Human naive epiblast cells possess unrestricted lineage potential.

Naive stem cell blastocyst model captures human embryo lineage segregation

Despite its importance, understanding the early phases of human development has been limited by availability of human samples. The recent emergence of stem-cell-derived embryo models, a new field aiming to use stem cells to construct in vitro models to recapitulate snapshots of the development of the mammalian conceptus, opens up exciting opportunities to promote fundamental understanding of human development and advance reproductive and regenerative medicine. This Review provides a summary of the current knowledge of early mammalian development, using mouse and human conceptuses as models, and emphasizes their similarities and critical differences. We then highlight existing embryo models that mimic different aspects of mouse and human development. We further discuss bioengineering tools used for controlling multicellular interactions and self-organization critical for the development of these models. We conclude with a discussion of the important next steps and exciting future opportunities of stem-cell-derived embryo models for fundamental discovery and translation. This Review highlights the recent emergence of stem-cell-derived embryo models for the purpose of advancing our understanding of mammalian embryology as well as their potential uses in regenerative and reproductive medicine.

Stem-cell-based embryo models for fundamental research and translation.

Understanding human development is of fundamental biological and clinical importance. Despite its significance, mechanisms behind human embryogenesis remain largely unknown. Here, we attempt to model human early embryo development with expanded pluripotent stem cells (EPSCs) in 3-dimensions. We define a protocol that allows us to generate self-organizing cystic structures from human EPSCs that display some hallmarks of human early embryogenesis. These structures mimic polarization and cavitation characteristic of pre-implantation development leading to blastocyst morphology formation and the transition to post-implantation-like organization upon extended culture. Single-cell RNA sequencing of these structures reveals subsets of cells bearing some resemblance to epiblast, hypoblast and trophectoderm lineages. Nevertheless, significant divergences from natural blastocysts persist in some key markers, and signalling pathways point towards ways in which morphology and transcriptional-level cell identities may diverge in stem cell models of the embryo. Thus, this stem cell platform provides insights into the design of stem cell models of embryogenesis. Human early development remains largely inaccessible, owing to technical and ethical limitations of working with natural embryos. Here the authors assess the extent to which human expanded pluripotent stem cells can specify distinct cell lineages and capture aspects of early human embryogenesis.

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Reconstructing aspects of human embryogenesis with pluripotent stem cells.

Mouse totipotent stem cells captured and maintained through spliceosomal repression

Totipotency is the ability of a single cell to give rise to all of the differentiated cell types that build the conceptus, yet how to capture this property in vitro remains incompletely understood. Defining totipotency relies on a variety of assays of variable stringency. Here, we describe criteria to define totipotency. We explain how distinct criteria of increasing stringency can be used to judge totipotency by evaluating candidate totipotent cell types in mice, including early blastomeres and expanded or extended pluripotent stem cells. Our data challenge the notion that expanded or extended pluripotent states harbour increased totipotent potential relative to conventional embryonic stem cells under in vitro and in vivo conditions.

Evaluating totipotency using criteria of increasing stringency.

Summary Reprogramming female mouse somatic cells into induced pluripotent stem cells (iPSCs) leads to X-chromosome reactivation. The extent to which increased X-chromosome dosage (X-dosage) in female iPSCs compared with male iPSCs leads to differences in the properties of iPSCs is still unclear. We show that chromatin accessibility in mouse iPSCs is modulated by X-dosage. Specific sets of transcriptional regulator motifs are enriched in chromatin with increased accessibility in XX or XY iPSCs. The transcriptome, growth and pluripotency exit are also modulated by X-dosage in iPSCs. To understand how increased X-dosage modulates the properties of mouse pluripotent stem cells, we used heterozygous deletions of the X-linked gene Dusp9. We show that X-dosage regulates the transcriptome, open chromatin landscape, growth, and pluripotency exit largely independently of global DNA methylation. Our results provide insights into how gene dosage modulates the epigenetic and genetic mechanisms that regulate cell identity.

X-Chromosome Dosage Modulates Multiple Molecular and Cellular Properties of Mouse Pluripotent Stem Cells Independently of Global DNA Methylation Levels.

During early mammalian development, the two X-chromosomes in female cells are active. Dosage compensation between XX female and XY male cells is then achieved by X-chromosome inactivation in female cells. Reprogramming female mouse somatic cells into induced pluripotent stem cells (iPSCs) leads to X-chromosome reactivation. The extent to which increased X-chromosome dosage (X-dosage) in female iPSCs leads to differences in the molecular and cellular properties of XX and XY iPSCs is still unclear. We show that chromatin accessibility in mouse iPSCs is modulated by X-dosage. Specific sets of transcriptional regulator motifs are enriched in chromatin with increased accessibility in XX or XY iPSCs. We show that the transcriptome, growth and pluripotency exit are also modulated by X-dosage in iPSCs. To understand the mechanisms by which increased X-dosage modulates the molecular and cellular properties of mouse pluripotent stem cells, we used heterozygous deletions of the X-linked gene Dusp9 in XX embryonic stem cells. We show that X-dosage regulates the transcriptome, open chromatin landscape, growth and pluripotency exit largely independently of global DNA methylation. Our results uncover new insights into X-dosage in pluripotent stem cells, providing principles of how gene dosage modulates the epigenetic and genetic mechanisms regulating cell identity.

https://www.biorxiv.org/content/biorxiv/early/2018/11/11/291450.full.pdf

X Chromosome Dosage Modulates Multiple Molecular and Cellular Properties of Mouse Pluripotent Stem Cells Independently of Global DNA Methylation Levels

Mouna El Bakkali

Papers

Evaluating totipotency using criteria of increasing stringency.

X-Chromosome Dosage Modulates Multiple Molecular and Cellular Properties of Mouse Pluripotent Stem Cells Independently of Global DNA Methylation Levels.

X Chromosome Dosage Modulates Multiple Molecular and Cellular Properties of Mouse Pluripotent Stem Cells Independently of Global DNA Methylation Levels