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Developmental Cell Jan 2022There has been recent renewed interest in studying human early embryonic development. The advent of improved culture conditions to maintain blastocysts in vitro for an... (Review)
Review
There has been recent renewed interest in studying human early embryonic development. The advent of improved culture conditions to maintain blastocysts in vitro for an extended period and the emerging stem-cell-based models of the blastocyst and peri-implantation embryos have provided new information that is relevant to early human embryogenesis. However, the mechanism of lineage development and embryonic patterning, and the molecular pathways involved in their regulation, are still not well understood. Interest in human embryonic development has been reinvigorated recently given numerous technical advances. In this review, Rossant and Tam discuss new insights into human embryogenesis gathered from successes in culturing human embryos in vitro and stem-cell-based embryo models. Then they outline what questions still need answering.
Topics: Blastocyst; Body Patterning; Cell Lineage; Embryo Culture Techniques; Embryo, Mammalian; Embryonic Development; Embryonic Stem Cells; Gastrulation; Gene Expression Regulation, Developmental; Humans
PubMed: 35077679
DOI: 10.1016/j.devcel.2021.12.022 -
Trends in Cell Biology Jan 2022The basic body plan of the mammalian embryo is established through gastrulation, a pivotal early postimplantation event during which the three major germ layers... (Review)
Review
The basic body plan of the mammalian embryo is established through gastrulation, a pivotal early postimplantation event during which the three major germ layers (endoderm, ectoderm, and mesoderm) are specified with cellular and spatial diversity. Despite its basic and clinical importance, human embryo development from peri-implantation to gastrulation remains shrouded in mystery. Recent advances in the elongated in vitro culture of rodent and non-primate embryos and the construction of embryo-like structures have helped to improve understanding of the mechanisms of human early embryonic development. Here, we review the recent advances and possible future directions in the development of in vitro models to better understand human embryogenesis from peri-implantation to gastrulation.
Topics: Animals; Embryo, Mammalian; Embryonic Development; Gastrulation; Humans; Mammals
PubMed: 34417090
DOI: 10.1016/j.tcb.2021.07.008 -
Developmental Biology Jun 2021Technical and ethical limitations create a challenge to study early human development, especially following the first 3 weeks of development after fertilization, when... (Review)
Review
Technical and ethical limitations create a challenge to study early human development, especially following the first 3 weeks of development after fertilization, when the fundamental aspects of the body plan are established through the process called gastrulation. As a consequence, our current understanding of human development is mostly based on the anatomical and histological studies on Carnegie Collection of human embryos, which were carried out more than half a century ago. Due to the 14-day rule on human embryo research, there have been no experimental studies beyond the fourteenth day of human development. Mutagenesis studies on animal models, mostly in mouse, are often extrapolated to human embryos to understand the transcriptional regulation of human development. However, due to the existence of significant differences in their morphological and molecular features as well as the time scale of their development, it is obvious that complete knowledge of human development can be achieved only by studying the human embryo. These studies require a cellular framework. Here we summarize the cellular, molecular, and temporal aspects associated with human gastrulation and discuss how they relate to existing human PSCs based models of early development.
Topics: Animals; Body Patterning; Embryo, Mammalian; Gastrulation; Humans; Models, Animal
PubMed: 33484705
DOI: 10.1016/j.ydbio.2021.01.006 -
Cell Aug 2023A human embryo's legal definition and its entitlement to protection vary greatly worldwide. Recently, human pluripotent stem cells have been used to form in vitro... (Review)
Review
A human embryo's legal definition and its entitlement to protection vary greatly worldwide. Recently, human pluripotent stem cells have been used to form in vitro models of early embryos that have challenged legal definitions and raised questions regarding their usage. In this light, we propose a refined legal definition of an embryo, suggest "tipping points" for when human embryo models could eventually be afforded similar protection to that of embryos, and then revisit basic ethical principles that might help to draft a roadmap for the gradual, justified usage of embryo models in a manner that aims to maximize benefits to society.
Topics: Humans; Embryo, Mammalian; Pluripotent Stem Cells; Embryo Research
PubMed: 37595564
DOI: 10.1016/j.cell.2023.07.028 -
Fertility and Sterility Nov 2020Embryo evaluation and selection embody the aggregate manifestation of the entire in vitro fertilization (IVF) process. It aims to choose the "best" embryos from the... (Review)
Review
Embryo evaluation and selection embody the aggregate manifestation of the entire in vitro fertilization (IVF) process. It aims to choose the "best" embryos from the larger cohort of fertilized oocytes, the majority of which will be determined to be not viable either as a result of abnormal development or due to chromosomal imbalances. Indeed, it is generally acknowledged that even after embryo selection based on morphology, time-lapse microscopic photography, or embryo biopsy with preimplantation genetic testing, implantation rates in the human are difficult to predict. Our pursuit of enhancing embryo evaluation and selection, as well as increasing live birth rates, will require the adoption of novel technologies. Recently, several artificial intelligence (AI)-based methods have emerged as objective, standardized, and efficient tools for evaluating human embryos. Moreover, AI-based methods can be implemented for other clinical aspects of IVF, such as assessing patient reproductive potential and individualizing gonadotropin stimulation protocols. As AI has the capability to analyze "big" data, the ultimate goal will be to apply AI tools to the analysis of all embryological, clinical, and genetic data in an effort to provide patient-tailored treatments. In this chapter, we present an overview of existing AI technologies in reproductive medicine and envision their potential future applications in the field.
Topics: Animals; Artificial Intelligence; Embryo Implantation; Embryo, Mammalian; Embryology; Fertilization in Vitro; Humans; Preimplantation Diagnosis
PubMed: 33160513
DOI: 10.1016/j.fertnstert.2020.09.157 -
Cell Stem Cell Apr 2023Human stem cell-derived blastoids display similar morphology and cell lineages to normal blastocysts. However, the ability to investigate their developmental potential...
Human stem cell-derived blastoids display similar morphology and cell lineages to normal blastocysts. However, the ability to investigate their developmental potential is limited. Here, we construct cynomolgus monkey blastoids resembling blastocysts in morphology and transcriptomics using naive ESCs. These blastoids develop to embryonic disk with the structures of yolk sac, chorionic cavity, amnion cavity, primitive streak, and connecting stalk along the rostral-caudal axis through prolonged in vitro culture (IVC). Primordial germ cells, gastrulating cells, visceral endoderm/yolk sac endoderm, three germ layers, and hemato-endothelial progenitors in IVC cynomolgus monkey blastoids were observed by single-cell transcriptomics or immunostaining. Moreover, transferring cynomolgus monkey blastoids to surrogates achieves pregnancies, as indicated by progesterone levels and presence of early gestation sacs. Our results reveal the capacity of in vitro gastrulation and in vivo early pregnancy of cynomolgus monkey blastoids, providing a useful system to dissect primate embryonic development without the same ethical concerns and access challenges in human embryo study.
Topics: Pregnancy; Animals; Female; Humans; Gastrulation; Macaca fascicularis; Embryo, Mammalian; Germ Layers; Embryonic Development; Endoderm; Cell Differentiation
PubMed: 37028403
DOI: 10.1016/j.stem.2023.03.009 -
Nature Oct 2023The human embryo undergoes morphogenetic transformations following implantation into the uterus, but our knowledge of this crucial stage is limited by the inability to...
The human embryo undergoes morphogenetic transformations following implantation into the uterus, but our knowledge of this crucial stage is limited by the inability to observe the embryo in vivo. Models of the embryo derived from stem cells are important tools for interrogating developmental events and tissue-tissue crosstalk during these stages. Here we establish a model of the human post-implantation embryo, a human embryoid, comprising embryonic and extraembryonic tissues. We combine two types of extraembryonic-like cell generated by overexpression of transcription factors with wild-type embryonic stem cells and promote their self-organization into structures that mimic several aspects of the post-implantation human embryo. These self-organized aggregates contain a pluripotent epiblast-like domain surrounded by extraembryonic-like tissues. Our functional studies demonstrate that the epiblast-like domain robustly differentiates into amnion, extraembryonic mesenchyme and primordial germ cell-like cells in response to bone morphogenetic protein cues. In addition, we identify an inhibitory role for SOX17 in the specification of anterior hypoblast-like cells. Modulation of the subpopulations in the hypoblast-like compartment demonstrates that extraembryonic-like cells influence epiblast-like domain differentiation, highlighting functional tissue-tissue crosstalk. In conclusion, we present a modular, tractable, integrated model of the human embryo that will enable us to probe key questions of human post-implantation development, a critical window during which substantial numbers of pregnancies fail.
Topics: Female; Humans; Pregnancy; Bone Morphogenetic Proteins; Cell Differentiation; Embryo Implantation; Embryo, Mammalian; Embryoid Bodies; Embryonic Development; Germ Layers; Human Embryonic Stem Cells; Models, Biological; Transcription Factors; Pluripotent Stem Cells
PubMed: 37369347
DOI: 10.1038/s41586-023-06368-y -
Cell Research Sep 2023Studies of cultured embryos have provided insights into human peri-implantation development. However, detailed knowledge of peri-implantation lineage development as well...
Studies of cultured embryos have provided insights into human peri-implantation development. However, detailed knowledge of peri-implantation lineage development as well as underlying mechanisms remains obscure. Using 3D-cultured human embryos, herein we report a complete cell atlas of the early post-implantation lineages and decipher cellular composition and gene signatures of the epiblast and hypoblast derivatives. In addition, we develop an embryo-like assembloid (E-assembloid) by assembling naive hESCs and extraembryonic cells. Using human embryos and E-assembloids, we reveal that WNT, BMP and Nodal signaling pathways synergistically, but functionally differently, orchestrate human peri-implantation lineage development. Specially, we dissect mechanisms underlying extraembryonic mesoderm and extraembryonic endoderm specifications. Finally, an improved E-assembloid is developed to recapitulate the epiblast and hypoblast development and tissue architectures in the pre-gastrulation human embryo. Our findings provide insights into human peri-implantation development, and the E-assembloid offers a useful model to disentangle cellular behaviors and signaling interactions that drive human embryogenesis.
Topics: Humans; Germ Layers; Embryo, Mammalian; Embryo Implantation; Endoderm; Mesoderm; Embryonic Development
PubMed: 37460804
DOI: 10.1038/s41422-023-00846-8 -
Cell Reports Jun 2022In human embryos, major zygotic genome activation (ZGA) initiates at the eight-cell (8C) stage. Abnormal ZGA leads to developmental defects and even contributes to the...
In human embryos, major zygotic genome activation (ZGA) initiates at the eight-cell (8C) stage. Abnormal ZGA leads to developmental defects and even contributes to the failure of human blastocyst formation or implantation. An in vitro cell model mimicking human 8C blastomeres would be invaluable to understanding the mechanisms regulating key biological events during early human development. Using the non-canonical promoter of LEUTX that putatively regulates human ZGA, we developed an 8C::mCherry reporter, which specifically marks the 8C state, to isolate rare 8C-like cells (8CLCs) from human preimplantation epiblast-like stem cells. The 8CLCs express a panel of human ZGA genes and have a unique transcriptome resembling that of the human 8C embryo. Using the 8C::mCherry reporter, we further optimize the chemical-based culture condition to increase and maintain the 8CLC population. Functionally, 8CLCs can self-organize to form blastocyst-like structures. The discovery and maintenance of 8CLCs provide an opportunity to recapitulate early human development.
Topics: Blastocyst; Embryonic Development; Gene Expression Regulation, Developmental; Genome; Humans; Zygote
PubMed: 35732112
DOI: 10.1016/j.celrep.2022.110994 -
Nature Oct 2023The ability to study human post-implantation development remains limited owing to ethical and technical challenges associated with intrauterine development after...
The ability to study human post-implantation development remains limited owing to ethical and technical challenges associated with intrauterine development after implantation. Embryo-like models with spatially organized morphogenesis and structure of all defining embryonic and extra-embryonic tissues of the post-implantation human conceptus (that is, the embryonic disc, the bilaminar disc, the yolk sac, the chorionic sac and the surrounding trophoblast layer) remain lacking. Mouse naive embryonic stem cells have recently been shown to give rise to embryonic and extra-embryonic stem cells capable of self-assembling into post-gastrulation structured stem-cell-based embryo models with spatially organized morphogenesis (called SEMs). Here we extend those findings to humans using only genetically unmodified human naive embryonic stem cells (cultured in human enhanced naive stem cell medium conditions). Such human fully integrated and complete SEMs recapitulate the organization of nearly all known lineages and compartments of post-implantation human embryos, including the epiblast, the hypoblast, the extra-embryonic mesoderm and the trophoblast layer surrounding the latter compartments. These human complete SEMs demonstrated developmental growth dynamics that resemble key hallmarks of post-implantation stage embryogenesis up to 13-14 days after fertilization (Carnegie stage 6a). These include embryonic disc and bilaminar disc formation, epiblast lumenogenesis, polarized amniogenesis, anterior-posterior symmetry breaking, primordial germ-cell specification, polarized yolk sac with visceral and parietal endoderm formation, extra-embryonic mesoderm expansion that defines a chorionic cavity and a connecting stalk, and a trophoblast-surrounding compartment demonstrating syncytium and lacunae formation. This SEM platform will probably enable the experimental investigation of previously inaccessible windows of human early post implantation up to peri-gastrulation development.
Topics: Humans; Embryo Implantation; Embryo, Mammalian; Embryonic Development; Fertilization; Gastrulation; Germ Layers; Human Embryonic Stem Cells; Trophoblasts; Yolk Sac; Giant Cells
PubMed: 37673118
DOI: 10.1038/s41586-023-06604-5