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Developmental Dynamics : An Official... Jul 1995We describe a series of stages for development of the embryo of the zebrafish, Danio (Brachydanio) rerio. We define seven broad periods of embryogenesis--the zygote,...
We describe a series of stages for development of the embryo of the zebrafish, Danio (Brachydanio) rerio. We define seven broad periods of embryogenesis--the zygote, cleavage, blastula, gastrula, segmentation, pharyngula, and hatching periods. These divisions highlight the changing spectrum of major developmental processes that occur during the first 3 days after fertilization, and we review some of what is known about morphogenesis and other significant events that occur during each of the periods. Stages subdivide the periods. Stages are named, not numbered as in most other series, providing for flexibility and continued evolution of the staging series as we learn more about development in this species. The stages, and their names, are based on morphological features, generally readily identified by examination of the live embryo with the dissecting stereomicroscope. The descriptions also fully utilize the optical transparancy of the live embryo, which provides for visibility of even very deep structures when the embryo is examined with the compound microscope and Nomarski interference contrast illumination. Photomicrographs and composite camera lucida line drawings characterize the stages pictorially. Other figures chart the development of distinctive characters used as staging aid signposts.
Topics: Animals; Blastocyst; Cleavage Stage, Ovum; Embryo, Nonmammalian; Gastrula; Morphogenesis; Temperature; Time Factors; Zebrafish; Zygote
PubMed: 8589427
DOI: 10.1002/aja.1002030302 -
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 -
Cell Sep 2022In vitro cultured stem cells with distinct developmental capacities can contribute to embryonic or extraembryonic tissues after microinjection into pre-implantation...
In vitro cultured stem cells with distinct developmental capacities can contribute to embryonic or extraembryonic tissues after microinjection into pre-implantation mammalian embryos. However, whether cultured stem cells can independently give rise to entire gastrulating embryo-like structures with embryonic and extraembryonic compartments remains unknown. Here, we adapt a recently established platform for prolonged ex utero growth of natural embryos to generate mouse post-gastrulation synthetic whole embryo models (sEmbryos), with both embryonic and extraembryonic compartments, starting solely from naive ESCs. This was achieved by co-aggregating non-transduced ESCs, with naive ESCs transiently expressing Cdx2 or Gata4 to promote their priming toward trophectoderm and primitive endoderm lineages, respectively. sEmbryos adequately accomplish gastrulation, advance through key developmental milestones, and develop organ progenitors within complex extraembryonic compartments similar to E8.5 stage mouse embryos. Our findings highlight the plastic potential of naive pluripotent cells to self-organize and functionally reconstitute and model the entire mammalian embryo beyond gastrulation.
Topics: Animals; Cell Differentiation; Embryo, Mammalian; Embryonic Development; Embryonic Stem Cells; Endoderm; Gastrulation; Mammals; Mice
PubMed: 35988542
DOI: 10.1016/j.cell.2022.07.028 -
Cell Stem Cell Oct 2022Several in vitro models have been developed to recapitulate mouse embryogenesis solely from embryonic stem cells (ESCs). Despite mimicking many aspects of early...
Several in vitro models have been developed to recapitulate mouse embryogenesis solely from embryonic stem cells (ESCs). Despite mimicking many aspects of early development, they fail to capture the interactions between embryonic and extraembryonic tissues. To overcome this difficulty, we have developed a mouse ESC-based in vitro model that reconstitutes the pluripotent ESC lineage and the two extraembryonic lineages of the post-implantation embryo by transcription-factor-mediated induction. This unified model recapitulates developmental events from embryonic day 5.5 to 8.5, including gastrulation; formation of the anterior-posterior axis, brain, and a beating heart structure; and the development of extraembryonic tissues, including yolk sac and chorion. Comparing single-cell RNA sequencing from individual structures with time-matched natural embryos identified remarkably similar transcriptional programs across lineages but also showed when and where the model diverges from the natural program. Our findings demonstrate an extraordinary plasticity of ESCs to self-organize and generate a whole-embryo-like structure.
Topics: Animals; Embryo, Mammalian; Embryonic Development; Embryonic Stem Cells; Mice; Mouse Embryonic Stem Cells; Neurulation
PubMed: 36084657
DOI: 10.1016/j.stem.2022.08.013 -
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 -
Journal of Assisted Reproduction and... Jul 2021Embryo selection within in vitro fertilization (IVF) is the process of evaluating qualities of fertilized oocytes (embryos) and selecting the best embryo(s) available...
Embryo selection within in vitro fertilization (IVF) is the process of evaluating qualities of fertilized oocytes (embryos) and selecting the best embryo(s) available within a patient cohort for subsequent transfer or cryopreservation. In recent years, artificial intelligence (AI) has been used extensively to improve and automate the embryo ranking and selection procedure by extracting relevant information from embryo microscopy images. The AI models are evaluated based on their ability to identify the embryo(s) with the highest chance(s) of achieving a successful pregnancy. Whether such evaluations should be based on ranking performance or pregnancy prediction, however, seems to divide studies. As such, a variety of performance metrics are reported, and comparisons between studies are often made on different outcomes and data foundations. Moreover, superiority of AI methods over manual human evaluation is often claimed based on retrospective data, without any mentions of potential bias. In this paper, we provide a technical view on some of the major topics that divide how current AI models are trained, evaluated and compared. We explain and discuss the most common evaluation metrics and relate them to the two separate evaluation objectives, ranking and prediction. We also discuss when and how to compare AI models across studies and explain in detail how a selection bias is inevitable when comparing AI models against current embryo selection practice in retrospective cohort studies.
Topics: Area Under Curve; Artificial Intelligence; Blastocyst; Calibration; Cryopreservation; Databases, Factual; Decision Making, Computer-Assisted; Embryo Transfer; Female; Fertilization in Vitro; Humans; Image Processing, Computer-Assisted; Pregnancy; Sample Size; Sensitivity and Specificity
PubMed: 34173914
DOI: 10.1007/s10815-021-02254-6 -
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 -
Cell Stem Cell Jun 2023Gastruloids are 3D structures generated from pluripotent stem cells recapitulating fundamental principles of embryonic pattern formation. Using single-cell genomic...
Gastruloids are 3D structures generated from pluripotent stem cells recapitulating fundamental principles of embryonic pattern formation. Using single-cell genomic analysis, we provide a resource mapping cell states and types during gastruloid development and compare them with the in vivo embryo. We developed a high-throughput handling and imaging pipeline to spatially monitor symmetry breaking during gastruloid development and report an early spatial variability in pluripotency determining a binary response to Wnt activation. Although cells in the gastruloid-core revert to pluripotency, peripheral cells become primitive streak-like. These two populations subsequently break radial symmetry and initiate axial elongation. By performing a compound screen, perturbing thousands of gastruloids, we derive a phenotypic landscape and infer networks of genetic interactions. Finally, using a dual Wnt modulation, we improve the formation of anterior structures in the existing gastruloid model. This work provides a resource to understand how gastruloids develop and generate complex patterns in vitro.
Topics: Mice; Animals; Embryo, Mammalian; Primitive Streak; Pluripotent Stem Cells; Embryonic Development
PubMed: 37209681
DOI: 10.1016/j.stem.2023.04.018 -
Cell Stem Cell May 2023Understanding the mechanisms of blastocyst formation and implantation is critical for improving farm animal reproduction but is hampered by a limited supply of embryos....
Understanding the mechanisms of blastocyst formation and implantation is critical for improving farm animal reproduction but is hampered by a limited supply of embryos. Here, we developed an efficient method to generate bovine blastocyst-like structures (termed blastoids) via assembling bovine trophoblast stem cells and expanded potential stem cells. Bovine blastoids resemble blastocysts in morphology, cell composition, single-cell transcriptomes, in vitro growth, and the ability to elicit maternal recognition of pregnancy following transfer to recipient cows. Bovine blastoids represent an accessible in vitro model for studying embryogenesis and improving reproductive efficiency in livestock species.
Topics: Pregnancy; Female; Cattle; Animals; Blastocyst; Trophoblasts; Embryo Implantation; Embryonic Development; Stem Cells; Cell Culture Techniques
PubMed: 37146582
DOI: 10.1016/j.stem.2023.04.003 -
Developmental Biology Jun 2021During development, a single cell is transformed into a highly complex organism through progressive cell division, specification and rearrangement. An important... (Review)
Review
During development, a single cell is transformed into a highly complex organism through progressive cell division, specification and rearrangement. An important prerequisite for the emergence of patterns within the developing organism is to establish asymmetries at various scales, ranging from individual cells to the entire embryo, eventually giving rise to the different body structures. This becomes especially apparent during gastrulation, when the earliest major lineage restriction events lead to the formation of the different germ layers. Traditionally, the unfolding of the developmental program from symmetry breaking to germ layer formation has been studied by dissecting the contributions of different signaling pathways and cellular rearrangements in the in vivo context of intact embryos. Recent efforts, using the intrinsic capacity of embryonic stem cells to self-assemble and generate embryo-like structures de novo, have opened new avenues for understanding the many ways by which an embryo can be built and the influence of extrinsic factors therein. Here, we discuss and compare divergent and conserved strategies leading to germ layer formation in embryos as compared to in vitro systems, their upstream molecular cascades and the role of extrinsic factors in this process.
Topics: Animals; Embryonic Stem Cells; Extraembryonic Membranes; Gastrulation; Germ Layers; Humans; Signal Transduction
PubMed: 33352181
DOI: 10.1016/j.ydbio.2020.12.014