Technical advances contribute to the study of genomic imprinting

Genomic imprinting in mammals was discovered over 30 years ago through elegant embryological and genetic experiments in mice. Imprinted genes show a monoallelic and parent of origin-specific expression pattern; the development of techniques that can distinguish between expression from maternal and paternal chromosomes in mice, combined with high-throughput strategies, has allowed for identification of many more imprinted genes, most of which are conserved in humans. Undoubtedly, technical progress has greatly promoted progress in the field of genomic imprinting. Here, we summarize the techniques used to discover imprinted genes, identify new imprinted genes, define imprinting regulation mechanisms, and study imprinting functions.

Pluripotent stem cells from maturing oocytes

Embryonic stem cells are mostly derived from mature oocytes that were either fertilized or activated parthenogenetically and then reached the blastocyst stage. From the cell cycle perspective, fertilization or activation induces the exit from meiosis, decondensation of oocyte chromosomes, and the entry into mitosis. Decondensation of oocyte chromatin with subsequent formation of nuclei can be, however, induced at any postgerminal vesicle breakdown meiotic maturation stage. In this article, we discuss the possibility of cleavage of transformed maturing oocytes and whether they can reach the blastocyst stage, from which pluripotent stem cell lines could be derived.

Differentiation potential of histocompatible parthenogenetic embryonic stem cells

Embryonic stem cells (ESCs) hold unique promise for the development of cell replacement therapies, but derivation of therapeutic products from ESCs is hampered by immunological barriers. Creation of HLA-typed ESC banks, or derivation of customized ESC lines by somatic cell nuclear transfer, have been envisioned for engineering histocompatible ESC-derived products. Proof of principle experiments in the mouse have demonstrated that autologous ESCs can be obtained via nuclear transfer and differentiated into transplantable tissues, yet nuclear transfer remains a technology with low efficiency. Parthenogenesis provides an additional means for deriving ESC lines. In parthenogenesis, artificial oocyte activation initiates development without sperm contribution and no viable offspring are produced in the absence of paternal gene expression. Development proceeds readily to the blastocyst stage, from which parthenogenetic ESC (pESC) lines can be derived with high efficiency. We have recently shown that when pESC lines are derived from hybrid mice, early recombination events produce heterozygosity at the major histocompatibility complex (MHC) loci in some of these lines, enabling the generation of histocompatible differentiated cells that can engraft immunocompetent MHC-matched mouse recipients. Here, we explore the differentiation potential of murine pESCs derived in our laboratory.

Differentiation potential of parthenogenetic embryonic stem cells is improved by nuclear transfer

Parthenogenesis is the process by which an oocyte develops into an embryo without being fertilized by a spermatozoon. Although such embryos lack the potential to develop to full term, they can be used to establish parthenogenetic embryonic stem (pES) cells for autologous cell therapy in females without needing to destroy normally competent embryos. Unfortunately, the capacity for further differentiation of these pES cells in vivo is very poor. In this study, we succeeded in improving the potential of pES cells using a nuclear transfer (NT) technique. The original pES cell nuclei were transferred into enucleated oocytes, and the resulting NT embryos were used to establish new NT-pES cell lines. We established 84 such lines successfully (78% from blastocysts, 12% from oocytes). All examined cell lines were positive for several ES cell markers and had a normal extent of karyotypes, except for one original pES cell line and its NT-pES cell derivatives, in which all nuclei were triploid. The DNA methylation status of the differentially methylated domain H19 and differentially methylated region IG did not change after NT. However, the in vivo and in vitro differentiation potentials of NT-pES cells were significantly (two to five times) better than the original pES cells, judged by the production of chimeric mice and by in vitro differentiation into neuronal and mesodermal cell lines. Thus, NT could be used to improve the potential of pES cells and may enhance that of otherwise poor-quality ES cells. It also offers a new tool for studying epigenetics.

Origins of mammalian hematopoiesis: in vivo paradigms and in vitro models

Though a topic of medical interest for centuries, our understanding of vertebrate hematopoietic or "blood-forming" tissue development has improved greatly only in recent years and given a series of scientific and technical milestones. Key among these observations was the description of procedures that allowed the transplantation of blood-forming activity. Beyond this, other advances include the creation of a variety of knock-out animals (mice and more recently zebrafish), microdissection of embryonic and fetal blood-forming tissues, hematopoietic stem (HSC) and progenitor cell (HPC) colony-forming assays, the discovery of cytokines with defined hematopoietic activities, gene transfer technologies, and the description of lineage-specific surface antigens for the identification and purification of pluripotent and differentiated blood cells. The availability of both murine and human embryonic stem cells (ESC) and the delineation of in vitro systems to direct their differentiation have now been added to this analytical arsenal. Such tools have allowed researchers to interrogate the complex developmental processes behind both primitive (yolk sac or extraembryonic) and definitive (intraembryonic) hematopoietic tissue formation. Using ES cells, we hope to not only gain additional basic insights into hematopoietic development but also to develop platforms for therapeutic use in patients suffering from hematological disease. In this review, we will focus on points of convergence and divergence between murine and human hematopoiesis in vivo and in vitro, and use these observations to evaluate the literature regarding attempts to create hematopoietic tissue from embryonic stem cells, the pitfalls encountered therein, and what challenges remain.

The conflict theory of genomic imprinting: how much can be explained?

In some mammalian genes, paternally and maternally derived alleles are expressed differently: this phenomenon is called genomic imprinting. Several-explanations have been proposed for the observed patterns of genomic imprinting, but the most successful explanation is the genetic conflict hypothesis--natural selection operating on the gene expression produces the parental origin-dependent gene expression--because the paternally derived allele tends to be less related to the siblings of the same mother than the maternal allele and hence the paternal allele should evolve to be more aggressive in obtaining maternal resources. The successes and failures of this argument have been examined in explaining the observed patterns of genomic imprinting in mammals. After a brief summary of the observations with some examples, a quantitative genetic model describing the evolution of the cis-regulating element of a gene affecting the maternal resource acquisition was presented. The model supports the verbal argument that the growth enhancer should evolve to show imprinting with the paternal allele expressed and the maternal allele inactive, whereas a growth suppressor gene tends to have an inactive paternal allele and an active maternal allele. There are four major problems of the genetic conflict hypothesis. (1) Some genes affect embryonic growth but are not imprinted (e.g., Igf1), which can be explained by considering recessive, deleterious mutations on the coding regions, (2) A gene exists that shows the pattern that is a perfect reversal (Mash2), which is needed for placental growth, and yet has an active maternal allele and an inactive paternal allele. This can be explained if the overproduction of this gene causes dose-sensitive abortion to occur in early gestation. (3) Paternal disomies are sometimes smaller than normal embryos. This is a likely outcome of evolution if imprinted genes control the allocation between placenta and embryo by modifying the cell developmental fate. (4) Genes on X chromosomes do not follow the predictions of the genetic conflict hypothesis. For genes on X chromosomes, two additional forces of natural selection (sex differentiation and dosage compensation) cause genomic imprinting, possibly in the opposite direction. Available evidence suggests that these processes are stronger than the natural selection caused by female multiple mating. Finally, the same formalism of evolution can handle an alternative nonconflict hypothesis: genomic imprinting might have evolved because it reduces the risk of the spontaneous development of parthenogenetic embryo, causing a serious threat to the life of the mother (ovarian time bomb hypothesis). This hypothesis can also explain major patterns of genomic imprinting. In conclusion, the genetic conflict hypothesis is very successful in explaining the observed patterns of imprinting for autosomal genes and probably is the most likely evolutionary explanation for them. However, for genes on X chromosomes, other processes of natural selection are more important. Considering that a nonconflict hypothesis can also explain the patterns in principle, we need a quantitative estimate of various parameters, such as the rate of dose-dependent abortion, the degree of female promiscuity, and the rate of spontaneous development of the parthenogenetic embryo, in order to make judgments on the relative importance of different forces of natural selection to form genomic imprinting.

Analysis and identification of imprinted genes

Genomic imprinting in mammals results in the unequal expression of the two parental alleles of specific genes. The existence of imprinting in the mouse emerged from nuclear transplantation studies and from the abnormal phenotypes associated with uniparental inheritance of particular chromosome segments. Over the past 5 years, 20 or so imprinted genes have been identified. This has emphasized the important roles played by some imprinted genes in development, permitted a description of the epigenetic properties associated with imprinting, and provided the first insights into the regulation of imprinting. In this article, we discuss the generation of experimental material in which imprinting effects can be analyzed, review the properties of imprinted genes, and discuss how to examine them using state-of-the-art techniques. Finally, we consider the means by which new imprinted genes can be identified.

Metabolism and cell allocation during parthenogenetic preimplantation mouse development

Diploid parthenogenetic postimplantation mouse embryos, containing two maternal genomes, are characterized by poor development of extraembryonic membranes derived from the trophectoderm and primitive endoderm of the blastocyst. This is thought to be caused by a deficiency of expression of paternally derived imprinted genes. Here we have compared the inner cell mass, from which the primitive endoderm and fetal lineages are derived, and the trophectoderm, which forms a major component of the placenta, in parthenogenetic and fertilized preimplantation embryos. We have also studied the metabolism from the 1-cell to the blastocyst stage. Cell numbers were reduced in the ICM and TE of parthenogenetic blastocysts compared to fertilized blastocysts. This was thought to be due to the increased levels of cell death observed in these lineages. Pyruvate and glucose uptake by parthenogenetic embryos was similar to that by fertilized embryos throughout preimplantation development. However, at the expanded blastocyst stage glucose uptake by parthenogenetic embryos was significantly higher than by fertilized embryos. The implications of the actions of imprinted genes and of X-inactivation is discussed.

A human parthenogenetic chimaera

In mice, parthenogenetic embryos die at the early postimplantation stage as a result of developmental requirements for paternally imprinted genes, particularly for formation of extraembryonic tissues. Chimaeric parthenogenetic<==>normal mice are viable, however, due to non-random differences in distribution of their two cell types. Species differences in imprinting patterns in embryo and extra-embryonic tissues mean that there are uncertainties in extrapolating these experimental studies to humans. Here, however, we demonstrate that parthenogenetic chimaerism can indeed result in viable human offspring, and suggest possible mechanisms of origin for this presumably rare event.

Peg1/Mest imprinted gene on chromosome 6 identified by cDNA subtraction hybridization

Parthenogenesis in the mouse is embryonic lethal partly because of imprinted genes that are expressed only from the paternal genome. In a systematic screen using subtraction hybridization between cDNAs from normal and parthenogenetic embryos, we initially identified two apparently novel imprinted genes, Peg1 and Peg3. Peg1 (paternally expressed gene 1) or Mest, the first imprinted gene found on the mouse chromosome 6, may contribute to the lethality of parthenogenones and of embryos with a maternal duplication for the proximal chromosome 6. Peg1/Mest is widely expressed in mesodermal tissues and belongs to the alpha/beta hydrolase fold family. A similar approach with androgenones can be used to identify imprinted genes that are expressed from the maternal genome only.

Genomic imprinting of Mash2, a mouse gene required for trophoblast development

The mouse gene Mash2 encodes a transcription factor required for development of trophoblast progenitors. Mash2-homozygous mutant embryos die at 10 days postcoitum from placental failure. Here we show that Mash2 is genomically imprinted. First, Mash2+/- embryos inheriting a wild-type allele from their father die at the same stage as -/- embryos, with a similar placental phenotype. Second, the Mash2 paternal allele is initially expressed by groups of trophoblast cells at 6.5 and 7.5 days post-coitum, but appears almost completely repressed by 8.5 days post-coitum. Finally, we have genetically and physically mapped Mash2 to the distal region of chromosome 7, within a cluster of imprinted genes, including insulin-2, insulin-like growth factor-2 and H19.

Expression of X-linked genes in androgenetic, gynogenetic, and normal mouse preimplantation embryos

A quantitative RT-PCR approach has been used to examine the expression of a number of X-linked genes during preimplantation development of normal mouse embryos and in androgenetic and gynogenetic mouse embryos. The data reveal moderately reduced expression of the Prps1, Hprt, and Pdha1 mRNAs in androgenetic eight-cell and morula stage embryos, but not in androgenetic blastocysts. Pgk1 mRNA abundance was severely reduced in androgenones at the eight-cell and morula stages and remained reduced, but to a lesser degree, in androgenetic blastocysts. These data indicate that paternally inherited X chromosomes are at least partially repressed in androgenones, as they are in normal XX embryos, and that the degree of this repression is chromosome position-dependent or gene-dependent. Gynogenetic embryos expressed elevated amounts of some mRNAs at the morula and blastocyst stages, indicative of a delay in dosage compensation that may be chromosome position-dependent. The Xist RNA was expressed at a greater abundance in androgenones than in gynogenones at the eight-cell and morula stages, consistent with previous studies. Xist expression was observed in both androgenones and gynogenones at the blastocyst stage. We conclude that the developmental arrest in early androgenones may be, in part, due to reduced expression of essential X-linked genes, particularly those near the X inactivation center, whereas the developmental defects of gynogenones and parthenogenones, by contrast, may be partially due to overexpression of X-linked genes in extraembryonic tissues, possibly those farthest away from the X inactivation center.

Loss of the imprinted IGF2/cation-independent mannose 6-phosphate receptor results in fetal overgrowth and perinatal lethality

Murine embryos that inherit a nonfunctional insulin-like growth factor-II/cation-independent mannose 6-phosphate receptor (Igf2r) gene from their fathers are viable and develop normally into adults. However, the majority of mice inheriting the same mutated allele from their mothers die around birth, as a consequence of major cardiac abnormalities. These mice do not express IGF2R in their tissues, are 25-30% larger than their normal siblings, have elevated levels of circulating IGF2 and IGF-binding proteins, and exhibit a slight kink in their tails. These results show that Igf2r is paternally imprinted and reveal that the receptor is crucial for regulating normal fetal growth, circulating levels of IGF2, and heart development.

Abnormal development of embryonic and extraembryonic cell lineages in parthenogenetic mouse embryos

Parthenogetically activated, diploid mouse oocytes can develop to midgestation stages in utero. However, even these advanced parthenogenones appear to die because of much reduced trophoblast and yolk sac development. Previous studies have compared the general features of parthenogenetic and androgenetic development and determined the fate of uniparental cells in chimeras with normal embryos. These studies led to the concept of genomic imprinting as the cause for developmental failure when either the maternal or the paternal genome is duplicated, with the corresponding deficiency of the other. Genomic imprinting appears to arise during gametogenesis and to act through dosage effects in a set of imprinted genes, whose expression depends on their parental origin. In this study we undertook a more detailed morphological analysis of parthenogenetic development in the mouse and established a classification system to quantify the developmental extent of parthenogenones. We found that the failure of parthenogenones occurred at different times during early postimplantation development, generating a spectrum of concepti which had developed to different extents, with only a small fraction of the embryos reaching advanced somite stages. In all parthenogenones differentiation and proliferation of the trophectoderm and primitive endoderm lineages (both extraembryonic) was abnormal, and in all, even the best-developed parthenogenones, we observed similar deficiencies in the embryonic lineages, especially the mesoderm. Common to all abnormally developed lineages was that the proportion of undifferentiated precursor cells was much reduced, while their differentiated descendants were relatively abundant. We propose, therefore, that the failure of parthenogenones to develop to term is due to abnormal regulation of differentiation and proliferation in both embryonic and extraembryonic lineages. In this hypothesis, the apparent tissue specific defects observed in parthenogenones arise as a consequence of the functional importance of certain tissues (like the trophoblast) early in development. The spectrum of parthenogenones thus appears to reflect critical events in early development, whose regulation are affected by genomic imprinting.

A functional analysis of imprinting in parthenogenetic embryonic stem cells

A detailed analysis of the developmental potential of parthenogenetic embryonic stem cells (PGES) was made in vivo and in vitro, and a comparison was made with the development of cells from parthenogenetic embryos (PG). In vivo, in chimeras with normal host cells (N), PGES cells showed a restricted tissue distribution consistent with that of PG cells, suggesting faithful imprinting in PGES cells with respect to genes involved in lineage allocation and differentiation. Restricted developmental potential was also observed in teratomas formed by ectopic transfer under the kidney capsule. In contrast, the classic phenotype of growth retardation normally observed in PG&lt;==&gt;N chimeras was not seen, suggesting aberrant regulation in PGES cells of genes involved in growth regulation. We also analysed the expression of known imprinted genes after ES cell differentiation. Igf2, H19 and Igf2r were all appropriately expressed in the PGES derived cells following induction of differentiation in vitro with all-trans retinoic acid or DMSO, when compared with control (D3) and androgenetic ES cells (AGES). Interestingly, H19 was found to be expressed at high levels following differentiation of the AGES cells. Due to the unexpected normal growth regulation of PGES&lt;==&gt;N chimeras we also examined Igf2 expression in PGES derived cells differentiated in vivo and found that this gene was still repressed. Our studies show that PGES cells provide a valuable in vitro model system to study the effects of imprinting on cell differentiation and they also provide invaluable material for extensive molecular studies on imprinted genes. In addition, the aberrant growth phenotype observed in chimeras has implications for mechanisms that regulate the somatic establishment and maintenance of some imprints. This is of particular interest as aberrant imprinting has recently been invoked in the etiology of some human diseases.

Spatially restricted imprinting of mouse chromosome 7

Three of the four known imprinted genes (Igf-2, H19, and Snrpn) map to mouse chromosome 7. We used mRNA phenotyping to examine the tissue-specific transcription of Igf-1r, H-ras-1, and Gabrb3, which map to chromosome 7 between Snrpn and the Igf-2/H19 domain, and Myod-1, which maps proximal to Snrpn. We found that all of these genes were expressed by both parental alleles in tissues from day 1 neonates. The fact that imprinted genes can flank or map closely to genes that escape such epigenetic modification suggests that autosomal imprinting is not manifested globally along imprinted chromosomes but rather is spatially restricted, perhaps even defined by specific DNA consensus sequences or an "imprint box" associated with imprintable genes.

Parthenogenetic stem cells in postnatal mouse chimeras

The ability of parthenogenetic (pg) cells to contribute to proliferating stem cell populations of postnatal aggregation chimeras was investigated. Using DNA in situ analysis, pg participation was observed in highly regenerative epithelia of various regions of the gastrointestinal tract, e.g., stomach, duodenum and colon, in the epithelia of tongue and uterus and in the epidermis. Pg cells also contributed to the epithelium of the urinary bladder, which is characterized by a relatively slow cellular turnover. Using a sensitive proliferation marker to determine division rate of pg and normal (wt) cells in tissues of a 24-day-old chimera, no significant differences between pg and fertilized cells were observed. However, in colon and uterus of a pg &lt;==&gt; wt chimera aged 101 days, a significant loss of proliferative capacity of pg cells was found. In the colon, this loss of proliferative potential was accompanied by an altered morphology of pg crypts. In general, they were situated at the periphery of the epithelium and lacked access to the lumen, with consequent cystic enlargement and flattened epithelium. No obvious morphological changes were observed in the pg-derived areas of the uterine epithelium of this chimera. Our results provide evidence that pg cells can persist as proliferating stem cells in various tissues of early postnatal chimeras. They suggest that pg-derived stem cells may cease to proliferate in restricted areas of the gastrointestinal tract and in the uterine epithelium of pg &lt;==&gt; wt chimeras of advanced age.(ABSTRACT TRUNCATED AT 250 WORDS)

Use of triple tissue blastocyst reconstitution to study the development of diploid parthenogenetic primitive ectoderm in combination with fertilization-derived trophectoderm and primitive endoderm

Diploid mouse conceptuses lacking a paternal genome can form morphologically normal but small fetuses of up to 25 somites, but they invariably fail to develop beyond mid-gestation. Such conceptuses differ from normal most notably in the poor development of extra-embryonic tissues which are largely of trophectodermal and primitive endodermal origin. However, it is not clear whether the demise of diploid parthenogenetic (P) or gynogenetic (G) conceptuses is attributable entirely to the defective development of these two tissues or whether differentiation of the primitive ectoderm, the precursor of the foetus, extra-embryonic mesoderm and amnion, is also impaired by the absence of a paternal genome. Therefore, a new blastocyst reconstitution technique was used which enabled primitive ectoderm from P blastocysts to be combined with primitive endoderm and trophectoderm from fertilization-derived (F) blastocysts. One third of the 'triple tissue' reconstituted blastocysts that implanted yielded foetuses. However, all foetuses recovered on the 11th or 12th day of gestation were small and, with one exception, either obviously retarded or arrested in development. The exception was a living 44 somite specimen which is the most advanced P foetus yet recorded. Foetuses were invariably degenerating in conceptuses recovered on the 13th day. In contrast, at least 16% of control reconstituted blastocysts with primitive ectoderm as well as primitive endoderm and trophectoderm of F origin developed normally on the 13th day of gestation or to term. Hence, the presence of a paternal genome seems to be essential for normal differentiation of all 3 primary tissues of the mouse blastocyst. The P foetuses that developed from reconstituted blastocysts were so closely invested by their membranes that they often showed abnormal flexure of the posterior region of the body. Several also showed a deficiency of allantoic tissue. Therefore, the possibility that the defect in development of P primitive ectoderms resided in their extra-embryonic tissues was investigated by analysing a series of chimaeras produced by injecting them into intact F blastocysts. The foregoing anomalies were not discernible even when P cells made a large contribution to the extra-embryonic mesoderm or amnion plus umbilical cord. Furthermore, selection against P cells was no greater in extra-embryonic derivatives of the primitive ectoderm than in the foetus itself.

Developmental consequences of imprinting of parental chromosomes by DNA methylation

Genomic imprinting by epigenetic modifications, such as DNA methylation, confers functional differences on parental chromosomes during development so that neither the male nor the female genome is by itself totipotential. We propose that maternal chromosomes are needed at the time when embryonic cells are totipotential or pluripotential, but paternal chromosomes are probably required for the proliferation of progenitor cells of differentiated tissues. Selective elimination or proliferation of embryonic cells may occur if there is an imbalance in the parental origin of some alleles. The inheritance of repressed and derepressed chromatin structures probably constitutes the initial germ-line-dependent 'imprints'. The subsequent modifications, such as changes in DNA methylation during early development, will be affected by the initial inheritance of epigenetic modifications and by the genotype-specific modifier genes. A significant number of transgene inserts are prone to reversible methylation imprinting so that paternally transmitted transgenes are undermethylated, whereas maternal transmission results in hypermethylation. Hence, allelic differences in epigenetic modifications can affect their potential for expression. The germ line evidently reverses the previously acquired epigenetic modifications before the introduction of new modifications. Errors in the reversal process could result in the transmission of epigenetic modifications to subsequent generation(s) with consequent cumulative phenotypic and grandparental effects.

Development of chimaeric two-cell mouse embryos produced by allogenic exchange of single nucleus from two- and eight-cell embryos

Synchronous or asynchronous chimaeras were produced by transplanting a single nucleus of two- and eight-cell embryos from CD-1xCD-1 or BALB/CxBALB/C albino strains into one enucleated blastomere of a late F1 (C57/BLxCBA) x F1 two-cell embryo. The cytoplasmic volume of the blastomere was reduced in some instances by 50%. These chimaeric embryos were cultured in vitro and transferred to pseudopregnant recipients. The distribution of each component to the pups and to the day-10 embryos after transfer to recipients was determined by examining their coat color and by glucose phosphate isomerase analysis, respectively. The contribution of progeny of the nuclear-transplanted cell with nonreduced cytoplast to the pups was 83% when synchronous; this proportion decreased to 43% when asynchronous because the progeny tended to migrate to the trophoblast and/or to the primitive endoderm. When the recipient cytoplast was reduced by 50%, the contribution of the nuclear-transplanted cell progeny to the pups was 79% when synchronous and 80% when asynchronous. This shows that allogenic exchange of a single nucleus at the two-cell stage by nuclear transfer is an effective procedure for producing highly asynchronous mouse chimaeras and suggests that larger and advanced blastomeres tend to be excluded from the inner cell mass of the embryo, but smaller, advanced blastomeres do not.

Preimplantation mammalian aggregation and injection chimeras

The preimplantation embryo is highly resilient to experimental manipulations. A specific manipulation that has revealed many clues to the developmental process is chimera production. Chimeras have been used to describe the importance of developmental characteristics of embryonic cells and how these characteristics are involved with developmental fate. These characteristics have been monopolized in the production of interspecific chimeras and the production of transgenic animals. This review attempts to discuss the major factors affecting preimplantation mammalian embryo chimera production.

Chimeras between parthenogenetic or androgenetic blastomeres and normal embryos: allocation to the inner cell mass and trophectoderm

A series of chimeras was generated by injecting single normal, parthenogenetic, or androgenetic blastomeres carrying transgenic markers under the zona pellucida of nontransgenic eight-cell embryos. These chimeras were cultured to the blastocyst stage and sectioned, and the transgenic component was detected by in situ hybridization. No statistically significant difference was found among the normal, parthenogenetic, and androgenetic chimeras in the number of chimeric blastocysts with a transgenic contribution to the inner cell mass (ICM), the trophectoderm, or both the ICM and trophectoderm. Since androgenetic and parthenogenetic cells were present in chimeras at a high frequency in both the ICM and trophectoderm at the blastocyst stage, but not in similar chimeras at late gastrulation, these cells must not respond normally to developmental signals subsequent to blastocyst formation and prior to late gastrulation.

The developmental fate of androgenetic, parthenogenetic, and gynogenetic cells in chimeric gastrulating mouse embryos

Both a maternal and a paternal genomic contribution are necessary for completion of embryonic development in the mouse. Parthenogenetic embryos, with only a maternally inherited genome, and androgenetic embryos, with only a paternally inherited genome, fail to develop to term, and these two types of isoparental embryos fail in development in characteristic ways. In this paper we describe the construction of chimeras between single androgenetic, parthenogenetic, and gynogenetic blastomeres and normal eight-cell embryos. We allow the development of the chimeras to reach the late-gastrulating-stage embryo and then analyze the tissue distributions of the isoparental component. The isoparental embryos are derived from a transgenic mouse line carrying plasmid and mouse beta-globin sequences. The isoparental cells are detected in histological sections of chimeras by DNA-DNA in situ hybridization to the transgene, using a biotinylated DNA probe with an enzymatic detection system. We found strong tissue preferences for the androgenetic, parthenogenetic, and gynogenetic cells in chimeras. Androgenetic cells contributed strongly to all trophectoderm-derived tissue, with only a rare contribution to any tissues of the embryo proper, extraembryonic mesoderm, or extraembryonic endoderm. Parthenogenetic cells shared a developmental fate similar to gynogenetic cells, contributing to all tissues of the embryo proper and to the extraembryonic mesoderm, but only rarely to the extraembryonic endoderm or to any trophectoderm-derived tissues.