The basal position of nuclei is one pre-requisite for asymmetric cell divisions in the early mouse embryo

Mechanical Forces, Nucleus, Chromosomes, and Chromatin

Individual cells and cells within the tissues and organs constantly face mechanical challenges, such as tension, compression, strain, shear stress, and the rigidity of cellular and extracellular surroundings. Besides the external mechanical forces, cells and their components are also subjected to intracellular mechanical forces, such as pulling, pushing, and stretching, created by the sophisticated force-generation machinery of the cytoskeleton and molecular motors. All these mechanical stressors switch on the mechanotransduction pathways, allowing cells and their components to respond and adapt. Mechanical force-induced changes at the cell membrane and cytoskeleton are also transmitted to the nucleus and its nucleoskeleton, affecting nucleocytoplasmic transport, chromatin conformation, transcriptional activity, replication, and genome, which, in turn, orchestrate cellular mechanical behavior. The memory of mechanoresponses is stored as epigenetic and chromatin structure modifications. The mechanical state of the cell in response to the acellular and cellular environment also determines cell identity, fate, and immune response to invading pathogens. Here, we give a short overview of the latest developments in understanding these processes, emphasizing their effects on cell nuclei, chromosomes, and chromatin.

Exploring Mechanical Forces Shaping Self-Organization and Morphogenesis During Early Embryo Development

Embryonic development is a dynamic process orchestrated by a delicate interplay of biochemical and biophysical factors. While the role of genetics and biochemistry in embryogenesis has been extensively studied, recent research has highlighted the significance of mechanical regulation in shaping and guiding this intricate process. Here, we provide an overview of the current understanding of the mechanical regulation of embryo development. We explore how mechanical forces generated by cells and tissues play a crucial role in driving the development of different stages. We examine key morphogenetic processes such as compaction, blastocyst formation, implantation, and egg cylinder formation, and discuss the mechanical mechanisms and cues involved. By synthesizing the current body of literature, we highlight the emerging concepts and open questions in the field of mechanical regulation. We aim to provide an overview of the field, inspiring future investigations and fostering a deeper understanding of the mechanical aspects of embryo development.

Revisiting trophectoderm-inner cell mass lineage segregation in the mammalian preimplantation embryo

In the first days of life, cells of the mammalian embryo segregate into two distinct lineages, trophectoderm and inner cell mass. Unlike nonmammalian species, mammalian development does not proceed from predetermined factors in the oocyte. Rather, asymmetries arise de novo in the early embryo incorporating cues from cell position, contractility, polarity, and cell-cell contacts. Molecular heterogeneities, including transcripts and non-coding RNAs, have now been characterized as early as the 2-cell stage. However, it's debated whether these early heterogeneities bias cells toward one fate or the other or whether lineage identity arises stochastically at the 16-cell stage. This review summarizes what is known about early blastomere asymmetries and our understanding of lineage allocation in the context of historical models. Preimplantation development is reviewed coupled with what is known about changes in morphology, contractility, and transcription factor networks. The addition of single-cell atlases of human embryos has begun to reveal key differences between human and mouse, including the timing of events and core transcription factors. Furthermore, the recent generation of blastoid models will provide valuable tools to test and understand fate determinants. Lastly, new techniques are reviewed, which may better synthesize existing knowledge with emerging data sets and reconcile models with the regulative capacity unique to the mammalian embryo.

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.

A stochastic model of homeostasis: The roles of noise and nuclear positioning in deciding cell fate

We study a population-based cellular model that starts from a single stem cell that divides stochastically to give rise to either daughter stem cells or differentiated daughter cells. There are three main components in the model: nucleus position, the underlying gene-regulatory network, and stochastic segregation of transcription factors in the daughter cells. The proportion of self-renewal and differentiated cell lines as a function of the nucleus position which in turn decides the plane of cleavage is studied. Both nuclear position and noise play an important role in determining the stem cell genealogies. We have observed both long and short genealogies in model simulation, and these compare well with experimental results from neuroblast and B-cell division. Symmetric divisions are observed in apical nuclei, while asymmetric division occurs when the nucleus is toward the base. In this model, the number of clones decreases over time, although the average clone size increases.

Microtubule-dependent subcellular organisation of pluripotent cells

With the advancement of cutting-edge live imaging technologies, microtubule remodelling has evolved as an integral regulator for the establishment of distinct differentiated cells. However, despite their fundamental role in cell structure and function, microtubules have received less attention when unravelling the regulatory circuitry of pluripotency. Here, we summarise the role of microtubule organisation and microtubule-dependent events required for the formation of pluripotent cells in vivo by deciphering the process of early embryogenesis: from fertilisation to blastocyst. Furthermore, we highlight current advances in elucidating the significance of specific microtubule arrays in in vitro culture systems of pluripotent stem cells and how the microtubule cytoskeleton serves as a highway for the precise intracellular movement of organelles. This Review provides an informed understanding of the intrinsic role of subcellular architecture of pluripotent cells and accentuates their regenerative potential in combination with innovative light-inducible microtubule techniques.

Is there a better evolutionary outcome in a 4-cell tetrahedron embryo?

With the growing understanding of in vitro fertilization and reproductive technology, the magnitude of studies related to embryonic evolution has also increased. The optimization of embryo selection is crucial to minimize the risk of multiple pregnancies and to guarantee successful implantation and pregnancy. On the second day of culture, the four-cell embryo can be shaped into different arrangements, such as tetrahedral and planar. Previous studies have shown that mammalian embryos have a tetrahedral shape and that any deviation from this ideal configuration can negatively affect blastocyst development. A few studies have also found that planar embryos would be linked to negative predictors of success for reaching the blastocyst stage and its good quality. Therefore, it seems that tetrahedral should be preferred over planar-shaped embryos for embryonic transfers, but there is still little understanding and evidence about this subject. Thus, the objective of the present paper was to review the available literature on study tendencies to compare tetrahedral and planar-shaped embryos considering their effect on implantation and pregnancy results.

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.

Cytoskeletal control of early mammalian development

The cytoskeleton - comprising actin filaments, microtubules and intermediate filaments - serves instructive roles in regulating cell function and behaviour during development. However, a key challenge in cell and developmental biology is to dissect how these different structures function and interact in vivo to build complex tissues, with the ultimate aim to understand these processes in a mammalian organism. The preimplantation mouse embryo has emerged as a primary model system for tackling this challenge. Not only does the mouse embryo share many morphological similarities with the human embryo during its initial stages of life, it also permits the combination of genetic manipulations with live-imaging approaches to study cytoskeletal dynamics directly within an intact embryonic system. These advantages have led to the discovery of novel cytoskeletal structures and mechanisms controlling lineage specification, cell-cell communication and the establishment of the first forms of tissue architecture during development. Here we highlight the diverse organization and functions of each of the three cytoskeletal filaments during the key events that shape the early mammalian embryo, and discuss how they work together to perform key developmental tasks, including cell fate specification and morphogenesis of the blastocyst. Collectively, these findings are unveiling a new picture of how cells in the early embryo dynamically remodel their cytoskeleton with unique spatial and temporal precision to drive developmental processes in the rapidly changing in vivo environment.

Blastomere cleavage plane orientation and the tetrahedral formation are associated with increased probability of a good-quality blastocyst for cryopreservation or transfer: a time-lapse study

OBJECTIVE

To determine whether blastomere spatial arrangement in early human embryos is reflective of embryonic potential.

DESIGN

Retrospective analysis of prospectively collected data.

SETTING

Single academic center.

PATIENT(S)

Patients undergoing a single blastocyst transfer.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Developmental kinetics, blastocyst quality, embryo dysmorphisms, and live birth rate.

RESULT(S)

A total of 716 embryos were examined in detail for cleavage plane orientation, blastomere arrangement, and morphokinetic behavior. Tetrahedral (TET) and nontetrahedral embryos (nTET) differed significantly in developmental kinetics. The frequency of dysmorphisms, multinucleation, and irregular chaotic division was higher in nTET embryos. Only 44% of nTET versus 62.9% of TET embryos were scored as top-quality blastocysts. After adjusting for age, our data indicated that having TET embryos significantly increased the odds of having a blastocyst for cryopreservation/transfer (odds ratio, 3.58; confidence interval, 2.42-5.28) when compared with nTET. A total of 164 fresh single ETs were performed with blastocyst-stage embryos. The implantation rate for TET- and nTET-derived blastocysts were similar (64.7% and 62%, respectively). The live birth rate was 55% in both groups. A meridonal first division was noted in 85% of the fresh SET blastocysts.

CONCLUSION(S)

Cleavage plane orientation during the first three divisions appeared to dictate final blastomere spatial arrangement. The TET formation at the four-cell stage was predictive for embryos most likely to develop into good-quality blastocysts for cryopreservation/transfer. Morphokinetic markers of embryo potential were significantly different between TET and nTET embryos.

Nestin-expressing progenitor cells: function, identity and therapeutic implications

The neuroepithelial stem cell protein, or Nestin, is a cytoskeletal intermediate filament initially characterized in neural stem cells. However, current extensive evidence obtained in in vivo models and humans shows presence of Nestin+ cells with progenitor and/or regulatory functions in a number of additional tissues, remarkably bone marrow. This review presents the current knowledge on the role of Nestin in essential stem cell functions, including self-renewal/proliferation, differentiation and migration, in the context of the cytoskeleton. We further discuss the available in vivo models for the study of Nestin+ cells and their progeny, their function and elusive nature in nervous system and bone marrow, and their potential mechanistic role and promising therapeutic value in preclinical models of disease. Future improved in vivo models and detection methods will allow to determine the true essence of Nestin+ cells and confirm their potential application as therapeutic target in a range of diseases.

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.

Mouse blastomeres acquire ability to divide asymmetrically before compaction

The mouse preimplantation embryo generates the precursors of trophectoderm (TE) and inner cell mass (ICM) during the 8- to 16-cell stage transition, when the apico-basal polarized blastomeres undergo divisions that give rise to cells with different fate. Asymmetric segregation of polar domain at 8–16 cell division generate two cell types, polar cells which adopt an outer position and develop in TE and apolar cells which are allocated to inner position as the precursors of ICM. It is still not know when the blastomeres of 8-cell stage start to be determined to undergo asymmetric division. Here, we analyze the frequency of symmetric and asymmetric divisions of blastomeres isolated from 8-cell stage embryo before and after compaction. Using p-Ezrin as the polarity marker we found that size of blastomeres in 2/16 pairs cannot be used as a criterion for distinguishing symmetric and asymmetric divisions. Our results showed that at early 8-cell stage, before any visible signs of cortical polarity, a subset of blastomeres had been already predestined to divide asymmetrically. We also showed that almost all of 8-cell stage blastomeres isolated from compacted embryo divide asymmetrically, whereas in intact embryos, the frequency of asymmetric divisions is significantly lower. Therefore we conclude that in intact embryo the frequency of symmetric and asymmetric division is regulated by cell-cell interactions.

Using Brownian motion to measure shape asymmetry in mesoscopic matter using optical tweezers

We propose a new method for quantifying shape asymmetry on the mesoscopic scale. The method takes advantage of the intrinsic coupling between rotational and translational Brownian motion (RBM and TBM, respectively) which happens in the case of asymmetric particles. We determine the coupling by measuring different correlation functions of the RBM and TBM for single, morphologically different, weakly trapped red blood cells in optical tweezers. The cells have different degrees of asymmetry that are controllably produced by varying the hypertonicity of their aqueous environment. We demonstrate a clear difference in the nature of the correlation functions both qualitatively and quantitatively for three types of cells having a varying degree of asymmetry. This method can have a variety of applications ranging from early stage disease diagnosis to quality control in microfabrication.

ESCs injected into the 8-cell stage mouse embryo modify pattern of cleavage and cell lineage specification

During mouse embryogenesis initial specification of the cell fates depends on the type of division during 8- to 16- and 16- to 32-cell stage transition. A conservative division of a blastomere creates two polar outer daughter cells, which are precursors of the trophectoderm (TE), whereas a differentiative division gives rise to a polar outer cell and an apolar inner (the presumptive inner cell mass - ICM) cell. We hypothesize that the type of division may depend on the interactions between blastomeres of the embryo. To investigate whether modification of these interactions influences divisions, we analyzed the pattern of blastomere division and cell lineage specification in chimeric embryos obtained by injection of a different number of mouse embryonic stem cells (ESCs) into 8-cell embryos. As the ESCs populate only the ICM of the resulting chimeric blastocysts, they emulated in our model additional inner cells. We found that introduction of ESCs decreased the number of inner, apolar blastomeres at the 8- to 16-cell stage transition and reduced the number of ICM cells of host embryo-origin during formation of the blastocyst. Moreover, we showed that the proportion of inner blastomeres and their fate (EPI or PE) in chimeric blastocysts was dependent on the number of ESCs injected. Our results suggest the existence of a regulative mechanism, which links number of inner cells with a proportion of conservative vs. differentiative blastomere divisions during the cleavage and thus dictates their developmental fate.

Polarity in Cell-Fate Acquisition in the Early Mouse Embryo

Establishing polarity is a fundamental part of embryogenesis and can be traced back to the earliest developmental stages. It can be achieved in one of two ways: through the preexisting polarization of germ cells before fertilization or via symmetry breaking after fertilization. In mammals, it seems to be the latter, and we will discuss the various cytological and molecular events that lead up to this event, its mechanisms and the consequences. In mammals, the first polarization event occurs in the preimplantation period, when the embryo is but a cluster of cells, free-floating in the oviduct. This provides a unique, autonomous system to study the de novo polarization that is essential to life. In this review, we will cover modern and past studies on the polarization of the early embryo, using the mouse as a model system, as well as hypothesizing the potential implications and functions of the biological events involved.

Mouse Embryo Compaction

Compaction is a critical first morphological event in the preimplantation development of the mammalian embryo. Characterized by the transformation of the embryo from a loose cluster of spherical cells into a tightly packed mass, compaction is a key step in the establishment of the first tissue-like structures of the embryo. Although early investigation of the mechanisms driving compaction implicated changes in cell-cell adhesion, recent work has identified essential roles for cortical tension and a compaction-specific class of filopodia. During the transition from 8 to 16 cells, as the embryo is compacting, it must also make fundamental decisions regarding cell position, polarity, and fate. Understanding how these and other processes are integrated with compaction requires further investigation. Emerging imaging-based techniques that enable quantitative analysis from the level of cell-cell interactions down to the level of individual regulatory molecules will provide a greater understanding of how compaction shapes the early mammalian embryo.

Lineage specification in the mouse preimplantation embryo

During mouse preimplantation embryo development, totipotent blastomeres generate the first three cell lineages of the embryo: trophectoderm, epiblast and primitive endoderm. In recent years, studies have shown that this process appears to be regulated by differences in cell-cell interactions, gene expression and the microenvironment of individual cells, rather than the active partitioning of maternal determinants. Precisely how these differences first emerge and how they dictate subsequent molecular and cellular behaviours are key questions in the field. As we review here, recent advances in live imaging, computational modelling and single-cell transcriptome analyses are providing new insights into these questions.

Polarity and cell division orientation in the cleavage embryo: from worm to human

Cleavage is a period after fertilization, when a 1-cell embryo starts developing into a multicellular organism. Due to a series of mitotic divisions, the large volume of a fertilized egg is divided into numerous smaller, nucleated cells—blastomeres. Embryos of different phyla divide according to different patterns, but molecular mechanism of these early divisions remains surprisingly conserved. In the present paper, we describe how polarity cues, cytoskeleton and cell-to-cell communication interact with each other to regulate orientation of the early embryonic division planes in model animals such as Caenorhabditis elegans, Drosophila and mouse. We focus particularly on the Par pathway and the actin-driven cytoplasmic flows that accompany it. We also describe a unique interplay between Par proteins and the Hippo pathway in cleavage mammalian embryos. Moreover, we discuss the potential meaning of polarity, cytoplasmic dynamics and cell-to-cell communication as quality biomarkers of human embryos.