Emerging in vitro platforms and omics technologies for studying the endometrium and early embryo-maternal interface in humans

Developing and characterising bovine decellularized extracellular matrix hydrogels to biofabricate female reproductive tissues

This study investigated the development and characterization of decellularized extracellular matrix (dECM) hydrogels tailored for the biofabrication of female reproductive tissues, specifically targeting ovarian cortex, endometrium, ovarian medulla, and oviduct tissues. We aimed to evaluate the cytocompatibility, biomechanical properties, and overall efficacy of these dECMs in promoting cell viability, proliferation, and morphology using the bovine model. Bovine species provide a valuable model due to their accessibility from slaughterhouse tissues, offering a practical alternative to human samples, which are often limited in availability. Additionally, bovine tissue closely mirrors certain physiological and biological characteristics of humans, making it a relevant model for translational research. Our findings revealed that these dECMs exhibited high biocompatibility with embryo development and cell viability, supporting micro vascularization and cellular morphology without the need for external growth factors. It is important to note that the addition of alginate was crucial for maintaining the structural integrity of the hydrogel during long-term cultures. These hydrogels displayed biomechanical properties that closely mimicked native tissues, which was vital for maintaining their functional integrity and supporting cellular activities. The printability assessments showed that dECMs, particularly those from cortex tissues, achieved high precision in replicating the intended structures, though challenges such as low porosity remained. The bioprinted constructs demonstrated robust cell growth, with over 97% viability observed by day 7, indicating their suitability for cell culture. This work represented a significant advancement in reproductive tissue biofabrication, demonstrating the potential of dECM-based hydrogels in creating structurally and viable tissue constructs. By tailoring each dECM to match the unique biomechanical properties of different tissues, we paved the way for more effective and reliable applications in reproductive medicine and tissue engineering. STATEMENT OF SIGNIFICANCE: This research explores the use of decellularized extracellular matrix (dECM) hydrogels as bio-inks for creating reproductive tissues. Ovarian cortex and medulla, oviduct and endometrium dECMs demonstrated biomechanical properties that mimicked native tissues, which is essential for maintaining functional integrity and supporting cellular processes. Notably, these hydrogels exhibited high biocompatibility with embryo development and cell viability, promoting microvascularization and cell differentiation without the need for supplemental growth factors. The successful bioprinting of these bio-inks underscores their potential for creating more complex models. This work represents a significant advancement in tissue engineering, offering promising new avenues for reproductive medicine.

Establishment of an In Vitro Embryo-Endometrium Model Using Alginate-Embedded Mouse Embryos and Human Embryoid Body

BACKGROUND

Embryo-endometrium cross-talk is one of the critical processes for implantation, and unsuccessful cross-talk leads to infertility. We established an endometrium-embryo (or embryoid bodies, hEBs) in vitro model in 2D and 3D conditions and assessed its potential through the fusion of embryos and the expression of specific markers.

METHODS

C57BL/6 mouse embryos and human embryoid body (hEB) derived from embryonic stem cells were prepared as embryo models. Mouse endometrium (EM) and human endometrium cell line, HEC-1-A, were prepared, and 2D or 3D EMs were generated. The viability of the 3D endometrium was analyzed, and the optimal ratio of the gelation was revealed. The invasion of the embryos or hEBs was examined by immunostaining and 3D image rendering.

RESULTS

The embryos and the alternative hEBs were effectively fused into 2D or 3D vitro EM models in both mouse and human models. The fused embryos and hEBs exhibited migration and further development. Notably, the established in vitro model expressed Oct4 and E-Cadherin, markers for early embryonic development; human CG Receptor and Progesterone Receptor, critical for implantation and pregnancy maintenance; and TSH Receptor, Epiregulin, and Prolactin, indicators of endometrial receptivity and embryo implantation.

CONCLUSION

This study marks a significant advancement in the field, as we have successfully established a novel in vitro model for studying embryo-endometrium cross-talk. This model, a crucial tool for understanding fertility and the causes of miscarriage due to failed implantation, provides a unique platform for investigating the complex processes of successful implantation and pregnancy, underscoring its potential impact on reproductive health.

Bioengineering approaches for the endometrial research and application

The endometrium undergoes a series of precise monthly changes under the regulation of dynamic levels of ovarian hormones that are characterized by repeated shedding and subsequent regeneration without scarring. This provides the potential for wound healing during endometrial injuries. Bioengineering materials highlight the faithful replication of constitutive cells and the extracellular matrix that simulates the physical and biomechanical properties of the endometrium to a larger extent. Significant progress has been made in this field, and functional endometrial tissue bioengineering allows an in-depth investigation of regulatory factors for endometrial and myometrial defects in vitro and provides highly therapeutic methods to alleviate obstetric and gynecological complications. However, much remains to be learned about the latest progress in the application of bioengineering technologies to the human endometrium. Here, we summarize the existing developments in biomaterials and bioengineering models for endometrial regeneration and improving the female reproductive potential.

The Placenta: A Maternofetal Interface

The placenta is the gatekeeper between the mother and the fetus. Over the first trimester of pregnancy, the fetus is nourished by uterine gland secretions in a process known as histiotrophic nutrition. During the second trimester of pregnancy, placentation has evolved to the point at which nutrients are delivered to the placenta via maternal blood (hemotrophic nutrition). Over gestation, the placenta must adapt to these variable nutrient supplies, to alterations in maternal physiology and blood flow, and to dynamic changes in fetal growth rates. Numerous questions remain about the mechanisms used to transport nutrients to the fetus and the maternal and fetal determinants of this process. Growing data highlight the ability of the placenta to regulate this process. As new technologies and omics approaches are utilized to study this maternofetal interface, greater insight into this unique organ and its impact on fetal development and long-term health has been obtained.

Shifting early embryology paradigms: Applications of stem cell-based embryo models in bioengineering

Technologies to reproduce specific aspects of early mammalian embryogenesis in vitro using stem cells have skyrocketed over the last several years. With these advances, we have gained new perspectives on how embryonic and extraembryonic cells self-organize to form the embryo. These reductionist approaches hold promise for the future implementation of precise environmental and genetic controls to understand variables affecting embryo development. Our review discusses recent progress in cellular models of early mammalian embryo development and bioengineering advancements that can be leveraged to study the embryo-maternal interface. We summarize current gaps in the field, emphasizing the importance of understanding how intercellular interactions at this interface contribute to reproductive and developmental health.

Micro-RNAs in Human Placenta: Tiny Molecules, Immense Power

Micro-RNAs (miRNAs) are short non-coding single-stranded RNAs that modulate the expression of various target genes after transcription. The expression and distribution of kinds of miRNAs have been characterized in human placenta during different gestational stages. The identified miRNAs are recognized as key mediators in the regulation of placental development and in the maintenance of human pregnancy. Aberrant expression of miRNAs is associated with compromised pregnancies in humans, and dysregulation of those miRNAs contributes to the occurrence and development of related diseases during pregnancy, such as pre-eclampsia (PE), fetal growth restriction (FGR), gestational diabetes mellitus (GDM), recurrent miscarriage, preterm birth (PTB) and small-for-gestational-age (SGA). Thus, having a better understanding of the expression and functions of miRNAs in human placenta during pregnancy and thereby developing novel drugs targeting the miRNAs could be a potentially promising method in the prevention and treatment of relevant diseases in future. Here, we summarize the current knowledge of the expression pattern and function regulation of miRNAs in human placental development and related diseases.

Early human trophoblast development: from morphology to function

Human pregnancy depends on the proper development of the embryo prior to implantation and the implantation of the embryo into the uterine wall. During the pre-implantation phase, formation of the morula is followed by internalization of blastomeres that differentiate into the pluripotent inner cell mass lineage, while the cells on the surface undergo polarization and differentiate into the trophectoderm of the blastocyst. The trophectoderm mediates apposition and adhesion of the blastocyst to the uterine epithelium. These processes lead to a stable contact between embryonic and maternal tissues, resulting in the formation of a new organ, the placenta. During implantation, the trophectoderm cells start to differentiate and form the basis for multiple specialized trophoblast subpopulations, all of which fulfilling specific key functions in placentation. They either differentiate into polar cells serving typical epithelial functions, or into apolar invasive cells that adapt the uterine wall to progressing pregnancy. The composition of these trophoblast subpopulations is crucial for human placenta development and alterations are suggested to result in placenta-associated pregnancy pathologies. This review article focuses on what is known about very early processes in human reproduction and emphasizes on morphological and functional aspects of early trophoblast differentiation and subpopulations.