The initial phase of embryonic patterning in mammals

Messenger RNAs in metaphase II oocytes correlate with successful embryo development to the blastocyst stage

The mRNAs accumulated in oocytes provide support for embryo development until embryo genomic activation. We hypothesized that the maternal mRNA stock present in bovine oocytes is associated with embryo development until the blastocyst stage. To test our hypothesis, we analyzed the transcriptome of the oocyte and correlated the results with the embryo development. Our goal was to identify genes expressed in the oocyte that correlate with its ability to develop to the blastocyst stage. A fraction of oocyte cytoplasm was biopsied using micro-aspiration and stored for further expression analysis. Oocytes were activated chemically, cultured individually and classified according to their capacity to develop in vitro to the blastocyst stage. Microarray analysis was performed on mRNA extracted from the oocyte cytoplasm fractions and correlated with its ability to develop to the blastocyst stage (good quality oocyte) or arrest at the 8-16-cell stage (bad quality oocyte). The expression of 4320 annotated genes was detected in the fractions of cytoplasm that had been collected from oocytes matured in vitro. Gene ontology classification revealed that enriched gene expression of genes was associated with certain biological processes: 'RNA processing', 'translation' and 'mRNA metabolic process'. Genes that are important to the molecular functions of 'RNA binding' and 'translation factor activity, RNA binding' were also enriched in oocytes. We identified 29 genes with differential expression between the two groups of oocytes compared (good versus bad quality). The content of mRNAs expressed in metaphase II oocytes influences the activation of the embryonic genome and enables further develop to the blastocyst stage.

Paternal Familial Twinning: Hypothesis and Genetic/Medical Implications

The phenomenon of paternally dependent familial twinning has been known in human and animal genetics since the 1920s, but still remains without any theoretical explanation and is indeed a neglected field of inquiry. Over the last two decades investigations in reproduction biology have discovered the significant role of multiple paternally dependent errors in fertilization including androgenic triploidy and moles. We suggest the hypothesis that the fathers of twins in the relevant families carry gene variants that increase the probability of dispermy, diplospermy and male pronucleus heterochrony as well as involvement of two male pronuclei in the fertilization of two female meiotic products. Any resulting twins would be an exceptional intermediate between MZ and DZ twins — and might properly be described as “sesquizygotic” (SZ). Paternal familial twinning may also go together with infertility due to triploidy, moles and chimerism. The hypothesis: (i) places the curiosities of paternally derived twinning within the framework of current knowledge of reproductive genetics and verifiable phenomena; (ii) predicts the existence of families in which twinning is associated with reproductive abnormalities; (iii) predicts an occurrence in relevant families of the third and intermediate category of SZ twins. Families with paternal twinning may thus provide the natural selective system for the search of unusual cases of primary chimeras, the frequency of which is still unknown.

The murine allantois: emerging paradigms in development of the mammalian umbilical cord and its relation to the fetus

The fertilized egg of the mammal gives rise to the embryo and its extraembryonic structures, all of which develop in intimate relation with each other. Yet, whilst the past several decades have witnessed a vast number of studies on the embryonic component of the conceptus, study of the extraembryonic tissues and their relation to the fetus have been largely ignored. The allantois, precursor tissue of the mature umbilical cord, is a universal feature of all placental mammals that establishes the vital vascular bridge between the fetus and its mother. The allantois differentiates into the umbilical blood vessels, which become secured onto the chorionic component of the placenta at one end and onto the fetus at the other. In this way, fetal blood is channeled through the umbilical cord for exchange with the mother. Despite the importance of this vascular bridge, little is known about how it is made. The aim of this review is to address current understanding of the biology of the allantois in the mouse and genetic control of its features and functions, and to highlight new paradigms concerning the developmental relationship between the fetus and its umbilical cord.

The axis of polarity of the mouse blastocyst is specified before blastulation and independently of the zona pellucida

BACKGROUND

Rather than being prepatterned, orientation of the embryonic-abembryonic (Em-Ab) axis of the mouse blastocyst has been claimed to depend on the conceptus being constrained by its zona pellucida (ZP) during blastulation. This hypothesis merited closer scrutiny, because it seemed at variance with observations on living conceptuses.

METHODS

Two-cell conceptuses with an oil drop injected into the lesser diameter (LD) of the ZP at the first cleavage plane were cultured until shortly before blastulation when the blastomere underlying the drop was labelled with carbocyanine dye. After removing the ZP, conceptuses were re-cultured to the blastocyst stage for recording the position along the axis of the centres of the patches of labelled cells.

RESULTS

These centres showed significant bias towards the equatorial (Eq) region of the axis compared with those resulting from labelling a blastomere at random, even following softening of the ZP at the 2-cell stage. This was also true if conceptuses were denuded at the 2-cell stage and the blastomere underlying an intact second polar body (PB) labelled in morulae.

CONCLUSIONS

These findings further support the view that the Em-Ab axis of the mouse blastocyst is normally prepatterned and provide no evidence of a role for the ZP in its specification.

Derivation of human embryonic stem cells from developing and arrested embryos

Human embryonic stem cells (hESC) hold huge promise in modern regenerative medicine, drug discovery, and as a model for studying early human development. However, usage of embryos and derivation of hESC for research and potential medical application has resulted in polarized ethical debates since the process involves destruction of viable developing human embryos. Here we describe that not only developing embryos (morulae and blastocysts) of both good and poor quality but also arrested embryos could be used for the derivation of hESC. Analysis of arrested embryos demonstrated that these embryos express pluripotency marker genes such OCT4, NANOG, and REX1. Derived hESC lines also expressed specific pluripotency markers (TRA-1-60, TRA-1-81, SSEA4, alkaline phosphatase, OCT4, NANOG, TERT, and REX1) and differentiated under in vitro and in vivo conditions into derivates of all three germ layers. All of the new lines, including lines derived from late arrested embryos, have normal karyotypes. These results demonstrate that arrested embryos are additional valuable resources to surplus and donated developing embryos and should be used to study early human development or derive pluripotent hESC.

Weaknesses in the case against prepatterning in the mouse

Serious weaknesses are exposed regarding recent claims to have demonstrated that there is no prepatterning of axes in the mouse. That the orientation of the axis of polarity of the blastocyst is dictated by the shape of the zona pellucida is contradicted by systematic observations on living conceptuses. Moreover, this 'mechanical constraint' hypothesis fails to account for key findings on which the case for prepatterning is based.

Experimental analysis of the transdifferentiation of visceral to parietal endoderm in the mouse

The visceral endoderm (VE) of isolated extraembryonic regions (ExEmbs) of 7 days postcoitum (dpc) prestreak mouse conceptuses have been shown to convert readily to parietal endoderm (PE). The present study addresses the following three unanswered questions. On what does conversion depend, how rapidly does it occur, and is it an enduring general property of a residual small population of relatively immature cells? In situ hybridization reveals that change in cell state occurs within 2 days of culture. Deprivation of the mesoderm also promotes it in later ExEmbs. Conversely, the conversion to PE in isolated 7 dpc ExEmbs is suppressed by grafting 8 dpc or 9 dpc mesoderm. Hence, the conversion provides an example of transdifferentiation that is promoted by the absence of extraembryonic mesoderm. The presence of mesoderm seems to be necessary to enable the VE to grow rather than convert to PE, as occurs if it retains contact with the extraembryonic ectoderm.

Embryogenesis of chimeras, twins and anterior midline asymmetries

Human spontaneous chimerism, with one body built from cells of both twins of a dizygotic (DZ) pair, is supposed to be extremely rare, arising from the exchange of blood cells through placental anastomoses. Mosaicism is supposed to be far more common, arising from single zygotes by embryonic mutation. Because typical diagnosis of mosaicism can neither identify nor exclude chimerism, 'mosaicism' may often be chimerism undiscovered. Evidence shows chimerism arises primarily from DZ embryo fusion and is not rare, although it has negligible probability under the hypothesis of independent double ovulation and independent embryogenesis. If, instead, DZ twin embryos begin development as a single cell mass, chimerism is likely. This would be consistent with observations that DZ twins develop as differently from singletons as monozygotic twins do with regard to embryogenic establishment of asymmetries of midline neural-crest-driven structures of brain, face and heart. Chimerism is a significant component of human embryonic development that deserves closer attention as a mechanism of developmental variation. The 'common knowledge' understanding of twinning mechanisms is at best inadequate. The importance of the difference lies in what we can learn from chimerism about human embryogenesis and the cellular origins of structures and functions basic to the business of becoming human.

Hematopoietic stem and progenitor cells: clinical and preclinical regeneration of the hematolymphoid system

A vast literature exists on the biology of blood formation and regeneration under experimental and clinical conditions. The field of hematopoiesis was recently advanced by the capacity to purify to homogeneity primitive hematopoietic stem and progenitor cells. Isolation of cells at defined maturational stages has enhanced the understanding of the fundamental nature of stem cells, including how cell fate decisions are made, and this understanding is relevant to the development of other normal as well as malignant tissues. This review updates the basic biology of hematopoietic stem cells (HSC) and progenitors, the evolving use of purified HSC as grafts for clinical hematopoietic cell transplantation (HCT) including immune tolerance induction, and the application of HSC biology to other stem cell fields.

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.

Gastrulation events in the prestreak pig embryo: ultrastructure and cell markers

The epithelial versus mesenchymal phenotypes of embryonic ectoderm and mesoderm cells of the prestreak stage pig embryos were examined by electron microscopy and molecular marker analysis. During this period the embryonic disc remained flat or slightly convex while becoming oval or pyriform in shape. Mesenchyme cells expressing vimentin were present between the embryonic disc and the underlying visceral endoderm before a primitive streak (or groove) was apparent. The migration of mesenchyme appeared to occur in lateral and posterior directions from a mass of quiescent cells located in the pointed end of the pyriform embryonic disc that expressed Brachyury; these cells are proposed to be the precursors of the primitive streak and/or form the equivalent of the mouse early gastrula organizer (EGO). Cells with the TEC-1 (or SSEA-1) epitope, the marker most frequently used to characterize pluripotent cells, were initially distributed randomly in the embryonic ectoderm and then were found to localize in an anterior crescent which may contain the precursor cells of ectoderm and neurectoderm. As mitotic figures were found only in the anterior crescent, it is proposed that at least some of these proliferating cells migrate toward the EGO. While cytokeratins were barely detectable in the embryonic ectoderm cells, vimentin expression was supposed to be associated with the migratory capacity of these cells. These findings indicate that the early step of gastrulation, migration of extraembryonic mesoderm, occurs at a prestreak stage during which the embryonic disc becomes polarized. genesis 38:13-25, 2004.

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.

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.

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.

Mouse early embryos obtained by natural breeding or in vitro fertilization display a differential sensitivity to extremely low-frequency electromagnetic fields

We have investigated the sensitivity of pre-implantation embryos obtained by natural breeding (NB) or in vitro fertilization (IVF) to extremely low-frequency magnetic fields (ELF-MF). Fertilized eggs obtained by NB were removed from mothers 12h after mating and cultured in vitro for 5 days under continuous ELF-MF exposure (constant strength of 50Hz and various intensities, i.e. 60, 120 and 220 microT). Alternatively, zygotes obtained by IVF were subjected to ELF-MF exposure (50Hz, 60 microT), starting 12h after IVF for 5 days. We found that ELF-MF exposure causes a small yet significant (P<0.05) decrease in the survival rate of NB-derived embryos at the latest stages of pre-implantation development, i.e. the eight cell-to-blastocyst transition. In embryos exposed to the highest field intensity (220 microT), the effect became apparent somewhat earlier. When IVF-derived embryos were exposed to ELF-MF, the reduction in the rate of embryo survival was more pronounced and the difference from controls was more significant (P<0.01). Moreover, the decreased survival rate in IVF embryos became apparent as early as the first cleavage and persisted throughout pre-implantation. These results suggest that IVF-derived embryos are more sensitive than NB-generated embryos to ELF-MF, and that this sensitivity occurs earlier in development.

Thoughts and observations on patterning in early mammalian development

The preimplantation mammalian conceptus shows an impressive ability to develop normally following the loss, gain or rearrangement of cells. This has prompted the view that, unlike in other species, patterning in mammals cannot depend on information that is already present in the zygote before it begins to cleave. However, various findings are hard to reconcile with this conclusion, including evidence that the incidence of monozygotic twinning is sensitive to the conditions to which eggs or very early concepti are exposed. Possible causes of early twinning are discussed, and it is argued that partial hatching of the conceptus through a hernia in the zona pellucida cannot account for all cases. Moreover, it remains questionable whether studies on aggregated morulae and isolated blastomeres really provide compelling evidence against the existence of indispensible patterning information in the egg. Finally, regularities in axial relationships between the blastocyst and zygote have been revealed employing strictly non-invasive techniques. These show that, at least in normal development, patterning begins before cleavage.

Differential pattern of Xist RNA accumulation in single blastomeres isolated from 8-cell stage mouse embryos following laser zona drilling

Xist gene expression begins at the late 2-cell stage in female mouse embryos and by the third division results in the accumulation of an average 100 copies of Xist RNA per cell, as measured by real-time reverse transcription-polymerase chain reaction (RT-PCR). In the blastocyst, the trophectoderm maintains the paternally imprinted pattern of Xist expression present during early development, while either the maternal or the paternal X chromosome can express Xist among cells of the inner mass. Fluorescent in situ hybridization (FISH) has previously established that Xist transcripts are localized on the silenced X chromosome, forming aggregates of variable dimensions in blastomeres of 8-cell embryos. This observation and the fact that Xist RNA accumulation per cell sharply decreases after morula stage raise the possibility that cells of cleaving embryos contain different levels of Xist RNA, perhaps linked to their subsequent developmental fates. We show here that Xist RNA is efficiently recovered from single blastomeres isolated from 8-cell embryos following laser zona drilling. Sexing of the samples and simultaneous quantification of Xist RNA in individual cells is achieved with a multiplex Xist/Sry real-time RT-PCR assay sensitive to the single-copy level. This analysis reveals that Xist RNA is indeed accumulated to substantially different levels in individual blastomeres of the same 8-cell embryo and that two blastomeres contain most of the molecules per embryo. These results support the conclusion that cells of the early mammalian embryo are not all functionally equivalent. Differential Xist gene expression could arise from differences in DNA methylation, or the order in which cells divide.

Patterning is initiated before cleavage in the mouse

Once experimental embryological studies revealed the striking ability of mammals to regulate their early development, the notion that pattern-formation might depend on information already present in the egg before cleavage was generally regarded as untenable. Mammals were therefore assumed to differ from almost all other animals in the way in which their embryonic patterning was set up. This view was justified by the profound way in which their early development is modified to meet the requirements of viviparity. However, it ignored various findings showing that exposure of gametes and very early conceptuses to altered conditions could perturb organisation of the fetus. Recent studies that place particular emphasis on non-invasive approaches have revealed hitherto overlooked regularities in early mouse development. They clearly show that specification of embryonic axes normally begins before cleavage in this species. Moreover, the relevant patterning processes seem to depend on intrinsic organisation of the egg rather than, as claimed recently, the site of entry of the fertilizing sperm. These new findings are of interest for two reasons. First, from an evolutionary perspective, it means that mammals retain common features with other animals in how their early development is controlled. Second, it raises the practical question whether the increasing use of in vitro manipulation of gametes and zygotes for assisting human reproduction carries a risk of perturbing development.

Directed differentiation of embryonic stem cells: genetic and epigenetic methods

Embryonic stem cells are derived from the inner cell mass of the pre-implantation blastocyst, and can both self-renew and differentiate into all the cells and tissues of the body. The embryonic stem cell is an unsurpassed starting material to begin to understand a critical, largely inaccessible, period of development, as well as an important source of cells for transplantation and gene therapy. Despite their potential, attempts to obtain specific cell types from embryonic stem cells have been only partially successful because many of the growth factor combinations and developmental control genes involved in cell type restricted differentiation are unknown. This article summarizes some of the recent advances in promoting lineage restricted differentiation of embryonic stem cells, focusing on growth factor manipulation, or genetically altering embryonic stem cells to produce a desired phenotype. The two approaches epitomize current scientific concerns regarding the therapeutic use of these cells; genetic alterations will produce more pure cells with the risk of increasing the likelihood of malignant transformation; epigenetic methods for the manipulation of stem cell phenotype are often incomplete and remaining pluripotent cells are likely to form teratomas. As more is known about lineage specification during development, it will be possible to more precisely control cell type specification.

Human pluripotent stem cells: a progress report

The derivation of diploid human pluripotent stem cell lines from either human blastocysts or embryonic gonads in 1998 attracted a great deal of interest because of the widespread potential applications of these cells in research and in regenerative medicine. Since the initial reports, there has been some progress in the characterisation of blastocyst-derived stem cells, and some technical advances in their manipulation. Conditions for differentiation in vitro of pluripotent stem cells from either blastocysts or gonads have been defined. In some studies, committed progenitor cell populations have been isolated from mixed cultures of differentiating ES cells.

Morphogenetic tissue movement and the establishment of body plan during development from blastocyst to gastrula in the mouse

In many animal species, the early development of the embryo follows a stereotypic pattern of cell cleavage, lineage allocation and generation of tissue asymmetry leading to delineation of the body plan with three primary embryonic axes. The mammalian embryo has been regarded as an exception and primary body axes of the mouse embryo were thought to develop after implantation. However, recent findings have challenged this view. Asymmetry in the fertilised oocyte, as defined by the position of the second polar body and the sperm entry point, can be correlated with the orientation of the animal-vegetal and the embryonic-abembryonic axes in the preimplantation blastocyst. Studies of the pattern of morphogenetic movement of cells and genetic activity in the peri-implantation embryo suggest that the animal-vegetal axis of the blastocyst might presage the orientation of the anterior-posterior axis of the gastrula. This suggests that the asymmetry of the zygote that is established at fertilisation and early cleavage has a lasting impact on the delineation of body axes during embryogenesis.

Specification of embryonic axes begins before cleavage in normal mouse development

Studies on the development of aggregated, isolated and rearranged blastomeres have engendered the view that in mammals, unlike most other animals, egg organization has no role in the genesis of asymmetries that are essential for cellular diversification and the specification of embryonic axes. Such asymmetries are assumed to arise post-zygotically through interactions between initially naive cells. However, various findings are difficult to reconcile with this view. Here, a consistent relationship between the structure of the blastocyst and the two-cell stage in the mouse has been found using a strictly non-invasive marking technique: injection of small oil drops into the substance of the zona pellicuda. This has revealed that both the embryonic-abembryonic axis of the blastocyst and its plane of bilateral symmetry are normally orthogonal to the plane of first cleavage. This relationship was also seen when denuded two-cell conceptuses were prevented from rotating during subsequent cleavage by immobilizing them in a gel. Therefore, during normal mouse development the axes of the blastocyst, which have been implicated in establishing those of the fetus, are already specified by the onset of cleavage.