Asymmetric distribution of PAR proteins in the mouse embryo begins at the 8-cell stage during compaction

Making the blastocyst: lessons from the mouse

Mammalian preimplantation development, which is the period extending from fertilization to implantation, results in the formation of a blastocyst with three distinct cell lineages. Only one of these lineages, the epiblast, contributes to the embryo itself, while the other two lineages, the trophectoderm and the primitive endoderm, become extra-embryonic tissues. Significant gains have been made in our understanding of the major events of mouse preimplantation development, and recent discoveries have shed new light on the establishment of the three blastocyst lineages. What is less clear, however, is how closely human preimplantation development mimics that in the mouse. A greater understanding of the similarities and differences between mouse and human preimplantation development has implications for improving assisted reproductive technologies and for deriving human embryonic stem cells.

Mechanisms of trophectoderm fate specification in preimplantation mouse development

During preimplantation mouse development, embryos establish two distinct cell lineages by the time of blastocyst formation: trophectoderm (TE) and inner cell mass (ICM). To explain the mechanism of this cell fate specification, two classical models, namely the inside-outside model and polarity model have been proposed based on experimental manipulation studies on embryos. This review summarizes recent findings on the molecular mechanisms of fate specification, and discusses how these findings fit into the classical models. TE development is regulated by a transcription factor cascade, the core transcription factors of which are Tead4 and Cdx2. The transcriptional activity of Tead4 is regulated by the position-dependent Hippo signaling pathway, thus supporting the inside-outside model. In contrast, several findings support the polarity model; some other findings suggest different mechanisms. We also discuss how the two classical models could be further developed in the light of recent molecular findings.

Establishment of trophectoderm and inner cell mass lineages in the mouse embryo

The first cell lineage specification in mouse embryo development is the formation of trophectoderm (TE) and inner cell mass (ICM) of the blastocyst. This article is to review and discuss the current knowledge on the cellular and molecular mechanisms of this particular event. Several transcription factors have been identified as the critical regulators of the formation or maintenance of the two cell lineages. The establishment of TE manifests as the formation of epithelium, and is dependent on many structural and regulatory components that are commonly found and that function in many epithelial tissues. Distinct epithelial features start to emerge at the late 8-cell stage, but the fates of blastomeres are not fixed as TE or ICM until around 32-cell stage. The location of blastomeres at this stage, that is, external or internal of the embryo, in effect defines the commitment towards the TE or ICM lineage, respectively. Some studies implicate the presence of a developmental bias among blastomeres at 2- or 4-cell stage, although it is unlikely to play a decisive role in the establishment of TE and ICM. The unique mode of cell lineage specification in the mouse embryo is further discussed in comparison with the formation of initial cell lineages, namely the three germ layers, in non-mammalian embryos.

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.

PAR6, a potential marker for the germ cells selected to form primordial follicles in mouse ovary

Partitioning-defective proteins (PAR) are detected to express mainly in the cytoplast, and play an important role in cell polarity. However, we showed here that PAR6, one kind of PAR protein, was localized in the nuclei of mouse oocytes that formed primordial follicles during the perinatal period, suggesting a new role of PAR protein. It is the first time we found that, in mouse fetal ovaries, PAR6 appeared in somatic cell cytoplasm and fell weak when somatic cells invaded germ cell cysts at 17.5 days post coitus (dpc). Meanwhile, the expression of PAR6 was observed in cysts, and became strong in the nuclei of some germ cells at 19.5 dpc and all primordial follicular oocytes at 3 day post parturition (dpp), and then obviously declined when the primordial follicles entered the folliculogenic growth phase. During the primordial follicle pool foundation, the number of PAR6 positive germ cells remained steady and was consistent with that of formed follicles at 3 dpp. There were no TUNEL (apoptosis examination) positive germ cells stained with PAR6 at any time studied. The number of follicles significantly declined when 15.5 dpc ovaries were treated with the anti-PAR6 antibody and PAR6 RNA interference. Carbenoxolone (CBX, a known blocker of gap junctions) inhibited the expression of PAR6 in germ cells and the formation of follicles. Our results suggest that PAR6 could be used as a potential marker of germ cells for the primordial follicle formation, and the expression of PAR6 by a gap junction-dependent process may contribute to the formation of primordial follicles and the maintenance of oocytes at the diplotene stage.

Inactivation of aPKClambda reveals a context dependent allocation of cell lineages in preimplantation mouse embryos

Background

During mammalian preimplantation development, lineage divergence seems to be controlled by the interplay between asymmetric cell division (once cells are polarized) and positional information. In the mouse embryo, two distinct cell populations are first observed at the 16-cell stage and can be distinguished by both their position (outside or inside) and their phenotype (polarized or non-polarized). Many efforts have been made during the last decade to characterize the molecular mechanisms driving lineage divergence.

Methodology/Principal Findings

In order to evaluate the importance of cell polarity in the determination of cell fate we have disturbed the activity of the apical complex aPKC/PAR6 using siRNA to down-regulate aPKCλ expression. Here we show that depletion of aPKCλ results in an absence of tight junctions and in severe polarity defects at the 16-cell stage. Importantly, we found that, in absence of aPKCλ, cell fate depends on the cellular context: depletion of aPKCλ in all cells results in a strong reduction of inner cells at the 16-cell stage, while inhibition of aPKCλ in only half of the embryo biases the progeny of aPKCλ defective blastomeres towards the inner cell mass. Finally, our study points to a role of cell shape in controlling cell position and thus lineage allocation.

Conclusion

Our data show that aPKCλ is dispensable for the establishment of polarity at the 8-cell stage but is essential for the stabilization of cell polarity at the 16-cell stage and for cell positioning. Moreover, this study reveals that in addition to positional information and asymmetric cell divisions, cell shape plays an important role for the control of lineage divergence during mouse preimplantation development. Cell shape is able to influence both the type of division (symmetric or asymmetric) and the position of the blastomeres within the embryo.

Making a firm decision: multifaceted regulation of cell fate in the early mouse embryo

The preimplantation mammalian embryo offers a striking opportunity to address the question of how and why apparently identical cells take on separate fates. Two cell fate decisions are taken before the embryo implants; these decisions set apart a group of pluripotent cells, progenitors for the future body, from the distinct extraembryonic lineages of trophectoderm and primitive endoderm. New molecular, cellular and developmental insights reveal the interplay of transcriptional regulation, epigenetic modifications, cell position and cell polarity in these two fate decisions in the mouse. We discuss how mechanisms proposed in previously distinct models might work in concert to progressively reinforce cell fate decisions through feedback loops.

Blastocyst lineage formation, early embryonic asymmetries and axis patterning in the mouse

The investigation into lineage allocation and early asymmetries in the pre- and peri-implantation mouse embryo is gaining momentum. As we review here, new insights have been gained into the cellular and molecular events that lead to the establishment of the three lineages of the blastocyst, to the determination of the origin and the fates of the visceral endoderm in the peri-implantation mouse embryo, and to the generation of cellular and molecular activities that accompany the emergence of asymmetries in the pre-gastrulation embryo. We also discuss the continuing debate that surrounds the relative impacts of early lineage bias versus the stochastic allocation of cells with respect to the events that pattern the blastocyst and initiate its later asymmetries.

Cell polarity emerges at first cleavage in sea urchin embryos

In protostomes, cell polarity is present after fertilization whereas most deuterostome embryos show minimal polarity during the early cleavages. We now show establishment of cell polarity as early as the first cleavage division in sea urchin embryos. We find, using the apical markers G(M1), integrins, and the aPKC-PAR6 complex, that cells are polarized upon insertion of distinct basolateral membrane at the first division. This early apical-basolateral polarity, similar to that found in much larger cleaving amphibian zygotes, reflects precocious functional epithelial cell polarity. Isolated cleavage blastomeres exhibit polarized actin-dependent fluid phase endocytosis only on the G(M1), integrin, microvillus-containing apical surface. A role for a functional PAR complex in cleavage plane determination was shown with experiments interfering with aPKC activity, which results in several spindle defects and compromised blastula development. These studies suggest that cell and embryonic polarity is established at the first cleavage, mediated in part by the Par complex of proteins, and is achieved by directed insertion of basolateral membrane in the cleavage furrow.

Cytokinesis and the establishment of early embryonic cell polarity

Cleavage divisions in many animals form a blastula made up of a simple polarized epithelium. This simple embryonic epithelium possesses an apical surface covered with microvilli and primary cilia separated from the basolateral surfaces by cell-cell junctions. The apical membrane proteins and lipids differ from those of the basolateral on these embryonic epithelial cells, as is found in adult epithelial cells. Formation of cell polarity in embryos at fertilization, including those from both protostomes and deuterostomes, uses the same molecules and signalling machinery as do polarizing epithelial cells that polarize upon cell-cell contact. In addition, the actin-myosin cytoskeleton plays an integral role in establishment and maintenance of this early cell polarity. However, early cleaving blastomeres from higher organisms including echinoderms and vertebrates have not been considered to exhibit cell polarity until formation of junctions at the third through to the fifth cleavage divisions. The role of new membrane addition into the late cleavage furrow during the early rounds of cytokinesis may play a key role in the early establishment of cell polarity in all animal embryos.

Crosstalk between small GTPases and polarity proteins in cell polarization

Cell polarization is crucial for the development of multicellular organisms, and aberrant cell polarization contributes to various diseases, including cancer. How cell polarity is established and how it is maintained remain fascinating questions. Conserved proteins of the partitioning defective (PAR), Scribble and Crumbs complexes guide the establishment of cell polarity in various organisms. Moreover, GTPases that regulate actin cytoskeletal dynamics have been implicated in cell polarization. Recent findings provide insights into polarization mechanisms and show intriguing crosstalk between small GTPases and members of polarity complexes in regulating cell polarization in different cellular contexts and cell types.

[Compaction and lineage divergence during mouse preimplantation embryo development]

The preimplantation embryo development leads to the formation of a blastocyst made of two cell lineages: an outer layer of epithelial cells, the trophectoderm, that will give rise to some embryonic annexes, and a mass of undifferentiated cells, the inner cell mass, that will form the foetus and the remaining embryonic annexes. The trophectoderm encloses the inner cell mass and protects it from the external medium. Moreover, after hatching, the trophectoderm invades the uterine tissue, a crucial step for the implantation of the embryo. Therefore, the divergence between these two lineages is of crucial importance for the emergence of the foetus itself and for the postimplantation development to take place correctly. The setting up of cell polarity during compaction at the eight-cell stage allows asymmetric divisions to take place, thereby leading to lineage divergence. Phenotypic properties of these two cell populations are progressively reinforced through cell-cell interactions, outer cells undergoing epithelial differentiation while inner cells remain undifferentiated. Although cellular mechanisms controlling the divergence of the first two lineages are quite well known, important efforts have been carried out this last decade to identify the molecular machinery involved in this process and will be presented in this review.

Polarization of the C. elegans embryo by RhoGAP-mediated exclusion of PAR-6 from cell contacts

Early embryos of some metazoans polarize radially to facilitate critical patterning events such as gastrulation and asymmetric cell division; however, little is known about how radial polarity is established. Early embryos of Caenorhabditis elegans polarize radially when cell contacts restrict the polarity protein PAR-6 to contact-free cell surfaces, where PAR-6 regulates gastrulation movements. We have identified a Rho guanosine triphosphatase activating protein (RhoGAP), PAC-1, which mediates C. elegans radial polarity and gastrulation by excluding PAR-6 from contacted cell surfaces. We show that PAC-1 is recruited to cell contacts, and we suggest that PAC-1 controls radial polarity by restricting active CDC-42 to contact-free surfaces, where CDC-42 binds and recruits PAR-6. Thus, PAC-1 provides a dynamic molecular link between cell contacts and PAR proteins that polarizes embryos radially.

Distribution of RNA binding protein MOEP19 in the oocyte cortex and early embryo indicates pre-patterning related to blastomere polarity and trophectoderm specification

We report the cloning and characterization of MOEP19, a novel 19 kDa RNA binding protein that marks a defined cortical cytoplasmic domain in oocytes and provides evidence of mammalian oocyte polarity and a form of pre-patterning that persists in zygotes and early embryos through the morula stage. MOEP19 contains a eukaryotic type KH-domain, typical of the KH-domain type I superfamily of RNA binding proteins, and both recombinant and native MOEP19 bind polynucleotides. By immunofluorescence, MOEP19 protein was first detected in primary follicles throughout the ooplasm. As oocytes expanded in size during oogenesis, MOEP19 increased in concentration. MOEP19 localized in the ovulated egg and early zygote as a symmetrical spherical cortical domain underlying the oolemma, deep to the zone of cortical granules. MOEP19 remained restricted to a cortical cytoplasmic crescent in blastomeres of 2-, 4- and 8-cell embryos. The MOEP19 domain was absent in regions underlying cell contacts. In morulae, the MOEP19 domain was found at the apex of outer, polarized blastomeres but was undetectable in blastomeres of the inner cell mass. In early blastocysts, MOEP19 localized in both mural and polar trophectoderm and a subset of embryos showed inner cell mass localization. MOEP19 concentration dramatically declined in late blastocysts. When blastomeres of 4- to 8-cell stages were dissociated, the polarized MOEP19 domain assumed a symmetrically spherical localization, while overnight culture of dissociated blastomeres resulted in formation of re-aggregated embryos in which polarity of the MOEP19 domain was re-established at the blastomere apices. MOEP19 showed no evidence of translation in ovulated eggs, indicating that MOEP19 is a maternal effect gene. The persistence during early development of the MOEP19 cortical oocyte domain as a cortical crescent in blastomers suggests an intrinsic pre-patterning in the egg that is related to the apical-basolateral polarity of the embryo. Although the RNAs bound to MOEP19 are presently unknown, we predict that the MOEP19 domain directs RNAs essential for normal embryonic development to specific locations in the oocyte and early embryo.

Morphogenesis of the mammalian blastocyst

The first 4 days of mouse pre-implantation development are characterized by a period of segmentation, including morphogenetic events that are required for the divergence of embryonic and extra-embryonic lineages. These extra-embryonic tissues are essential for the implantation into the maternal uterus and for the development of the foetus. In this review, we first discuss data showing unambiguously that no essential axis of development is set up before the late blastocyst stage, and explain why the pre-patterning described during the early phases (segmentation) of development in other vertebrates cannot apply to mammalian pre-implantation period. Then, we describe important cellular and molecular events that are required for the morphogenesis of the blastocyst.

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.

Stochastic patterning in the mouse pre-implantation embryo

Mouse pre-implantation development gives rise to the blastocyst, which is made up of at least three distinct cell types: the trophectoderm (TE) that surrounds a cavity, and an inner cell mass (ICM) comprising the primitive endoderm (PE) and epiblast (EPI). However, the underlying mechanisms involved in patterning the cleavage-stage embryo are still unresolved. By analyzing the distribution of the transcription factors Oct4 (Pou5f1), Cdx2 and Nanog at precisely defined stages in pre-implantation development, we were able to identify critical events leading to the divergence of TE, EPI and PE lineages. We found that Oct4 is present in all cells until late blastocyst, gradually disappearing from the TE thereafter. The expression patterns of both Cdx2 and Nanog exhibit two specific phases, culminating in their restriction to TE and EPI, respectively. In the first phase, starting after compaction, blastomeres show highly variable Cdx2 and Nanog protein levels. Importantly, the variability in Nanog levels is independent of position within the morula, whereas Cdx2 variability may originate from asymmetric cell divisions at the 8-cell stage in a non-stereotypic way. Furthermore, there is initially no reciprocal relationship between Cdx2 and Oct4 or between Cdx2 and Nanog protein levels. In the second phase, a definite pattern is established, possibly by a sorting process that accommodates intrinsic and extrinsic cues. Based on these results, we propose a model in which early embryonic mouse patterning includes stochastic processes, consistent with the highly regulative capacity of the embryo. This may represent a feature unique to early mammalian development.

Polarity complex proteins

The formation of functional epithelial tissues involves the coordinated action of several protein complexes, which together produce a cell polarity axis and develop cell-cell junctions. During the last decade, the notion of polarity complexes emerged as the result of genetic studies in which a set of genes was discovered first in Caenorhabditis elegans and then in Drosophila melanogaster. In epithelial cells, these complexes are responsible for the development of the apico-basal axis and for the construction and maintenance of apical junctions. In this review, we focus on apical polarity complexes, namely the PAR3/PAR6/aPKC complex and the CRUMBS/PALS1/PATJ complex, which are conserved between species and along with a lateral complex, the SCRIBBLE/DLG/LGL complex, are crucial to the formation of apical junctions such as tight junctions in mammalian epithelial cells. The exact mechanisms underlying their tight junction construction and maintenance activities are poorly understood, and it is proposed to focus in this review on establishing how these apical polarity complexes might regulate epithelial cell morphogenesis and functions. In particular, we will present the latest findings on how these complexes regulate epithelial homeostasis.

Conditional knock-out reveals that zygotic vezatin-null mouse embryos die at implantation

Vezatin, a protein associated to adherens junctions in epithelial cells, is already expressed in mouse oocytes and during pre-implantation development. Using a floxed strategy to generate a vezatin-null allele, we show that the lack of zygotic vezatin is embryonic lethal, indicating that vezatin is an essential gene. Homozygous null embryos are able to elicit a decidual response but as early as day 6.0 post-coitum mutant implantation sites are devoid of embryonic structures. Mutant blastocysts are morphologically normal, but only half of them are able to hatch upon in vitro culture and the blastocyst outgrowths formed after 3.5 days in culture exhibit severe abnormalities, in particular disrupted intercellular adhesion and clear signs of cellular degeneration. Notably, the junctional proteins E-cadherin and beta-catenin are delocalized and not observed at the plasma membrane anymore. These in vitro observations reinforce the idea that homozygous vezatin-null mutants die at the time of implantation because of a defect in intercellular adhesion. Together these results indicate that the absence of zygotic vezatin is deleterious for the implantation process, most likely because cadherin-dependent intercellular adhesion is impaired in late blastocysts when the maternal vezatin is lost.

Extending from PARs in Caenorhabditis elegans to homologues in Haemonchus contortus and other parasitic nematodes

Signal transduction molecules play key roles in the regulation of developmental processes, such as morphogenesis, organogenesis and cell differentiation in all organisms. They are organized into 'pathways' that represent a coordinated network of cell-surface receptors and intracellular molecules, being involved in sensing environmental stimuli and transducing signals to regulate or modulate cellular processes, such as gene expression and cytoskeletal dynamics. A particularly important group of molecules implicated in the regulation of the cytoskeleton for the establishment and maintenance of cell polarity is the PAR proteins (derived from partition defective in asymmetric cell division). The present article reviews salient aspects of PAR proteins involved in the early embryonic development and morphogenesis of the free-living nematode Caenorhabditis elegans and some other organisms, with an emphasis on the molecule PAR-1. Recent advances in the knowledge and understanding of PAR-1 homologues from the economically important parasitic nematode, Haemonchus contortus, of small ruminants is summarized and discussed in the context of exploring avenues for future research in this area for parasitic nematodes.

Tyrosine phosphorylated Par3 regulates epithelial tight junction assembly promoted by EGFR signaling

The conserved polarity complex, comprising the partitioning-defective (Par) proteins Par3 and Par6, and the atypical protein kinase C, functions in various cell-polarization events and asymmetric cell divisions. However, little is known about whether and how external stimuli-induced signals may regulate Par3 function in epithelial cell polarity. Here, we found that Par3 was tyrosine phosphorylated through phosphoproteomic profiling of pervanadate-induced phosphotyrosine proteins. We also demonstrated that the tyrosine phosphorylation event induced by multiple growth factors including epidermal growth factor (EGF) was dependent on activation of Src family kinase (SFK) members c-Src and c-Yes. The tyrosine residue 1127 (Y1127) of Par3 was identified as the major EGF-induced phosphorylation site. Moreover, we found that Y1127 phosphorylation reduced the association of Par3 with LIM kinase 2 (LIMK2), thus enabling LIMK2 to regulate cofilin phosphorylation dynamics. Substitution of Y1127 for phenylalanine impaired the EGF-induced Par3 and LIMK2 dissociation and delayed epithelial tight junction (TJ) assembly considerably. Collectively, these data suggest a novel, phosphotyrosine-dependent fine-tuning mechanism of Par3 in epithelial TJ assembly controlled by the EGF receptor-SFK signaling pathway.

Cell shape and cell division

The correlation between cell shape elongation and the orientation of the division axis described by early cell biologists is still used as a paradigm in developmental studies. However, analysis of early embryo development and tissue morphogenesis has highlighted the role of the spatial distribution of cortical cues able to guide spindle orientation. In vitro studies of cell division have revealed similar mechanisms. Recent data support the possibility that the orientation of cell division in mammalian cells is dominated by cell adhesion and the associated traction forces developed in interphase. Cell shape is a manifestation of these adhesive and tensional patterns. These patterns control the spatial distribution of cortical signals and thereby guide spindle orientation and daughter cell positioning. From these data, cell division appears to be a continuous transformation ensuring the maintenance of tissue mechanical integrity.

Embryology: does prepatterning occur in the mouse egg?

A recurring question in developmental biology has been whether localized determinants play any role in mammalian preimplantation development. This is a controversial issue that brings back the idea of prepatterning and is explored further by Plusa et al., who claim it is the first cleavage of the mouse zygote that predicts the blastocyst axis, rather than the animal pole or sperm entry point, as previously suggested. However, other evidence indicates that the blasotcyst axis is not predetermined and there is no prepatterning in the mouse egg. Here we investigate the origin of these different views and conclude that they arise from differences in the data themselves and in their interpretation.

Cell and molecular regulation of the mouse blastocyst

Animals use diverse strategies to specify tissue lineages during development. A common strategy is to partition maternally supplied and localized lineage determinants into progenitor cells. The mouse embryo appears to use a different, more regulative strategy to specify the first three lineages: the epiblast (EPI: future embryo), the trophectoderm (TE: future placenta), and the primitive endoderm (PE: future yolk sac). These lineages are specified during two successive differentiation steps leading to formation of the blastocyst. Here, we review classic and contemporary models of early lineage specification in the mouse, and describe recent efforts to understand the molecular regulation of these events. We describe evidence that trophectoderm differentiation bears resemblance to the process of epithelialization and describe the importance of apical/basal protein complexes in regulating this process. Next, we present a revised model of PE specification, and describe evidence that PE cells in the inner cell mass sort out to occupy their ultimate position on the surface of the EPI. Finally, we describe factors that reinforce these lineages and three distinct stem cell types that can be isolated from them. Together, these mechanisms guide the differentiation of the first lineages of the mouse and thereby set up tissues that will be important for the first steps of embryonic body patterning.