Site of the previous meiotic division defines cleavage orientation in the mouse embryo

Blastomere size in the human 2-cell embryo predicts the division order that leads to imbalanced lineage contribution to the future body

Retrospective tracing of somatic mutations predicted that most cells in the human body could be traced back to a single cell of the 2-cell stage embryo. Accordingly, a recent prospective study of the developmental trajectory of blastomeres in human embryos confirmed that progeny of the first 2-cell stage blastomere to divide generates more epiblast cells (future body). How the 2-cell blastomeres differ is unknown. Here, we show that 2-cell stage blastomeres in human embryos are asymmetric; they differ in size and the bigger blastomere divides first to 4-cell stage. We propose that this asymmetry might originate differences in cell fate.

Maternal factors regulating preimplantation development in mice

Mammalian embryogenesis depends on maternal factors accumulated in eggs prior to fertilization and on placental transfers later in gestation. In this review, we focus on initial events when the organism has insufficient newly synthesized embryonic factors to sustain development. These maternal factors regulate preimplantation embryogenesis both uniquely in pronuclear formation, genome reprogramming and cell fate determination and more universally in regulating cell division, transcription and RNA metabolism. Depletion, disruption or inappropriate persistence of maternal factors can result in developmental defects in early embryos. To better understand the origins of these maternal effects, we include oocyte maturation processes that are responsible for their production. We focus on recent publications and reference comprehensive reviews that include earlier scientific literature of early mouse development.

Fertilization and Cleavage Axes Differ In Primates Conceived By Conventional (IVF) Versus Intracytoplasmic Sperm Injection (ICSI)

With nearly ten million babies conceived globally, using assisted reproductive technologies, fundamental questions remain; e.g., How do the sperm and egg DNA unite? Does ICSI have consequences that IVF does not? Here, pronuclear and mitotic events in nonhuman primate zygotes leading to the establishment of polarity are investigated by multidimensional time-lapse video microscopy and immunocytochemistry. Multiplane videos after ICSI show atypical sperm head displacement beneath the oocyte cortex and eccentric para-tangential pronuclear alignment compared to IVF zygotes. Neither fertilization procedure generates incorporation cones. At first interphase, apposed pronuclei align obliquely to the animal-vegetal axis after ICSI, with asymmetric furrows assembling from the male pronucleus. Furrows form within 30° of the animal pole, but typically, not through the ICSI injection site. Membrane flow drives polar bodies and the ICSI site into the furrow. Mitotic spindle imaging suggests para-tangential pronuclear orientation, which initiates random spindle axes and minimal spindle:cortex interactions. Parthenogenetic pronuclei drift centripetally and assemble astral spindles lacking cortical interactions, leading to random furrows through the animal pole. Conversely, androgenotes display cortex-only pronuclear interactions mimicking ICSI. First cleavage axis determination in primates involves dynamic cortex-microtubule interactions among male pronuclei, centrosomal microtubules, and the animal pole, but not the ICSI site.

Double Strand Break DNA Repair occurs via Non-Homologous End-Joining in Mouse MII Oocytes

The unique biology of the oocyte means that accepted paradigms for DNA repair and protection are not of direct relevance to the female gamete. Instead, preservation of the integrity of the maternal genome depends on endogenous protein stores and/or mRNA transcripts accumulated during oogenesis. The aim of this study was to determine whether mature (MII) oocytes have the capacity to detect DNA damage and subsequently mount effective repair. For this purpose, DNA double strand breaks (DSB) were elicited using the topoisomerase II inhibitor, etoposide (ETP). ETP challenge led to a rapid and significant increase in DSB (P = 0.0002) and the consequential incidence of metaphase plate abnormalities (P = 0.0031). Despite this, ETP-treated MII oocytes retained their ability to participate in in vitro fertilisation, though displayed reduced developmental competence beyond the 2-cell stage (P = 0.02). To account for these findings, we analysed the efficacy of DSB resolution, revealing a significant reduction in DSB lesions 4 h post-ETP treatment. Notably, this response was completely abrogated by pharmacological inhibition of key elements (DNA-PKcs and DNA ligase IV) of the canonical non-homologous end joining DNA repair pathway, thus providing the first evidence implicating this reparative cascade in the protection of the maternal genome.

Tracing the origin of heterogeneity and symmetry breaking in the early mammalian embryo

A fundamental question in developmental and stem cell biology concerns the origin and nature of signals that initiate asymmetry leading to pattern formation and self-organization. Instead of having prominent pre-patterning determinants as present in model organisms (worms, sea urchin, frog), we propose that the mammalian embryo takes advantage of more subtle cues such as compartmentalized intracellular reactions that generate micro-scale inhomogeneity, which is gradually amplified over several cellular generations to drive pattern formation while keeping developmental plasticity. It is therefore possible that by making use of compartmentalized information followed by its amplification, mammalian embryos would follow general principle of development found in other organisms in which the spatial cue is more robustly presented.

The Role of Maternal-Effect Genes in Mammalian Development: Are Mammalian Embryos Really an Exception?

The essential contribution of multiple maternal factors to early mammalian development is rapidly altering the view that mammals have a unique pattern of development compared to other species. Currently, over 60 maternal-effect mutations have been described in mammalian systems, including critical determinants of pluripotency. This data, combined with the evidence for lineage bias and differential gene expression in early blastomeres, strongly suggests that mammalian development is to some extent mosaic from the four-cell stage onward.

Epigenetic Control of Early Mouse Development

Although the genes sequentially transcribed in the mammalian embryo prior to implantation have been identified, understanding of the molecular processes ensuring this transcription is still in development. The genomes of the sperm and egg are hypermethylated, hence transcriptionally silent. Their union, in the prepared environment of the egg, initiates their epigenetic genomic reprogramming into a totipotent zygote, in which the genome gradually becomes transcriptionally activated. During gametogenesis, sex-specific processes result in sperm and eggs with disparate epigenomes, both of which require drastic reprogramming to establish the totipotent genome of the zygote and the pluripotent inner cell mass of the blastocyst. Herein, we describe the factors, DNA and histone modifications, activation and repression of retrotransposons, and cytoplasmic localizations, known to influence the activation of the mammalian genome at the initiation of new life.

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.

Preformation Versus Epigenesis in Early Mammalian Development

Whether or not early mammalian development results from preformation or epigenesis remains an unresolved issue. Evidence for or against either is weak, inconclusive, and often misinterpreted. Yet, one can parsimoniously conceptualize formation of the mouse blastocyst as a series of random, stochastic events stemming from initial and sequential small asymmetries in egg, zygote, and cleavage stages. Differential compartmentalized gene expression does not lead but follows the morphogenesis and cell fate allocation in the mammalian blastocyst.

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.

Developmental bias in cleavage-stage mouse blastomeres

BACKGROUND

The cleavage-stage mouse embryo is composed of superficially equivalent blastomeres that will generate both the embryonic inner cell mass (ICM) and the supportive trophectoderm (TE). However, it remains unsettled whether the contribution of each blastomere to these two lineages can be accounted for by chance. Addressing the question of blastomere cell fate may be of practical importance, because preimplantation genetic diagnosis requires removal of blastomeres from the early human embryo. To determine whether blastomere allocation to the two earliest lineages is random, we developed and utilized a recombination-mediated, noninvasive combinatorial fluorescent labeling method for embryonic lineage tracing.

RESULTS

When we induced recombination at cleavage stages, we observed a statistically significant bias in the contribution of the resulting labeled clones to the trophectoderm or the inner cell mass in a subset of embryos. Surprisingly, we did not find a correlation between localization of clones in the embryonic and abembryonic hemispheres of the late blastocyst and their allocation to the TE and ICM, suggesting that TE-ICM bias arises separately from embryonic-abembryonic bias. Rainbow lineage tracing also allowed us to demonstrate that the bias observed in the blastocyst persists into postimplantation stages and therefore has relevance for subsequent development.

CONCLUSIONS

The Rainbow transgenic mice that we describe here have allowed us to detect lineage-dependent bias in early development. They should also enable assessment of the developmental equivalence of mammalian progenitor cells in a variety of tissues.

Assessing pre-implantation embryo development in mice provides a rationale for understanding potential adverse effects of ART and PGD procedures

Although the molecular events controlling human pre-implantation development remain unclear, mechanisms have been identified by analyzing these stages in mice. Through this approach, considerable insight has been gained into the events that operate to determine the first two cell fate decisions, occurring from zygote formation to the blastocyst prior to implantation. These mechanisms are related to cell polarization, cell division, cell-cell contact, and cell spatial position. Two developmental stages are essential for these processes to proceed adequately. Firstly, the second polar body must anchor to the external membrane during the first mitotic divisions of the embryo as its position is strongly biased to determine the plane of polarity. This in turn has important influence on the fate of the early blastomeres. Secondly, in the transition from the 8- to 16-cell stage, the cells that will form the inner cell mass are determined. Moreover, analyses performed on human oocytes and embryos have identified similar processes to those reported in mice and thus are evolutionarily conserved. Therefore, the understanding of mice pre-implantation embryo development provides a rationale to interpret current results of potential long-term adverse outcomes of Assisted Reproductive Technologies and Pre-implantation Genetic Diagnosis (PGD).

Preimplantation mouse embryo: developmental fate and potency of blastomeres

During the past decade we have witnessed great progress in the understanding of cellular, molecular, and epigenetic aspects of preimplantation mouse development. However, some of the issues, especially those regarding the nature and regulation of mouse development, are still unresolved and controversial and raise heated discussion among mammalian embryologists. This chapter presents different standpoints and various research approaches aimed at examining the fate and potency of cells (blastomeres) of mouse preimplantation embryo. In dealing with this subject, it is important to recognize the difference between the fate of blastomere and the prospective potency of blastomere, with the first being its contribution to distinct tissues during normal development, and the second being a full range of its developmental capabilities, which can be unveiled only by experimental perturbation of the embryo. Studies of the developmental potential and the fate of blastomeres are of the utmost importance as they may lead to future clinical application in reproductive and regenerative medicine.

From mouse egg to mouse embryo: polarities, axes, and tissues

This review describes the three classical models (mosaic, positional, and polarization) proposed to explain blastocyst formation and summarizes the evidence concerning them. It concludes that the polarization model incorporates elements of the other two models and best explains most known information. I discuss key requirements of a molecular basis for the generation and stabilization of polarity and identify ezrin/E-cadherin, PAR proteins, and Cdx2 as plausible key molecular players. I also discuss the idea of a network process operating to build cell allocations progressively into committed differences. Finally, this review critically considers the possibility of developmental information being encoded within the oocyte and zygote. No final decision can be reached on a mechanism of action underlying any encoded information, but a cell interaction process model is preferred over one that relies solely on differential inheritance.

Epidemiology of acephalus/acardius monozygotic twins: new insights into an epigenetic causal hypothesis

Apart from a series of 10 acephalus/acardius (Ac/Ac) cases described from a pathological point of view, and the analysis of a review of published cases, we have been unable to find any epidemiological studies on Ac/Ac. Using data from the Spanish Collaborative Study of Congenital Malformations (ECEMC), we present here what seems to be the first epidemiological analysis of a consecutive series of the Ac/Ac type of monozygotic twins (MZT). Among a total of 2,281,604 consecutive births, 11 cases of Ac/Ac MZT were detected, giving a frequency of 0.48 per 100,000 births. However, we consider the period 1980-1985 as the baseline for our data, as in this period voluntary termination of pregnancy was not possible in Spain, and the frequency of Ac/Ac MZT was 0.49 per 100,000 births. Nonetheless, this frequency should be considered as a minimal estimation. The characteristics of these Ac/Ac cases indicate that they are more frequent in males (sex ratio 2.67). In addition, gestational age in Ac/Ac cases was 2.41 and 3.12 weeks lower than in malformed and control twins, respectively. Similarly, their mothers are 4.54 and 4.68 years younger than mothers of separate malformed and control twins, respectively. To understand the biological basis behind the occurrence of MZT in the context of recent observations, we evaluate the hypothesis that the epigenetic processes involved in the early cleavage of the embryo, and in blastocyst formation during development, may be implicated in twinning.

Assisted fertilization and embryonic axis formation in higher primates

In naturally fertilized embryos of various organisms, the spermatozoon provides a localized cue to initiate early embryonic patterning. In mice, the sperm entry point (SEP) may reorient the first cleavage division, which separates the zygote into two halves that follow distinct fates. However, it is unknown whether the mechanical injection of spermatozoa into an oocyte by intracytoplasmic sperm injection (ICSI), a technique commonly used in human assisted reproduction, possesses such a role. Rhesus macaque embryos fertilized by ICSI were examined in order to determine the consequences of placing the spermatozoon at specific positions in the ooplasm and whether this can provide new information about patterning in mammalian eggs. The SEP specified by the injected spermatozoa was most often localized near the first cleavage plane and was mainly distributed along the boundary zone that separates the embryonic and abembryonic parts of the monkey blastocyst. Moreover, the ICSI data, when compared with naturally fertilized mouse embryos, showed a similar outcome in terms of cleavage axes and first embryonic axis specification. As there are no studies to date regarding sperm entry in human oocytes and its influence on embryonic development, this investigation using the rhesus macaque as a clinical model is noteworthy.

Role of Cdx2 and cell polarity in cell allocation and specification of trophectoderm and inner cell mass in the mouse embryo

Genesis of the trophectoderm and inner cell mass (ICM) lineages occurs in two stages. It is initiated via asymmetric divisions of eight- and 16-cell blastomeres that allocate cells to inner and outer positions, each with different developmental fates. Outside cells become committed to the trophectoderm at the blastocyst stage through Cdx2 activity, but here we show that Cdx2 can also act earlier to influence cell allocation. Increasing Cdx2 levels in individual blastomeres promotes symmetric divisions, thereby allocating more cells to the trophectoderm, whereas reducing Cdx2 promotes asymmetric divisions and consequently contribution to the ICM. Furthermore, both Cdx2 mRNA and protein levels are heterogeneous at the eight-cell stage. This heterogeneity depends on cell origin and has developmental consequences. Cdx2 expression is minimal in cells with unrestricted developmental potential that contribute preferentially to the ICM and is maximal in cells with reduced potential that contribute more to the trophectoderm. Finally, we describe a mutually reinforcing relationship between cellular polarity and Cdx2: Cdx2 influences cell polarity by up-regulating aPKC, but cell polarity also influences Cdx2 through asymmetric distribution of Cdx2 mRNA in polarized blastomeres. Thus, divisions generating inside and outside cells are truly asymmetric with respect to cell fate instructions. These two interacting effects ensure the generation of a stable outer epithelium by the blastocyst stage.

Dynamic rearrangement of surface proteins is essential for cytokinesis

Cytokinesis is a complex process that involves dynamic cortical rearrangement. Our recent time-lapse recordings of the mouse egg unexpectedly revealed a high motility of the second polar body (2pb). Experiments to address its underlying mechanism show that neither mechanical compression by the zona pellucida nor the connection via the mid-body is required for the 2pb movement. Time-lapse recordings establish that the 2pb moves together with the cell membrane. These recordings, in which cell surface proteins are labeled with fluorescent latex-microbeads or monovalent antibodies against whole mouse proteins, indicate that the majority of the surface proteins dynamically accumulate in the cleavage furrow at every cell division. Comparable dynamics of the cell surface proteins, and specifically of E-cadherin, are also observed in cultured epithelial cells. The surface protein dynamics are closely correlated with, and dependent on, those of the underlying cortical actin. The cortical actin network may form a scaffold for membrane proteins and thereby transfer them during contractile ring formation toward the cleavage furrow. Immobilization of surface proteins by tetravalent lectin-mediated crosslinking results in the failure of cleavage, demonstrating that the observed protein dynamics are essential for cytokinesis. We propose that dynamic rearrangement of the cell surface proteins is a common feature of cytokinesis, playing a key role in modifying the mechanical properties of the cell membrane during cortical ingression.

Axial specification in mice: ten years of advances and controversies

The definitive axes of the mouse embryo can be unequivocally identified in embryos dissected at 5.5 days of gestation. However, how and when are these axes established remains an open question. At pre-implantation stages, different approaches have been aimed at determining if events occurring in the zygote influence the geometrical arrangement of the blastocyst. An intense debate has focused on whether the mouse embryo is a pre-patterned or a regulative structure. At post-implantation stages, the efforts have been concentrated in understanding how extra-embryonic tissues affect the formation of the primitive streak, the caudal marker of the anteroposterior axis. Here I summarize the last 10 years of research in this field.

Subcellular distribution of mitochondrial ribosomal RNA in the mouse oocyte and zygote

Mitochondrial ribosomal RNAs (mtrRNAs) have been reported to translocate extra-mitochondrially and localize to the germ cell determinant of oocytes and zygotes in some metazoa except mammals. To address whether the mtrRNAs also localize in the mammals, expression and distribution of mitochondrion-encoded RNAs in the mouse oocytes and zygotes was examined by whole-mount in situ hybridization (ISH). Both 12S and 16S rRNAs were predominantly distributed in the animal hemisphere of the mature oocyte. This distribution pattern was rearranged toward the second polar body in zygotes after fertilization. The amount of mtrRNAs decreased around first cleavage, remained low during second cleavage and increased after third cleavage. Staining intensity of the 12S rRNA was weaker than that of the 16S rRNA throughout the examined stages. Similar distribution dynamics of the 16S rRNA was observed in strontium-activated haploid parthenotes, suggesting the distribution rearrangement does not require a component from sperm. The distribution of 16S rRNAs did not coincide with that of mitochondrion-specific heat shock protein 70, suggesting that the mtrRNA is translocated from mitochondria. The ISH-scanning electron microscopy confirms the extra-mitochondrial mtrRNA in the mouse oocyte. Chloramphenicol (CP) treatment of late pronuclear stage zygotes perturbed first cleavage as judged by the greater than normal disparity in size of blastomeres of 2-cell conceptuses. Two-third of the CP-treated zygotes arrested at either 2-cell or 3-cell stage even after the CP was washed out. These findings indicate that the extra-mitochondrial mtrRNAs are localized in the mouse oocyte and implicated in correct cytoplasmic segregation into blastomeres through cleavages of the zygote.

Regulation of dynamic events by microfilaments during oocyte maturation and fertilization

Actin filaments (microfilaments) regulate various dynamic events during oocyte meiotic maturation and fertilization. In most species, microfilaments are not required for germinal vesicle breakdown and meiotic spindle formation, but they mediate peripheral nucleus (chromosome) migration, cortical spindle anchorage, homologous chromosome separation, cortex development/maintenance, polarity establishment, and first polar body emission during oocyte maturation. Peripheral cortical granule migration is controlled by microfilaments, while mitochondria movement is mediated by microtubules. During fertilization, microfilaments are involved in sperm incorporation, spindle rotation (mouse), cortical granule exocytosis, second polar body emission and cleavage ring formation, but are not required for pronuclear apposition (except for the mouse). Many of the events are driven by the dynamic interactions between myosin and actin filaments whose polymerization is regulated by RhoA, Cdc42, Arp2/3 and other signaling molecules. Studies have also shown that oocyte cortex organization and polarity formation mediated by actin filaments are regulated by mitogen-activated protein kinase, myosin light-chain kinase, protein kinase C and its substrate p-MARKS as well as PAR proteins. The completion of several dynamic events, including homologous chromosome separation, spindle anchorage, spindle rotation, vesicle organelle transport and pronuclear apposition (mouse), requires interactions between microfilaments and microtubules, but determination of how the two systems of the cytoskeleton precisely cross-link, and which proteins link microfilaments to microtubules to perform functions in eggs, requires further studies. Finally, the meaning of microfilament-mediated oocyte polarity versus embryo polarity and embryo development in different species (Drosophila, Xenopus and mouse) is discussed.

The first cell-fate decisions in the mouse embryo: destiny is a matter of both chance and choice

Development of the early mouse embryo has always been classified as regulative, meaning that when parts or blastomeres of the embryo are isolated they change their developmental fate and can even reconstruct the whole. However, regulative development does not mean that, in situ, these parts or blastomeres are equivalent; it does not mean that the early mammalian embryo is a ball of identical cells without any bias. Regulative development simply means that whatever bias the regions of the embryo might have they still remain flexible and can respond to experimental interference by changes of fate. This realization -- that regulative development and patterning can co-exist -- has led to a renaissance of interest in the first days of development of the mouse embryo, and several laboratories have provided evidence for some early bias. Now the challenge is to gain some understanding of the molecular basis of this bias.

The aPKC-PAR-6-PAR-3 cell polarity complex localizes to the centrosome attracting body, a macroscopic cortical structure responsible for asymmetric divisions in the early ascidian embryo

Posterior blastomeres of 8-cell stage ascidian embryos undergo a series of asymmetric divisions that generate cells of unequal sizes and segregate muscle from germ cell fates. These divisions are orchestrated by a macroscopic cortical structure, the ;centrosome attracting body' (CAB) which controls spindle positioning and distribution of mRNA determinants. The CAB is composed of a mass of cortical endoplasmic reticulum containing mRNAs (the cER-mRNA domain) and an electron dense matrix, but little is known about its precise structure and functions. We have examined the ascidian homologues of PAR proteins, known to regulate polarity in many cell types. We found that aPKC, PAR-6 and PAR-3 proteins, but not their mRNAs, localize to the CAB during the series of asymmetric divisions. Surface particles rich in aPKC concentrate in the CAB at the level of cortical actin microfilaments and form a localized patch sandwiched between the plasma membrane and the cER-mRNA domain. Localization of aPKC to the CAB is dependent on actin but not microtubules. Both the aPKC layer and cER-mRNA domain adhere to cortical fragments prepared from 8-cell stage embryos. Astral microtubules emanating from the proximal centrosome contact the aPKC-rich cortical domain. Our observations indicate that asymmetric division involves the accumulation of the aPKC-PAR-6-PAR-3 complex at the cortical position beneath the pre-existing cER-mRNA domain.

Roadmap to embryo implantation: clues from mouse models

Implantation involves an intricate discourse between the embryo and uterus and is a gateway to further embryonic development. Synchronizing embryonic development until the blastocyst stage with the uterine differentiation that takes place to produce the receptive state is crucial to successful implantation, and therefore to pregnancy outcome. Although implantation involves the interplay of numerous signalling molecules, the hierarchical instructions that coordinate the embryo-uterine dialogue are not well understood. This review highlights our knowledge about the molecular development of preimplantation and implantation and the future challenges of the field. A better understanding of periimplantation biology could alleviate female infertility and help to develop novel contraceptives.

PAR-1 and the microtubule-associated proteins CLASP2 and dynactin-p50 have specific localisation on mouse meiotic and first mitotic spindles

The site of second meiotic division, marked by the second polar body, is an important reference point in the early mouse embryo. To study its formation, we look at the highly asymmetric meiotic divisions. For extrusion of the small polar bodies during meiosis, the spindles must be located cortically. The positioning of meiotic spindles is known to involve the actin cytoskeleton, but whether microtubules are also involved is not clear. In this study we investigated the patterns of localisation of microtubule regulatory proteins in mouse oocytes. PAR-1 is a member of the PAR (partitioning-defective) family with known roles in regulation of microtubule stability and spindle positioning in other model systems. Here we show its specific localisation on mouse meiotic and first mitotic spindles. In addition, the microtubule-associated proteins CLASP2 (a CLIP associating protein) and dynactin-p50 are found on kinetochores and a subset of microtubule-organising centres. Thus we show specific localisation of microtubule regulatory proteins in mouse oocytes, which could indicate roles in meiotic spindle organisation.

Cleavage pattern and emerging asymmetry of the mouse embryo

Early mammalian development is regulative - it is flexible and responsive to experimental intervention. This flexibility could be explained if embryogenesis were originally completely unbiased and disordered; order and determination of cells only arising later. Alternatively, regulative behaviour could be consistent with the embryo having some order or bias from the very beginning, with inflexibility and cell determination increasing steadily over time. Recent evidence supports the second view and indicates that the sequence and the orientations of cell divisions help to build the first asymmetries.

An investigation of the origin and significance of bilateral symmetry of the pronuclear zygote in the mouse

BACKGROUND

Preliminary observations revealed that advanced zygotes of the PO strain mouse are often bilaterally symmetrical, and suggested that both the plane of first cleavage and features of the blastocyst bear a consistent relationship to the zygote's bilateral plane.

METHODS

Spaced oil drops were injected into the zona pellucida to delineate the bilateral plane in pronuclear zygotes, and a distinct cluster of drops then placed over the second polar body. Such non-invasive marking was combined with gelation of the perivitelline space to prevent rotation of the zygotes within the zona pellucida.

RESULTS

Nearly two-thirds of advanced pronuclear stage zygotes were bilaterally symmetrical and, regardless of whether first cleavage was meridional, it was almost invariably orthogonal to the bilateral plane. Moreover, both the axis of polarity and bilateral plane of the blastocyst bore a consistent relationship to the zygote's bilateral plane. Haploid parthenotes also exhibited bilateral symmetry, although in the absence of fertilization, first cleavage was less consistently orthogonal to the bilateral plane.

CONCLUSIONS

Bilateral symmetry may be an intrinsic property of the oocyte that is induced by its activation and, from the reproducible way it maps on both the 2-cell conceptus and blastocyst, seems to play a role in early patterning.

Cell polarity during folliculogenesis and oogenesis

Polarity is an important aspect of oogenesis and early development for many animal groups, but only recently it has become relevant to the study of mammals. Mammalian oocyte development occurs through tight coordination and interaction between all ovarian structures. In fact, bi-directional communication between the oocyte and its companion granulosa cells (GC) in the ovarian follicle seems essential for GC proliferation, differentiation, and production of a functional female gamete. The transzonal projections (TZP), which are specialized extensions from granulosa cells that terminate on the oolema after crossing the zona pellucida, are major structural components necessary for oocyte-GC interaction. Granulosa cell polarity seems to be a necessary requisite for appropriate function of TZP, and the role of FSH as modulator of a polarized phenotype on GC is discussed. This article also discusses oocyte polarity with special reference to the partial loss of polarity that occurs during in-vitro oocyte maturation and possible implications in the modulation of oocyte competencies. Cytoskeletal markers that may account for oocyte quality were defined and found to be distinct in in-vivo and in-vitro matured oocytes. Implications of partial loss of oocyte polarity during in-vitro maturation, reflected by distinct distribution of these markers, are further discussed. It is also proposed that expression of both somatic and germ cell polarity in the ovarian follicle will ultimately determine acquisition of meiotic, fertilization and developmental competences by the oocyte.

Determination of first cleavage plane: the relationships between the orientation of the mitotic apparatus for first cleavage and the position of meiotic division-related structures in starfish eggs

In order to understand when the orientation of the first cleavage plane is fixed along the animal-vegetal axis in starfish eggs, the behavior of the sperm aster was examined by indirect immunofluorescence staining. After duplication, the sperm aster organizes the mitotic apparatus for first cleavage perpendicular to the cleavage plane. The sperm aster located in the egg periphery just after fertilization and moved to the site close to the animal pole rather than the egg center by meiosis II. At early metaphase II, duplication of the sperm aster was detected but the axis of the resultant sperm diaster randomly pointed. Subsequently, its axis had already turned perpendicular to the animal-vegetal axis before pronucleus fusion. These results indicate that the orientation processes of the sperm diaster consist of positioning before its duplication and successive determining its azimuth. Furthermore, the azimuth and position of the mitotic apparatus for first cleavage did not change by shifting or eliminating the meiotic division-related structures such as the germinal vesicle, meiotic spindle, and female pronucleus by micromanipulation. These results show that none of them determines the first cleavage plane. Therefore, we discuss the pointing mechanism of the first cleavage plane without the influence of these meiotic division-related structures.

Spatial arrangement of individual 4-cell stage blastomeres and the order in which they are generated correlate with blastocyst pattern in the mouse embryo

In the unperturbed development of the mouse embryo one of the 2-cell blastomeres tends to contribute its progeny predominantly to the embryonic and the other to the abembryonic part of the blastocyst. However, a significant minority of embryos (20-30%) do not show this correlation. In this study, we have used non-invasive lineage tracing to determine whether development of blastocyst pattern shows any correlation with the orientation and order of the second cleavage divisions that result in specific positioning of blastomeres at the 4-cell stage. Although the orientation and order of the second cleavages are not predetermined, in the great majority (80%) of embryos the spatial arrangement of 4-cell blastomeres is consistent with one of the second cleavages occurring meridionally and the other equatorially or obliquely with respect to the polar body. In such cleaving embryos, one of the 2-cell stage blastomeres tends to contribute to embryonic while the other contributes predominantly to abembryonic part of the blastocyst. Thus, in these embryos the outcome of the first cleavage tends to correlate with the orientation of the blastocyst embryonic-abembryonic axis. However, the order of blastomere divisions predicts a specific polarity for this axis only when the earlier 2-cell blastomere to divide does so meridionally. In contrast to the above two groups, in those embryos in which both second cleavage divisions occur in a similar orientation, either meridionally or equatorially, we do not observe any tendency for the 2-cell blastomeres to contribute to specific blastocyst parts. We find that all these groups of embryos develop to term with similar success, with the exception of those in which both second cleavage divisions occur equatorially whose development can be compromised. We conclude that the orientations and order of the second cleavages are not predetermined; they correlate with the development of blastocyst patterning; and that the majority, but not all, of these cleavage patterns allow equally successful development.

The GAPs, GEFs, and GDIs of heterotrimeric G-protein alpha subunits

The heterotrimeric G-protein alpha subunit has long been considered a bimodal, GTP-hydrolyzing switch controlling the duration of signal transduction by seven-transmembrane domain (7TM) cell-surface receptors. In 1996, we and others identified a superfamily of “regulator of G-protein signaling” (RGS) proteins that accelerate the rate of GTP hydrolysis by Gα subunits (dubbed GTPase-accelerating protein or “GAP” activity). This discovery resolved the paradox between the rapid physiological timing seen for 7TM receptor signal transduction in vivo and the slow rates of GTP hydrolysis exhibited by purified Gα subunits in vitro. Here, we review more recent discoveries that have highlighted newly-appreciated roles for RGS proteins beyond mere negative regulators of 7TM signaling. These new roles include the RGS-box-containing, RhoA-specific guanine nucleotide exchange factors (RGS-RhoGEFs) that serve as Gα effectors to couple 7TM and semaphorin receptor signaling to RhoA activation, the potential for RGS12 to serve as a nexus for signaling from tyrosine kinases and G-proteins of both the Gα and Ras-superfamilies, the potential for R7-subfamily RGS proteins to couple Gα subunits to 7TM receptors in the absence of conventional Gβγ dimers, and the potential for the conjoint 7TM/RGS-box Arabidopsis protein AtRGS1 to serve as a ligand-operated GAP for the plant Gα AtGPA1. Moreover, we review the discovery of novel biochemical activities that also impinge on the guanine nucleotide binding and hydrolysis cycle of Gα subunits: namely, the guanine nucleotide dissociation inhibitor (GDI) activity of the GoLoco motif-containing proteins and the 7TM receptor-independent guanine nucleotide exchange factor (GEF) activity of Ric‑8/synembryn. Discovery of these novel GAP, GDI, and GEF activities have helped to illuminate a new role for Gα subunit GDP/GTP cycling required for microtubule force generation and mitotic spindle function in chromosomal segregation.

The developmental origins of mammalian oocyte polarity

Drosophila has been an excellent model system to study the cell and molecular determinants of oocyte axis specification, a problem which is little known in mammalian species. Recent evidence supports the notion that mammalian oocytes utilize axis-orienting properties during the course of oogenesis. Among these, axis specification in relation to the oocyte cortex, germinal vesicle (GV) position, anchoring of GV and spindle, and patterning of follicle cell/oocyte attachments are proposed as conserved features of oogenesis in mammals that may be important to the survival and development of the preimplantation embryo.

First cell fate decisions and spatial patterning in the early mouse embryo

A growing body of evidence indicates that although the early mouse embryo retains flexibility in responding to perturbations, its patterning is initiated at the earliest developmental stages. There are a few spatial cues that are able to influence the pattern of cleavage divisions: one of these lies in the vicinity of the previous meiotic division, the second is associated with the sperm entry and, related to this, the third is the cell shape. Furthermore, the first cleavage separates the zygote into two cells that tend to follow distinguishable fates: one contributes mainly to the embryonic part of the blastocyst, and the other to the abembryonic. The cumulative effect of the early asymmetries generated through cleavage might lead to asymmetric interactions between the first lineages of cells. This could influence development of patterning after implantation. These early polarity cues serve to bias patterning and not as definitive determinants.

The first cleavage of the mouse zygote predicts the blastocyst axis

One of the unanswered questions in mammalian development is how the embryonic-abembryonic axis of the blastocyst is first established. It is possible that the first cleavage division contributes to this process, because in most mouse embryos the progeny of one two-cell blastomere primarily populate the embryonic part of the blastocyst and the progeny of its sister populate the abembryonic part. However, it is not known whether the embryonic-abembryonic axis is set up by the first cleavage itself, by polarity in the oocyte that then sets the first cleavage plane with respect to the animal pole, or indeed whether it can be divorced entirely from the first cleavage and established in relation to the animal pole. Here we test the importance of the orientation of the first cleavage by imposing an elongated shape on the zygote so that the division no longer passes close to the animal pole, marked by the second polar body. Non-invasive lineage tracing shows that even when the first cleavage occurs along the short axis imposed by this experimental treatment, the progeny of the resulting two-cell blastomeres tend to populate the respective embryonic and abembryonic parts of the blastocyst. Thus, the first cleavage contributes to breaking the symmetry of the embryo, generating blastomeres with different developmental characteristics.

Mitotic Spindles and Cleavage Planes Are Oriented Randomly in the Two-Cell Mouse Embryo

Most experimental embryological studies performed on the early mouse embryo have led to the conclusion that there are no mosaically distributed developmental determinants in the zygote and early embryo (for example see [1-6]). It has been suggested recently that "the cleavage pattern of the early mouse embryo is not random and that the three-dimensional body plan is pre-patterned in the egg" (in [7] for review see [8-10]). Two major spatial cues influencing the pattern of cleavage divisions have been proposed: the site of the second meiotic division [11, 12] and the sperm entry point [13-14], although the latter is controversial [15-17]. An implication of this hypothesis is that the orientations of the first few cleavage divisions are stereotyped. Such a define cleavage pattern, leading to the segregation of developmental determinants, is observed in many species [18]. Recently, it was shown that the first cleavage plane is not predetermined but defined by the topology of the two apposing pronuclei [19]. Because the position of the female pronucleus is dependent upon the site of polar body extrusion and the position of the male pronuclei is dependent upon the sperm entry point [19-20], this observation leaves open the possibility that the sperm may provide some kind of directionality [7]. But, even if asymmetries were set up only after fertilization, a stereotyped cleavage pattern should take place during the following cleavage divisions. Thus, we studied the cleavage pattern of two-cell embryos by videomicroscopy to distinguish between the two hypotheses. After the mitotic spindle formed, its orientation did not change until cleavage. During late metaphase and anaphase, the spindle poles appear to be anchored to the cortex through astral microtubules and PARD6a. Only at the time of cleavage, during late anaphase, do the forming daughter cells change their relative positions. These studies show that cleavage planes are oriented randomly in two-cell embryos. This argues against a prepatterning of the mouse embryo before compaction.

Downregulation of Par3 and aPKC function directs cells towards the ICM in the preimplantation mouse embryo

Generation of inside cells that develop into inner cell mass (ICM) and outside cells that develop into trophectoderm is central to the development of the early mouse embryo. Critical to this decision is the development of cell polarity and the associated asymmetric (differentiative) divisions of the 8-cell-stage blastomeres. The underlying molecular mechanisms for these events are not understood. As the Par3/aPKC complex has a role in establishing cellular polarity and division orientation in other systems, we explored its potential function in the developing mouse embryo. We show that both Par3 and aPKC adopt a polarized localization from the 8-cell stage onwards and that manipulating their function re-directs cell positioning and consequently influences cell fate. Injection of dsRNA against Par3 or mRNA for a dominant negative form of aPKC into a random blastomere at the 4-cell stage directs progeny of the injected cell into the inside part of the embryo. This appears to result from both an increased frequency by which such cells undertake differentiative divisions and their decreased probability of retaining outside positions. Thus, the natural spatial allocation of blastomere progeny can be over-ridden by downregulation of Par3 or aPKC, leading to a deceased tendency for them to remain outside and so develop into trophectoderm. In addition, this experimental approach illustrates a powerful means of manipulating gene expression in a specific clonal population of cells in the preimplantation embryo.

The case for prepatterning in the mouse

In studies from several laboratories using a variety of different techniques, features of the zygote and two-cell conceptus have been found to map nonrandomly on the blastocyst with respect to both its axis of polarity and bilateral plane. This is not what would be expected if, as is widely believed, early patterning depends entirely on positional relationships and interactions among the progeny of blastomeres that are equipotential until at least the eight-cell stage. Rather, the implication of these findings is that prepatterning is a normal facet of development in mammals, just as it is in most other metazoa. Nevertheless, there is still no general consensus regarding the extent to which such prepatterning depends on intrinsic organization of the oocyte, as opposed to events that are contingent on fertilization.

Lineage allocation and cell polarity during mouse embryogenesis

The first developmental lineage allocation during the generation of the mouse blastocyst is to outer trophoblast or to inner pluriblast (inner cell mass; ICM) cells. This allocation seems to be initiated at the 8-cell stage, when blastomeres polarise. Polarisation is followed by differentiative divisions at the subsequent two cleavage divisions to generate polar outer and non-polar inner 16- and 32-cells. The key events in polarisation are regulated post-translationally through a cell contact-mediated pathway, which imposes a heritable determinant-like organisation on the blastomere cortex. Two proteins in particular, E-cadherin and ezrin, are intimately involved in the generation and stabilisation of developmentally significant information. Transcriptional differences between lineages appear to follow and may coincide with the lineage commitment of cells.

Emerging Asymmetry and Embryonic Patterning in Early Mouse Development

Recent studies have revealed asymmetries in the mouse zygote and preimplantation embryo, well before the establishment of anterior-posterior polarity after implantation. Whether these asymmetries are causally related to embryonic patterning or are coincidental outcomes of the topology of normal development remains uncertain.

First cleavage plane of the mouse egg is not predetermined but defined by the topology of the two apposing pronuclei

Studies of experimentally manipulated embryos have led to the long-held conclusion that the polarity of the mouse embryo remains undetermined until the blastocyst stage. However, recent studies reporting that the embryonic-abembryonic axis of the blastocyst arises perpendicular to the first cleavage plane, and hence to the animal-vegetal axis of the zygote, have led to the claim that the axis of the mouse embryo is already specified in the egg. Here we show that there is no specification of the axis in the egg. Time-lapse recordings show that the second polar body does not mark a stationary animal pole, but instead, in half of the embryos, moves towards a first cleavage plane. The first cleavage plane coincides with the plane defined by the two apposing pronuclei once they have moved to the centre of the egg. Pronuclear transfer experiments confirm that the first cleavage plane is not determined in early interphase but rather is specified by the newly formed topology of the two pronuclei. The microtubule networks that allow mixing of parental chromosomes before dividing into two may be involved in these processes.

Ca2+ signalling and cortical re-organisation during the transition from meiosis to mitosis in mammalian oocytes

In mammals, the mature ovulated egg is arrested in metaphase II of the first meiotic division. The signal that triggers the transition from meiosis to mitosis is provided by the fertilising sperm and takes the form of a series of Ca(2+) oscillations. The pattern of Ca(2+) oscillations is imposed by maternal control mechanisms that ensure Ca(2+) transients occur during M-phase of meiosis II and during the first mitotic division. The transition from meiosis to mitosis involves a major re-organisation. The unfertilised egg is polarised with the meiotic spindle located in the cortex of the animal pole and clusters of endoplasmic reticulum in the vegetal hemisphere. By the time of the first mitotic division some 20h later the spindle has formed in the centre of the embryo and is surrounded by endoplasmic reticulum. These changes in organisation have implications for the inheritance of ER in meiotic and mitotic cell divisions and may reflect different roles and requirements for Ca(2+) in meiosis and mitosis.

Two PAR6 proteins become asymmetrically localized during establishment of polarity in mouse oocytes

Meiotic maturation in mammals is characterized by two asymmetric divisions, leading to the formation of two polar bodies and the female gamete. Whereas the mouse oocyte is a polarized cell, molecules implicated in the establishment of this polarity are still unknown. PAR proteins have been demonstrated to play an important role in cell polarity in many cell types, where they control spindle positioning and asymmetric distribution of determinants. Here we show that two PAR6-related proteins have distinct polarized distributions in mouse oocytes. mPARD6a is first localized on the spindle and then accumulates at the pole nearest the cortex during spindle migration. In the absence of microtubules, the chromosomes still migrate to the cortex, and mPARD6a was found associated with the chromosomes and was facing the cortex. mPARD6a is the first identified protein to associate with the spindle during spindle migration and to relocalize to the chromosomes in the absence of microtubule behavior, suggesting a role in spindle migration. The other protein, mPARD6b, was found on spindle microtubules until entry into meiosis II and relocalized to the cortex at the animal pole during metaphase II arrest. mPARD6b is the first identified protein to localize to the animal pole of the mouse oocyte and likely contributes to the polarization of the cortex.

A genome-wide study of gene activity reveals developmental signaling pathways in the preimplantation mouse embryo

The preimplantation development of the mammalian embryo encompasses a series of critical events: the transition from oocyte to embryo, the first cell divisions, the establishment of cellular contacts, the first lineage differentiation-all the first subtle steps toward a future body plan. Here, we use microarrays to explore gene activity during preimplantation development. We reveal robust and dynamic patterns of stage-specific gene activity that fall into two major phases, one up to the 2-cell stage (oocyte-to-embryo transition) and one after the 4-cell stage (cellular differentiation). The mouse oocyte and early embryo express components of multiple signaling pathways including those downstream of Wnt, BMP, and Notch, indicating that conserved regulators of cell fate and pattern formation are likely to function at the earliest embryonic stages. Overall, these data provide a detailed temporal profile of gene expression that reveals the richness of signaling processes in early mammalian development.

Embryonic Axes: The Long and Short of It in the Mouse

Contrary to conventional wisdom, recent studies indicate that, during mouse development, the embryo's anterior-posterior axis shifts from the shorter transverse axis to the orthogonal longer one as the shape of the pre-gastrula-stage embryo is remodelled.

First Cleavage of the Mouse Embryo Responds to Change in Egg Shape at Fertilization

Although mouse development is regulative, the cleavage pattern of the embryo is not random. The first cleavage tends to relate to the site of the previous meiosis. Sperm entry might provide a second cue, but evidence for and against this is indirect and has been debated. To resolve whether sperm entry position relates to the first cleavage, we have followed development from fertilization by time-lapse imaging. This directly showed cytokinesis passes close to the site of the previous meiosis and to both the sperm entry site and trajectory of the male pronucleus in a significant majority of eggs. We detected asymmetric distribution of Par6 protein in relation to the site of meiosis, but not sperm entry. Unexpectedly, we found the egg becomes flattened upon fertilization in an actin-mediated process. The sperm entry position tends to lie at one end of the short axis along which cleavage will pass. When we manipulated eggs to change their shape, this repositioned the cleavage plane such that eggs divided along their experimentally imposed short axis. Such manipulated eggs were able to develop to term, emphasizing the regulative nature of their development.

The evolution of developmental mechanisms

Over the past two to three decades, developmental biology has demonstrated that all multicellular organisms in the animal kingdom share many of the same molecular building blocks and many of the same regulatory genetic pathways. Yet we still do not understand how the various organisms use these molecules and pathways to assume all the forms we know today. Evolutionary developmental biology tackles this problem by comparing the development of one organism to another and comparing the genes involved and gene functions to understand what makes one organism different from another. In this review, we revisit a set of seven concepts defined by Lewis Wolpert (fate maps, asymmetric division, induction, competence, positional information, determination, and lateral inhibition) that describe the characters of many developmental systems and supplement them with three additional concepts (developmental genomics, genetic redundancy, and genetic networks). We will discuss examples of comparative developmental studies where these concepts have guided observations on the advent of a developmental novelty. Finally, we identify a set of evolutionary frameworks, such as developmental constraints, cooption, duplication, parallel and convergent evolution, and homoplasy, to adequately describe the evolutionary properties of developmental systems.

The basis and significance of pre-patterning in mammals

The second polar body (Pb) provides an enduring marker of the animal pole of the zygote, thereby revealing that the axis of bilateral symmetry of the early blastocyst is aligned with the zygote's animal-vegetal axis. That this relationship is biologically significant appeared likely when subsequent studies showed that the equator of the blastocyst tended to correspond with the plane of first cleavage. However, this cleavage plane varies both with respect to the position of the second Pb and to the distribution of components of the fertilizing sperm that continue to mark the point where it entered the egg. It also maps too variably on the blastocyst to play a causal role in early patterning. The zygote has been found transiently to exhibit bilateral symmetry before regaining an essentially spherical shape prior to first cleavage. Marking experiments indicate that the plane of bilateral symmetry of the blastocyst is aligned with, and the plane of first cleavage is typically orthogonal to, the zygote's bilateral plane. The bilateral symmetry of the zygote bears no consistent relationship either to the point of sperm entry or to the distribution of the pronuclei, and may therefore be a manifestation of intrinsic organization of the egg. Finally, the two-cell blastomere inheriting the sperm entry point has not been found to differ consistently in fate from the one that does not.