Mouse blastomeres acquire ability to divide asymmetrically before compaction

Spatial positioning of preimplantation mouse embryo cells is regulated by mTORC1 and m7G-cap-dependent translation at the 8- to 16-cell transition

Preimplantation mouse embryo development involves temporal-spatial specification and segregation of three blastocyst cell lineages: trophectoderm, primitive endoderm and epiblast. Spatial separation of the outer-trophectoderm lineage from the two other inner-cell-mass (ICM) lineages starts with the 8- to 16-cell transition and concludes at the 32-cell stages. Accordingly, the ICM is derived from primary and secondary contributed cells; with debated relative EPI versus PrE potencies. We report generation of primary but not secondary ICM populations is highly dependent on temporal activation of mammalian target of Rapamycin (mTOR) during 8-cell stage M-phase entry, mediated via regulation of the 7-methylguanosine-cap (m7G-cap)-binding initiation complex (EIF4F) and linked to translation of mRNAs containing 5' UTR terminal oligopyrimidine (TOP-) sequence motifs, as knockdown of identified TOP-like motif transcripts impairs generation of primary ICM founders. However, mTOR inhibition-induced ICM cell number deficits in early blastocysts can be compensated by the late blastocyst stage, after inhibitor withdrawal; compensation likely initiated at the 32-cell stage when supernumerary outer cells exhibit molecular characteristics of inner cells. These data identify a novel mechanism specifically governing initial spatial segregation of mouse embryo blastomeres, that is distinct from those directing subsequent inner cell formation, contributing to germane segregation of late blastocyst lineages.

Developmental capacity is unevenly distributed among single blastomeres of 2-cell and 4-cell stage mouse embryos

During preimplantation development, mammalian embryo cells (blastomeres) cleave, gradually losing their potencies and differentiating into three primary cell lineages: epiblast (EPI), trophectoderm (TE), and primitive endoderm (PE). The exact moment at which cells begin to vary in their potency for multilineage differentiation still remains unknown. We sought to answer the question of whether single cells isolated from 2- and 4-cell embryos differ in their ability to generate the progenitors and cells of blastocyst lineages. We revealed that twins were often able to develop into blastocysts containing inner cell masses (ICMs) with PE and EPI cells. Despite their capacity to create a blastocyst, the twins differed in their ability to produce EPI, PE, and TE cell lineages. In contrast, quadruplets rarely formed normal blastocysts, but instead developed into blastocysts with ICMs composed of only one cell lineage or completely devoid of an ICM altogether. We also showed that quadruplets have unequal capacities to differentiate into TE, PE, and EPI lineages. These findings could explain the difficulty of creating monozygotic twins and quadruplets from 2- and 4-cell stage mouse embryos.

Cell fate determination and Hippo signaling pathway in preimplantation mouse embryo

First cell fate determination plays crucial roles in cell specification during early phases of embryonic development. Three classical concepts have been proposed to explain the lineage specification mechanism of the preimplantation embryo: inside-outside, pre-patterning, and polarity models. Transcriptional effectors of the Hippo signal pathway are YAP and TAZ activators that can create a shuttle between the cytoplasm and the nucleus. Despite different localizations of YAP in the cell, it determines the fate of ICM and TE. How the decisive cue driving factors that determine YAP localization are coordinated remains a central unanswered question. How can an embryonic cell find its position? The objective of this review is to summarize the molecular and mechanical aspects in cell fate decision during mouse preimplantation embryonic development. The findings will reveal the relationship between cell-cell adhesion, cell polarity, and determination of cell fate during early embryonic development in mice and elucidate the inducing/inhibiting mechanisms that are involved in cell specification following zygotic genome activation and compaction processes. With future studies, new biophysical and chemical cues in the cell fate determination will impart significant spatiotemporal effects on early embryonic development. The achieved knowledge will provide important information to the development of new approaches to be used in infertility treatment and increase the success of pregnancy.

Epithelial-Mesenchymal Transition Drives Three-Dimensional Morphogenesis in Mammalian Early Development

From fertilization to onset of gastrulation, a mammalian embryo goes through several rounds of cellular morphogenesis resembling phenomena of epithelial-mesenchymal transition (EMT) and mesenchymal-epithelial transition (MET), collectively referred to as EMTs. How these EMT events play a role in shaping the three-dimensional (3-D) architecture of the developing embryo is not well-understood. In this review, we present a model in which cellular morphogenesis, represented primarily by dynamic changes in its epithelialization status, is the driving force of embryonic 3-D organization. This is achieved through the integration of three key components of mammalian early development, the pluripotency regulation, morphogenetic signaling, and biomechanical force anisotropy. Although cells in an early embryo do not exhibit full mesenchymal characteristics, our model underscores the importance of investigating molecular regulation of epithelial cell polarity and partial EMT/MET in understanding mammalian early development.

The unknown human trophectoderm: implication for biopsy at the blastocyst stage

Trophectoderm biopsy is increasingly performed for pre-implantation genetic testing of aneuploidies and considered a safe procedure on short-term clinical outcome, without strong assessment of long-term consequences. Poor biological information on human trophectoderm is available due to ethical restrictions. Therefore, most studies have been conducted in vitro (choriocarcinoma cell lines, embryonic and pluripotent stem cells) and on murine models that nevertheless poorly reflect the human counterpart. Polarization, compaction, and blastomere differentiation (e.g., the basis to ascertain trophectoderm origin) are poorly known in humans. In addition, the trophectoderm function is poorly known from a biological point of view, although a panoply of questionable and controversial microarray studies suggest that important genes overexpressed in trophectoderm are involved in pluripotency, metabolism, cell cycle, endocrine function, and implantation. The intercellular communication system between the trophectoderm cells and the inner cell mass, modulated by cell junctions and filopodia in the murine model, is obscure in humans. For the purpose of this paper, data mainly on primary cells from human and murine embryos has been reviewed. This review suggests that the trophectoderm origin and functions have been insufficiently ascertained in humans so far. Therefore, trophectoderm biopsy should be considered an experimental procedure to be undertaken only under approved rigorous experimental protocols in academic contexts.

A Survey of Essential Genome Stability Genes Reveals That Replication Stress Mitigation Is Critical for Peri-Implantation Embryogenesis

Murine development demands that pluripotent epiblast stem cells in the peri-implantation embryo increase from approximately 120 to 14,000 cells between embryonic days (E) 4.5 and E7.5. This is possible because epiblast stem cells can complete cell cycles in under 3 h in vivo. To ensure conceptus fitness, epiblast cells must undertake this proliferative feat while maintaining genome integrity. How epiblast cells maintain genome health under such an immense proliferation demand remains unclear. To illuminate the contribution of genome stability pathways to early mammalian development we systematically reviewed knockout mouse data from 347 DDR and repair associated genes. Cumulatively, the data indicate that while many DNA repair functions are dispensable in embryogenesis, genes encoding replication stress response and homology directed repair factors are essential specifically during the peri-implantation stage of early development. We discuss the significance of these findings in the context of the unique proliferative demands placed on pluripotent epiblast stem cells.

Building Principles for Constructing a Mammalian Blastocyst Embryo

The self-organisation of a fertilised egg to form a blastocyst structure, which consists of three distinct cell lineages (trophoblast, epiblast and hypoblast) arranged around an off-centre cavity, is unique to mammals. While the starting point (the zygote) and endpoint (the blastocyst) are similar in all mammals, the intervening events have diverged. This review examines and compares the descriptive and functional data surrounding embryonic gene activation, symmetry-breaking, first and second lineage establishment, and fate commitment in a wide range of mammalian orders. The exquisite detail known from mouse embryogenesis, embryonic stem cell studies and the wealth of recent single cell transcriptomic experiments are used to highlight the building principles underlying early mammalian embryonic development.

The first cell-fate decision of mouse preimplantation embryo development: integrating cell position and polarity

During the first cell-fate decision of mouse preimplantation embryo development, a population of outer-residing polar cells is segregated from a second population of inner apolar cells to form two distinct cell lineages: the trophectoderm and the inner cell mass (ICM), respectively. Historically, two models have been proposed to explain how the initial differences between these two cell populations originate and ultimately define them as the two stated early blastocyst stage cell lineages. The 'positional' model proposes that cells acquire distinct fates based on differences in their relative position within the developing embryo, while the 'polarity' model proposes that the differences driving the lineage segregation arise as a consequence of the differential inheritance of factors, which exhibit polarized subcellular localizations, upon asymmetric cell divisions. Although these two models have traditionally been considered separately, a growing body of evidence, collected over recent years, suggests the existence of a large degree of compatibility. Accordingly, the main aim of this review is to summarize the major historical and more contemporarily identified events that define the first cell-fate decision and to place them in the context of both the originally proposed positional and polarity models, thus highlighting their functional complementarity in describing distinct aspects of the developmental programme underpinning the first cell-fate decision in mouse embryogenesis.