Cynomolgus monkey embryo model captures gastrulation and early pregnancy

Advancing primatology through ethical and scientific perspectives on rhesus monkey (Macaca mulatta) cloning

A critical turning point was reached in research with the recent success in cloning rhesus monkeys (Macaca mulatta), a major advancement in primatology. This breakthrough marks the beginning of a new age in biomedical research, ushered by improved somatic cell nuclear transfer techniques and creative trophoblast replacement strategies. The successful cloning of rhesus monkeys presents the possibility of producing genetically homogeneous models that are highly advantageous for studying complex biological processes, testing drugs, and researching diseases. However, this achievement raises important ethical questions, particularly regarding animal welfare and the broader ramifications of primate cloning. Approaching the future of primate research with balance is critical, as the scientific world stands on the brink of these revolutionary breakthroughs. This paper aims to summarise the consequences, ethical challenges and possible paths forward in primatology arising from rhesus monkey cloning.

Why study human embryo development?

Understanding the processes and mechanisms underlying early human embryo development has become an increasingly active and important area of research. It has potential for insights into important clinical issues such as early pregnancy loss, origins of congenital anomalies and developmental origins of adult disease, as well as fundamental insights into human biology. Improved culture systems for preimplantation embryos, combined with the new tools of single cell genomics and live imaging, are providing new insights into the similarities and differences between human and mouse development. However, access to human embryo material is still restricted and extended culture of early embryos has regulatory and ethical concerns. Stem cell-derived models of different phases of human development can potentially overcome these limitations and provide a scalable source of material to explore the early postimplantation stages of human development. To date, such models are clearly incomplete replicas of normal development but future technological improvements can be envisaged. The ethical and regulatory environment for such studies remains to be fully resolved.

Current strategies using 3D organoids to establish in vitro maternal-embryonic interaction

IMPORTANCE

The creation of robust maternal-embryonic interactions and implantation models is important for comprehending the early stages of embryonic development and reproductive disorders. Traditional two-dimensional (2D) cell culture systems often fail to accurately mimic the highly complex in vivo conditions. The employment of three-dimensional (3D) organoids has emerged as a promising strategy to overcome these limitations in recent years. The advancements in the field of organoid technology have opened new avenues for studying the physiology and diseases affecting female reproductive tract.

OBSERVATIONS

This review summarizes the current strategies and advancements in the field of 3D organoids to establish maternal-embryonic interaction and implantation models for use in research and personalized medicine in assisted reproductive technology. The concepts of endometrial organoids, menstrual blood flow organoids, placental trophoblast organoids, stem cell-derived blastoids, and in vitro-generated embryo models are discussed in detail. We show the incorportaion of organoid systems and microfluidic technology to enhance tissue performance and precise management of the cellular surroundings.

CONCLUSIONS AND RELEVANCE

This review provides insights into the future direction of modeling maternal-embryonic interaction research and its combination with other powerful technologies to interfere with this dialogue either by promoting or hindering it for improving fertility or methods for contraception, respectively. The merging of organoid systems with microfluidics facilitates the creation of sophisticated and functional organoid models, enhancing insights into organ development, disease mechanisms, and personalized medical investigations.

Role and ethics of cynomolgus monkey (Macaca fascicularis) blastoids in primate developmental biology research

This review on cynomolgus monkey (Macaca fascicularis) blastoids discusses a breakthrough in modeling early non-human primate embryogenesis, offering insights into embryonic development and implantation processes. It acknowledges ethical challenges and animal welfare considerations in developmental biology, suggests potential applications in human reproductive medicine, and highlights the need for ongoing ethical and technical refinement.

Human stem-cell-derived embryo models: When bioethical normativity meets biological ontology

The use of human stem-cell-derived embryo models in biomedical research has recently sparked intense bioethical debates. In this article, we delve into the ethical complexities surrounding these models and advocate for a deeper exploration of their biological ontology to discuss their bioethical normativity. We examine the ethical considerations arising from the implementation of these models, emphasizing varying viewpoints on their ethical standing and the ethical obligations associated with their development and utilization. We contend that a nuanced comprehension of their biological ontology is crucial for navigating these ethical quandaries. Furthermore, we underscore the indispensability of interdisciplinary cooperation among bioethicists, biologists, and philosophers to unravel the complex interplay between biological ontology and the normative framework of bioethics. Moreover, this article introduces a novel combinatorial approach to resolve the ethical dilemma surrounding these models. We propose a distinction between models that closely emulate natural embryos, based on the status of synthetic embryos, and those capable of reproducing specific dimensions of embryonic development. Such differentiation allows for nuanced ethical considerations while harnessing the value of these models in scientific research, paving the way for a more comprehensive ethical framework in the context of evolving biotechnologies.

Using embryo models to understand the development and progression of embryonic lineages: a focus on primordial germ cell development

BACKGROUND

Recapitulating mammalian cell type differentiation in vitro promises to improve our understanding of how these processes happen in vivo, while bringing additional prospects for biomedical applications. The establishment of stem cell-derived embryo models and embryonic organoids, which have experienced explosive growth over the last few years, open new avenues for research due to their scale, reproducibility, and accessibility. Embryo models mimic various developmental stages, exhibit different degrees of complexity, and can be established across species. Since embryo models exhibit multiple lineages organised spatially and temporally, they are likely to provide cellular niches that, to some degree, recapitulate the embryonic setting and enable "co-development" between cell types and neighbouring populations. One example where this is already apparent is in the case of primordial germ cell-like cells (PGCLCs).

SUMMARY

While directed differentiation protocols enable the efficient generation of high PGCLC numbers, embryo models provide an attractive alternative as they enable the study of interactions of PGCLCs with neighbouring cells, alongside the regulatory molecular and biophysical mechanisms of PGC competency. Additionally, some embryo models can recapitulate post-specification stages of PGC development (including migration or gametogenesis), mimicking the inductive signals pushing PGCLCs to mature and differentiate, and enabling the study of PGCLC development across stages. Therefore, in vitro models may allow us to address questions of cell type differentiation, and PGC development specifically, that have hitherto been out of reach with existing systems.

KEY MESSAGE

This review evaluates the current advances in stem cell-based embryo models, with a focus on their potential to model cell type-specific differentiation in general, and in particular to address open questions in PGC development and gametogenesis.

Stem cell-derived embryo models: moral advance or moral obfuscation?

Stem cell-derived embryo models (SCEMs) are model embryos used in scientific research to gain a better understanding of early embryonic development. The way humans develop from a single-cell zygote to a complex multicellular organism remains poorly understood. However, research looking at embryo development is difficult because of restrictions on the use of human embryos in research. Stem cell embryo models could reduce the need for human embryos, allowing us to both understand early development and improve assisted reproductive technologies. There have been several rapid advances in creating SCEMs in recent years. These advances potentially provide a new avenue to study early human development. The benefits of SCEMs are predicated on the claim that they are different from embryos and should, therefore, be exempt from existing regulations that apply to embryos (such as the 14-day rule). SCEMs are proposed as offering a model that can capture the inner workings of the embryo but lack its moral sensitivities. However, the ethical basis for making this distinction has not been clearly explained. In this current controversy, we focus on the ethical justification for treating SCEMs differently to embryos, based on considerations of moral status.

Potentiality switches and epistemic uncertainty: the Argument from Potential in times of human embryo-like structures

Recent advancements in developmental biology enable the creation of embryo-like structures from human stem cells, which we refer to as human embryo-like structures (hELS). These structures provide promising tools to complement-and perhaps ultimately replace-the use of human embryos in clinical and fundamental research. But what if these hELS-when further improved-also have a claim to moral status? What would that imply for their research use? In this paper, we explore these questions in relation to the traditional answer as to why human embryos should be given greater protection than other (non-)human cells: the so-called Argument from Potential (AfP). According to the AfP, human embryos deserve special moral status because they have the unique potential to develop into persons. While some take the development of hELS to challenge the very foundations of the AfP, the ongoing debate suggests that its dismissal would be premature. Since the AfP is a spectrum of views with different moral implications, it does not need to imply that research with human embryos or hELS that (may) have 'active' potential should be completely off-limits. However, the problem with determining active potential in hELS is that this depends on development passing through 'potentiality switches' about the precise coordinates of which we are still in the dark. As long as this epistemic uncertainty persists, extending embryo research regulations to research with specific types of hELS would amount to a form of regulative precaution that as such would require further justification.

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.

Retrotransposon renaissance in early embryos

Despite being the predominant genetic elements in mammalian genomes, retrotransposons were often dismissed as genomic parasites with ambiguous biological significance. However, recent studies reveal their functional involvement in early embryogenesis, encompassing crucial processes such as zygotic genome activation (ZGA) and cell fate decision. This review underscores the paradigm shift in our understanding of retrotransposon roles during early preimplantation development, as well as their rich functional reservoir that is exploited by the host to provide cis-regulatory elements, noncoding RNAs, and functional proteins. The rapid advancement in long-read sequencing, low input multiomics profiling, advanced in vitro systems, and precise gene editing techniques encourages further dissection of retrotransposon functions that were once obscured by the intricacies of their genomic footprints.

The many dimensions of germline competence

Primordial germ cell (PGC) specification is the first step in the development of the germline. Recent work has elucidated human-mouse differences in PGC differentiation and identified cell states with enhanced competency for PGC-like cell (PGCLC) differentiation in vitro in both species. However, it remains a subject of debate how different PGC competent states in vitro relate to each other, to embryonic development, and to the origin of PGCs in vivo. Here we review recent literature on human PGCLC differentiation in the context of mouse and non-human primate models. In contrast to what was previously thought, recent work suggests human pluripotent stem cells (hPSCs) are highly germline competent. We argue that paradoxical observations regarding the origin and signaling requirements of hPGCLCs may be due to local cell interactions. These confound assays of competence by generating endogenous signaling gradients and spatially modulating the ability to receive exogenous inductive signals. Furthermore, combinatorial signaling suggests that there is no unique germline competent state: rather than a one-dimensional spectrum of developmental progression, competence should be considered in a higher dimensional landscape of cell states.

An expedition in the jungle of pluripotent stem cells of non-human primates

For nearly three decades, more than 80 embryonic stem cell lines and more than 100 induced pluripotent stem cell lines have been derived from New World monkeys, Old World monkeys, and great apes. In this comprehensive review, we examine these cell lines originating from marmoset, cynomolgus macaque, rhesus macaque, pig-tailed macaque, Japanese macaque, African green monkey, baboon, chimpanzee, bonobo, gorilla, and orangutan. We outline the methodologies implemented for their establishment, the culture protocols for their long-term maintenance, and their basic molecular characterization. Further, we spotlight any cell lines that express fluorescent reporters. Additionally, we compare these cell lines with human pluripotent stem cell lines, and we discuss cell lines reprogrammed into a pluripotent naive state, detailing the processes used to attain this. Last, we present the findings from the application of these cell lines in two emerging fields: intra- and interspecies embryonic chimeras and blastoids.

Human embryo models made from pluripotent stem cells are not synthetic; they aren't embryos, either

Embryo models are potentially highly impactful for human health research because their development recapitulates otherwise inaccessible events in a poorly understood area of biology, the first few weeks of human life. Casual reference to these models as "synthetic embryos" is misleading and should be approached with care and deliberation.

Seven days in the life cycle of Homo sapiens

The second week of embryonic development is a critical phase of the human life cycle and one that has been largely inaccessible to scientific investigation. Recent studies of human embryo models built from stem cells promise to yield dramatic insights into the key events of cell specification and morphogenesis that occur during this brief window of embryogenesis.

Human and Pig Pluripotent Stem Cells: From Cellular Products to Organogenesis and Beyond

Pluripotent stem cells (PSCs) are important for studying development and hold great promise in regenerative medicine due to their ability to differentiate into various cell types. In this review, we comprehensively discuss the potential applications of both human and pig PSCs and provide an overview of the current progress and challenges in this field. In addition to exploring the therapeutic uses of PSC-derived cellular products, we also shed light on their significance in the study of interspecies chimeras, which has led to the creation of transplantable human or humanized pig organs. Moreover, we emphasize the importance of pig PSCs as an ideal cell source for genetic engineering, facilitating the development of genetically modified pigs for pig-to-human xenotransplantation. Despite the achievements that have been made, further investigations and refinement of PSC technologies are necessary to unlock their full potential in regenerative medicine and effectively address critical healthcare challenges.

Stem cell-derived embryo models: a frontier of human embryology

Studying human development remains difficult due to limited accessibility to human embryonic tissues. Prompted by the availability of human stem cells that share molecular and cellular similarities with embryonic and extraembryonic cells in peri-implantation human embryos, researchers have now successfully developed stem cell-based human embryo models that are promising as experimental tools for studying early human development. In this Perspective, we discuss the current progress in mouse and human stem cell-derived embryo models and highlight their promising applications in advancing the fundamental understanding of mammalian development.

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.

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.

Current state of stem cell research in non-human primates: an overview

The remarkable similarity between non-human primates (NHPs) and humans establishes them as essential models for understanding human biology and diseases, as well as for developing novel therapeutic strategies, thereby providing more comprehensive reference data for clinical treatment. Pluripotent stem cells such as embryonic stem cells and induced pluripotent stem cells provide unprecedented opportunities for cell therapies against intractable diseases and injuries. As continue to harness the potential of these biotechnological therapies, NHPs are increasingly being employed in preclinical trials, serving as a pivotal tool to evaluate the safety and efficacy of these interventions. Here, we review the recent advancements in the fundamental research of stem cells and the progress made in studies involving NHPs.

Prolonged 3D culture unlocks black box of primate embryogenesis

By developing in vitro 3D culture systems and identifying morphological and molecular events of early organogenesis, two recent studies1,2 reported exciting research advances in non-human primate embryo 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.

New monkey embryo models—it’s getting complicated

Li et al.¹ report on the generation of cynomolgus monkey models of blastocyst-stage embryos (called "blastoids") using naive cynomolgus embryonic stem cells. These blastoids recapitulate gastrulation in vitro and induce early pregnancy responses when transferred into cynomolgus monkey surrogates, prompting consideration of the policy implications for human blastoid research.