Bovine blastocyst-like structures derived from stem cell cultures

Human blastoid as an in vitro model of human blastocysts

Human development is a highly coordinated process, with any abnormalities during the early embryonic stages that can often have detrimental consequences. The complexity and nuances of human development underpin its significance in embryo research. However, this research is often hindered by limited availability and ethical considerations associated with the use of donated blastocysts from in vitro fertilization (IVF) surplus. Human blastoids offer promising alternatives as they can be easily generated and manipulated in the laboratory while preserving key characteristics of human blastocysts. In this way, they hold the potential to serve as a scalable and ethically permissible resource in embryology research. By utilizing such human embryo models, we can establish a transformative platform that complements the study with IVF embryos, ultimately enhancing our understanding of human embryogenesis.

Self-renewing human naïve pluripotent stem cells dedifferentiate in 3D culture and form blastoids spontaneously

Human naïve pluripotent stem cells (hnPSCs) can generate integrated models of blastocysts termed blastoids upon switch to inductive medium. However, the underlying mechanisms remain obscure. Here we report that self-renewing hnPSCs spontaneously and efficiently give rise to blastoids upon three dimensional (3D) suspension culture. The spontaneous blastoids mimic early stage human blastocysts in terms of structure, size, and transcriptome characteristics and are capable of progressing to post-implantation stages. This property is conferred by the glycogen synthase kinase-3 (GSK3) signalling inhibitor IM-12 present in 5iLAF self-renewing medium. IM-12 upregulates oxidative phosphorylation-associated genes that underly the capacity of hnPSCs to generate blastoids spontaneously. Starting from day one of self-organization, hnPSCs at the boundary of all 3D aggregates dedifferentiate into E5 embryo-like intermediates. Intermediates co-express SOX2/OCT4 and GATA6 and by day 3 specify trophoblast fate, which coincides with cavity and blastoid formation. In summary, spontaneous blastoid formation results from 3D culture triggering dedifferentiation of hnPSCs into earlier embryo-like intermediates which are then competent to segregate blastocyst fates.

Understanding conceptus-maternal interactions: what tools do we need to develop?

Communication between the maternal endometrium and developing embryo/conceptus is critical to support successful pregnancy to term. Studying the peri-implantation period of pregnancy is critical as this is when most pregnancy loss occurs in cattle. Our current understanding of these interactions is limited, due to the lack of appropriate in vitro models to assess these interactions. The endometrium is a complex and heterogeneous tissue that is regulated in a transcriptional and translational manner throughout the oestrous cycle. While there are in vitro models to study endometrial function, they are static and 2D in nature or explant models and are limited in how well they recapitulate the in vivo endometrium. Recent developments in organoid systems, microfluidic approaches, extracellular matrix biology, and in silico approaches provide a new opportunity to develop in vitro systems that better model the in vivo scenario. This will allow us to investigate in a more high-throughput manner the fundamental molecular interactions that are required for successful pregnancy in cattle.

Molecular and cellular programs underlying the development of bovine pre-implantation embryos

Early embryonic mortality is a major cause of infertility in cattle, yet the underlying molecular causes remain a mystery. Over the past half century, assisted reproductive technologies such as in vitro fertilisation and somatic cell nuclear transfer have been used to improve cattle reproductive efficiency; however, reduced embryo developmental potential is seen compared to their in vivo counterparts. Recent years have seen exciting progress across bovine embryo research, including genomic profiling of embryogenesis, new methods for improving embryo competence, and experimenting on building bovine embryos from stem cell cultures. These advances are beginning to define bovine embryo molecular and cellular programs and could potentially lead to improved embryo health. Here, I highlight the current status of molecular determinants and cellular programs of bovine embryo development and new opportunities to improve the bovine embryo health.

Modeling post-implantation stages of human development into early organogenesis with stem-cell-derived peri-gastruloids

In vitro stem cell models that replicate human gastrulation have been generated, but they lack the essential extraembryonic cells needed for embryonic development, morphogenesis, and patterning. Here, we describe a robust and efficient method that prompts human extended pluripotent stem cells to self-organize into embryo-like structures, termed peri-gastruloids, which encompass both embryonic (epiblast) and extraembryonic (hypoblast) tissues. Although peri-gastruloids are not viable due to the exclusion of trophoblasts, they recapitulate critical stages of human peri-gastrulation development, such as forming amniotic and yolk sac cavities, developing bilaminar and trilaminar embryonic discs, specifying primordial germ cells, initiating gastrulation, and undergoing early neurulation and organogenesis. Single-cell RNA-sequencing unveiled transcriptomic similarities between advanced human peri-gastruloids and primary peri-gastrulation cell types found in humans and non-human primates. This peri-gastruloid platform allows for further exploration beyond gastrulation and may potentially aid in the development of human fetal tissues for use in regenerative medicine.

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.

Human 8-cell embryo-like cells from pluripotent stem cells

The totipotent embryo initiates transcription during zygotic or embryonic genome activation (EGA, ZGA). ZGA occurs at the 8-cell stage in humans and its failure leads to developmental arrest. Understanding the molecular pathways underlying ZGA and totipotency is essential to comprehend human development. Recently, human 8-cell-like cells (8CLCs) have been discovered in vitro that resemble the 8-cell embryo. 8CLCs exist among naive pluripotent stem cells and can be induced genetically or chemically. Their ZGA-like transcriptome, transposable element activation, 8-cell embryo-specific protein expression, and developmental properties make them an exceptional model system to study early embryonic cell-state transitions and human totipotency programs in vitro.

Recent advances in stem cell-based blastocyst models

Early embryo development is a highly dynamic process that plays a crucial role in determining the health and characteristics of an organism. For many years, embryonic and extraembryonic stem cell lines representing various developmental stages have served as valuable models for studying early embryogenesis. As our understanding of stem cell culture and embryo development has advanced, researchers have been able to create more sophisticated 3D structures mimicking early embryos, such as blastocyst-like structures (blastoids). These innovative models represent a significant leap forward in the field. In this mini-review, we will discuss the latest progress in stem cell-based embryo models, explore potential future directions, and examine how these models contribute to a deeper understanding of early mammalian development.

Technical challenges of studying early human development

Recent years have seen exciting progress across human embryo research, including new methods for culturing embryos, transcriptional profiling of embryogenesis and gastrulation, mapping lineage trajectories, and experimenting on stem cell-based embryo models. These advances are beginning to define the dynamical principles of development across stages, tissues and organs, enabling a better understanding of human development before birth in health and disease, and potentially leading to improved treatments for infertility and developmental disorders. However, there are still significant roadblocks en route to this goal. Here, we highlight technical challenges to studying early human development and propose ways and means to overcome some of these constraints.

Establishment of bovine trophoblast stem cells

Here, we report that a chemical cocktail (LCDM: leukemia inhibitory factor [LIF], CHIR99021, dimethinedene maleate [DiM], minocycline hydrochloride), previously developed for extended pluripotent stem cells (EPSCs) in mice and humans, enables de novo derivation and long-term culture of bovine trophoblast stem cells (TSCs). Bovine TSCs retain developmental potency to differentiate into mature trophoblast cells and exhibit transcriptomic and epigenetic (chromatin accessibility and DNA methylome) features characteristic of trophectoderm cells from early bovine embryos. The bovine TSCs established in this study will provide a model to study bovine placentation and early pregnancy failure.