphogenesis and remodeling. Del-1 has been studied as a potential anti-angiogenesis drug targeting solid tumors. In this study, osteosarcoma cells were cultured in a fibrous-bed bioreactor (FBB) to continuously produce Del-1. The FBB was constructed by packing a polyester fibrous matrix into a 1.5-l spinner flask. The effects of media composition, including the serum content in the medium, and dilution rate on cell growth, metabolism, and Del-1 production were studied. A gradual reduction of serum content from 10% (v/v) to 0.5% (v/v) caused no loss in Del-1 production. However, the production of Del-1 decreased significantly in a serum-free medium, suggesting some nutrients present in the serum were important to culture viability and Del-1 production. The continuous FBB culture was stable for long-term production of Del-1, with a higher Del-1 titer than that normally obtained in T-flask cultures and overall productivity similar to the total production from 300 25-cm(2) T-flasks. Reducing geneticin in the medium from 250 microg ml(-1) to zero at later culturing stages had no significant effect on Del-1 production. The FBB was operated for a period of more than 4 months without any notable degeneration, and reached a final cell density of 3 x 10(8) cells ml(-1) of packing volume with >90% cell viability. The good reactor performance can be attributed to the three-dimensional environment provided by the fibrous matrix that allows for efficient mass transfer and cell immobilization and growth. Scanning electron microscopic and confocal scanning laser microscopic studies of the cell-matrix showed that cells formed large aggregates in the fibrous matrix and cell density was relatively uniform in the matrix.

Perfusion culture of hepatocytes within galactose-derivatized biodegradable poly(lactide-co-glycolide) scaffolds prepared by gas foaming of effervescent salts

Galactose, a specific ligand for asialoglycoprotein receptor in hepatocytes, was immobilized onto the internal surface of highly porous biodegradable poly(D,L-lactic-co-glycolic acid) scaffolds prepared by gas foaming of effervescent salts. Rat hepatocytes seeded within the scaffolds were cultivated by using a continuous flow and perfusion reactor system. Flow rate of medium circulating through the closed loop bioreactor system was optimized to minimize the extent of cell washout from the scaffold/cell construct while satisfying the oxygen transport rate to the seeded hepatocytes. Using the flow culture system, the scaffolds immobilized with galactose onto its internal surface retained a greater number of hepatocytes than those with unmodified or immobilized with glucose due to specific interactions between seeded hepatocytes and galactose moieties exposed onto the surface of the scaffolds. The perfusion culture system based on galactose-modified macroporous scaffolds, under optimal flow conditions, resulted in much higher albumin secretion rate, approximately 70 pg/cell/day for 7 days, compared to that with glucose modified scaffolds used as a negative control. The enhanced functional activity of hepatocytes seeded within the galactose modified scaffolds was likely caused by the formation of aggregated hepatocytes within the scaffolds.

ED(27) trophoblast-like cells isolated from first-trimester chorionic villi are genetically identical to HeLa cells yet exhibit a distinct phenotype

ED(27) trophoblast-like cells were prepared from human chorionic villus samples obtained at 9 weeks gestation and have been grown continuously in vitro without phenotypic drift for nearly a decade. These cells express many trophoblast markers, including cytokeratin, placental alkaline phosphatase (PLAP), secretion of 17beta-estradiol, and a microvillous apical surface. The ED(27) cell line is a useful model system for studies of placental cell biology and has been distributed to laboratories world-wide. However, experiments to investigate their relationship to primary villous cytotrophoblast have shown that these cells do not secrete detectable amounts of human chorionic gonadotropin in culture and, when digested with trypsin, disperse into individual cells. Furthermore, immunocytochemical studies demonstrated that, unlike villous cytotrophoblasts, ED(27) cells were immunoreactive with monoclonal antibodies recognizing some HLA Class I antigens. This was not HLA-G, however, as would be expected if these cells originated from extravillous cytotrophoblasts, but rather classical HLA-A, B which is thought not to be expressed by any trophoblast subpopulations. These inconsistencies prompted us to question the authenticity of the continuous cell line as it now exists. Genetic haplotype analysis using the polymerase chain reaction (PCR) revealed that ED(27) was genetically identically to the HeLa cell line. Inasmuch as HeLa cells have never been grown in the laboratory (DAK), the only possible origin of HeLa cell contamination of ED(27) cells was the WISH cell line, and further PCR analysis revealed that this cell line was also genetically identical to HeLa. Like ED(27) cells, HeLa cells and WISH cells synthesized small amounts of estrogen and were found to express PLAP and antigens recognized by the monoclonal antibodies ED822, directed against the syncytiotrophoblast, and J1B5 directed against villous cytotrophoblast. These results point out the need for adherence to rigorous and consistent quality control measures to assure the authenticity of cell lines used as in vitro model systems.

Oxygen tension influences proliferation and differentiation in a tissue-engineered model of placental trophoblast-like cells

A considerable oxygen gradient exists in vivo, which exerts regulatory effects on tissue development and function. The objective of this study was to evaluate the feasibility of controlling cell proliferation and differentiation by regulating oxygen tension in a tissue-engineered bioreactor model. The effects of oxygen tension on proliferation and differentiation of first-trimester human trophoblast cells (known as ED(27) cells) were studied in a fiber-bed perfusion bioreactor system in which cells were grown in polyethylene terephthalate (PET) nonwoven fibrous matrix. By varying the oxygen tension between 2% and 20%, differential responses of trophoblasts in their proliferation and differentiation activities were observed. There was no significant difference in the rates of glucose consumption and lactate production, and lactate dehydrogenase (LDH) level in the culture media for both 2% and 20% oxygen tension cultures, indicating that cell metabolic activities were not limited by low oxygen tension. However, 2% oxygen stimulated cell proliferation but impeded the secretion of a functional hormone, 17beta-estradiol. In contrast, 20% oxygen tension reduced cell proliferation, but yielded higher hormone secretion. A step change in oxygen tension from 2% to 20% caused cells in the bioreactor to increase 17beta-estradiol secretion and shifted cell cycle from proliferation to differentiation, which were verified with the expression levels of cyclin B1 and p27(kip1). However, no significant response to a change from 6% to 20% oxygen tension was observed. It is concluded that changes in oxygen tension can be an effective strategy to control cell cycle and long-term tissue development. This work also demonstrated the important role of oxygen tension in regulating placental trophoblast tissue development and the feasibility of using the bioreactor under well-controlled physiological environment for tissue engineering applications.

Three-dimensional cell-scaffold constructs promote efficient gene transfection: implications for cell-based gene therapy

To date, introduction of gene-modified cells in vivo is still a critical limitation for cell-based gene therapy. In this study, based on tissue engineering techniques, we developed a three-dimensional (3-D) transfection system to be cell-based gene delivery vehicle. Human trophoblast-like ED(27) and fibroblastic NIH3T3 cells were used as model cell lines. Cells were seeded onto PET fibrous matrices and plated on polyethylene terephathalate (PET) films as 2-D transfection control. The cell-matrices and cell-films were transfected with pCMV-betagal and pEGFP (green fluorescent protein) reporter gene vectors using LipofectAmine reagent. Gene expression on 3-D versus 2-D growth surface were investigated. The effects of seeding method, seeding density, porosity of the PET matrix, and culturing time of the cell-matrix complex on cDNA transfection and expression in the 3-D cell-matrix complex were also investigated. The beta-gal assay and GFP detection showed that 3-D transfection promoted a higher gene expression level and longer expression time as compared to 2-D transfection. There existed an optimal initial cell seeding density for gene transfection of 3-D cell-matrix complex. Cells seeded on PET matrices with a lower porosity ( approximately 87%) had higher gene expression activities than cells in the matrices with a higher porosity ( approximately 90%). Also, Higher gene expression levels of beta-gal were obtained for the more uniformly seeded matrices that were seeded with a depth-filtration method. The results from this study demonstrate the potential utility of cells seeded onto 3-D fibrous matrices as cell-based gene delivery vehicle for in vitro study of gene expression or in vivo gene therapy.

Effects of pore size in 3-D fibrous matrix on human trophoblast tissue development

The effects of pore size in a 3-D polyethylene terephthalate (PET) nonwoven fibrous matrix on long-term tissue development of human trophoblast ED27 cells were studied. Thermal compression was used to modify the porosity and pore size of the PET matrix. The pore size distributions in PET matrices were quantified using a liquid extrusion method. Cell metabolic activities, estradiol production, and cell proliferation and differentiation were studied for ED27 cells cultured in the thermally compressed PET matrices with known pore structure characteristics. In general, metabolic activities and proliferation rate were higher initially for cultures grown in the low-porosity (LP) PET matrix (porosity of 0.849, average pore size of 30 microm in diameter) than those in the high-porosity (HP) matrix (porosity of 0.896, average pore size of 39 microm in diameter). However, 17beta-estradiol production and cell differentiation activity in the HP matrix surpassed those in the LP matrix after 12 days. The expression levels of cyclin B1 and p27kip1 in cells revealed progressively decreasing proliferation and increasing differentiation activities for cells grown in PET matrices. Also, difference in pore size controlled the cell spatial organization in the PET matrices and contributed to the tissue development in varying degrees of proliferation and differentiation. It was also found that cells grown on the 2-D surface behaved differently in cell cycle progression and did not show increased differentiation activities after growth had stopped and proliferation activities had lowered to a minimal level. The results from this study suggest that the 3-D cell organization guided by the tissue scaffold is important to tissue formation in vitro.