Karyotypically Normal and Abnormal Human Embryonic Stem Cell Lines Derived from PGD-Analyzed Embryos

Chromosomal mosaicism in human blastocysts: the ultimate diagnostic dilemma

BACKGROUND

Trophectoderm (TE) biopsy and next generation sequencing (NGS) are currently the preferred techniques for preimplantation genetic testing for aneuploidies (PGT-A). Although this approach delivered important improvements over previous testing strategies, increased sensitivity has also prompted a rise in diagnoses of uncertain clinical significance. This includes reports of chromosomal mosaicism, suggesting the presence of karyotypically distinct cells within a single TE biopsy. Given that PGT-A relies on the chromosomal constitution of the biopsied cells being representative of the entire embryo, the prevalence and clinical implications of blastocyst mosaicism continue to generate considerable controversy.

OBJECTIVE AND RATIONALE

The objective of this review was to evaluate existing scientific evidence regarding the prevalence and impact of chromosomal mosaicism in human blastocysts. We discuss insights from a biological, technical and clinical perspective to examine the implications of this diagnostic dilemma for PGT-A.

SEARCH METHODS

The PubMed and Google Scholar databases were used to search peer-reviewed publications using the following terms: ‘chromosomal mosaicism’, ‘human’, ‘embryo’, ‘blastocyst’, ‘implantation’, ‘next generation sequencing’ and ‘clinical management’ in combination with other keywords related to the subject area. Relevant articles in the English language, published until October 2019 were critically discussed.

OUTCOMES

Chromosomal mosaicism predominately results from errors in mitosis following fertilization. Although it appears to be less pervasive at later developmental stages, establishing the true prevalence of mosaicism in human blastocysts remains exceedingly challenging. In a clinical context, blastocyst mosaicism can only be reported based on a single TE biopsy and has been ascribed to 2–13% of embryos tested using NGS. Conversely, data from NGS studies disaggregating whole embryos suggests that mosaicism may be present in up to ~50% of blastocysts. However, differences in testing and reporting strategies, analysis platforms and the number of cells sampled inherently overshadow current data, while added uncertainties emanate from technical artefacts. Moreover, laboratory factors and aspects of in vitro culture generate further variability. Outcome data following the transfer of blastocysts diagnosed as mosaic remain limited. Current studies suggest that the transfer of putative mosaic embryos may lead to healthy live births, but also results in significantly reduced ongoing pregnancy rates compared to the transfer of euploid blastocysts. Observations that a subset of mosaic blastocysts has the capacity to develop normally have sparked discussions regarding the ability of embryos to self-correct. However, there is currently no direct evidence to support this assumption. Nevertheless, the exclusion of mosaic blastocysts results in fewer embryos available for transfer, which may inevitably compromise treatment outcomes.

WIDER IMPLICATIONS

Chromosomal mosaicism in human blastocysts remains a perpetual diagnostic and clinical dilemma in the context of PGT-A. This review offers an important scientific resource, informing about the challenges, risks and value of diagnosing mosaicism. Elucidating these uncertainties will ultimately pave the way towards improved clinical and patient management.

WNT Inhibition and Increased FGF Signaling Promotes Derivation of Less Heterogeneous Primed Human Embryonic Stem Cells, Compatible with Differentiation

Human embryonic stem cells (hESCs) hold great value for future clinical applications. However, standard culture conditions maintain hESCs in a primed state, which bears heterogeneity in pluripotency and a tendency for spontaneous differentiation. To counter these drawbacks, primed hESCs have been converted to a naive state, but this has restricted the efficiency of existing directed differentiation protocols. In mouse, WNT inhibition by inhibitor of WNT production-2, together with a higher dose of fibroblast growth factor 2 (12 ng/mL) in DMEM/F12 basal medium (DhiFI), markedly improved derivation and maintenance of primed mouse epiblast stem cells. In this study, we show that DhiFI conditions similarly improved primed hESC traits, such as conferring a primed transcriptional signature with high levels of pluripotency markers and reduced levels of differentiation markers. When triggered to differentiate to neuronal and cardiac lineages, DhiFI hESCs and isogenic primed hESCs progressed similarly. Moreover, DhiFI conditions supported the derivation of hESC lines from a post-inner cell mass intermediate (PICMI). DhiFI-derived hESCs showed less spontaneous differentiation and expressed significantly lower levels of lineage-specific markers, compared to primed-derived lines from the same PICMI. Overall, DhiFI hESCs retained advantages of both primed and naive pluripotency and may ultimately represent a more favorable starting point for differentiation toward clinically desired cell types.

Establishment of human-embryonic-stem-cell line from mosaic trisomy 9 embryo

OBJECTIVE

Human-embryonic-stem-cell (hESC) lines derived from chromosomally or genetically abnormal embryos obtained following preimplantation genetic diagnosis are valuable in investigating genetic disorders.

MATERIALS AND METHODS

In this study, a new hESC line, Center of Clinical Reproductive Medicine 8 (CCRM8) was established by isolation, culture, and passaging of the inner cell mass of mosaic trisomy 9 embryos.

RESULTS

A karyotype analysis showed that the hESC line possessed a euploid (46 chromosomes). The undifferentiated hESCs exhibited long-term proliferation capacity and expressed typical markers of OCT4, TRA-1-60, and TRA-1-81. In vitro embryoid-body (EB) formation, differentiation, and in vivo teratoma production confirmed the pluripotency of the hESC line. The data represented here are the first detailed report on the characterization and differentiation of one Chinese hESC line generated from mosaic trisomy 9 embryos.

CONCLUSION

Our study showed that chromosomally aberrant embryos could generate a normal hESC line, which would be useful in investigating gene function and embryo development.

A Euploid Line of Human Embryonic Stem Cells Derived from a 43,XX,dup(9q),+12,-14,-15,-18,-21 Embryo

Aneuploid embryos diagnosed by FISH-based preimplantation genetic screening (PGS) have been shown to yield euploid lines of human embryonic stem cells (hESCs) with a relatively high frequency. Given that the diagnostic procedure is usually based on the analysis of 1–2 blastomeres of 5 to 10-cell cleavage-stage embryos, mosaicism has been a likely explanation for the phenomena. However, FISH-based PGS can have a significant rate of misdiagnosis, and therefore some of those lines may have been derived from euploid embryos misdiagnosed as aneuploid. More recently, coupling of trophectoderm (TE) biopsy at the blastocyst stage and array-CGH lead to a more informative form of PGS. Here we describe the establishment of a new line of hESCs from an embryo with a 43,XX,dup(9q),+12,-14,-15,-18,-21 chromosomal content based on array-CGH of TE biopsy. We show that, despite the complex chromosomal abnormality, the corresponding hESC line BR-6 is euploid (46,XX). Single nucleotide polymorphism analysis showed that the embryo´s missing chromosomes were not duplicated in BR-6, suggesting the existence of extensive mosaicism in the TE lineage.

Cell lines from morphologically abnormal discarded IVF embryos are typically euploid and unaccompanied by intrachromosomal aberrations

Routine IVF practices result in the discarding of a significant proportion of embryos due to their unsuitability for transfer or cryopreservation. The present study plated clinically unusable human blastocysts to derive cellular outgrowths for aneuploidy studies and genome-wide analysis of DNA copy number variations, and to evaluate their potential as a source for pluripotent stem cells. Just 79 cellular outgrowths were obtained from 1026 abnormal blastocysts (7.7%), reflecting their low developmental potential. Of these, 13 (16.5%) were karyotypically abnormal and included trisomies frequently detected in miscarriages, each of which was uniform (nonmosaic) and the result of meiotic nondisjunction. Evaluation of submicroscopic DNA gains and losses in 10 diploid cellular outgrowths did not identify increased rates of copy number variations. Five of these outgrowths were shown to express pluripotency markers and could be developed into cell lineages representative of the three germ layers. These data suggest that embryos with chromosomal abnormalities resist cell-line derivation, and mosaic aneuploidy produced from mitotic nondisjunction, common in preimplantation embryos, is likely to be diminished or lost under conditions of diploid cell competition. Furthermore, this work demonstrated that abnormal embryos discarded in IVF programmes can provide a valuable source for pluripotent stem cell lines. During IVF, a large proportion of embryos are clinically unsuitable due to abnormal development and these embryos only have a small chance of achieving a pregnancy. Here we used these abnormal embryos to create cell lines for genetic testing and to determine their potential as stem cells. Of the 1026 abnormal embryos used, 79 (7.7%) created cell lines, reflecting their low developmental potential. Of those, only 16.5% had chromosomal anomalies, a much lower number than expected. This included chromosome abnormalities frequently observed in miscarriages, all of which were found in each cell tested (nonmosaic) and originated from the egg or the sperm as opposed to cell division. In-depth testing of 10 normal cell lines for small DNA gains and losses did not reveal an increased frequency of mutations. Furthermore, five of the cell lines were examined for stem cell properties and found to exhibit the hallmark features of stem cells including their ability to make mature cells from different parts of the body. Our data suggest that embryos with abnormal chromosomes resist making cell lines and that abnormalities that arise during cell division are likely to be lost due to competition with normal cells. We also demonstrated that abnormal embryos usually discarded in IVF programmes can provide a valuable source for stem cell lines.

Derivation of Human Embryonic Stem Cell Lines from Vitrified Human Blastocysts

Human embryonic stem cells are pluripotent cells typically derived from blastulating embryos that have become excess to clinical needs in assisted reproduction programs. They provide cellular models for embryonic development and disease, and are thought to be useful for future cell replacement therapies and regenerative medicine. Here we describe methods to derive human embryonic stem cell lines. This includes blastocyst cryopreservation using a highly efficient vitrification protocol, the production and use of fibroblast feeder cells, embryo plating and passaging of resulting cellular outgrowths, and cryopreservation of putative stem cells lines.

Establishment, characterization, and differentiation of a karyotypically normal human embryonic stem cell line from a trisomy-affected embryo

Derivation of human embryonic stem cell (hESC) lines from chromosomally or genetically abnormal embryos obtained following preimplantation genetic diagnosis (PGD) is of immense interest to study various kinds of genetic disorders. In this study, we have established a new hESC line Relicell(®)hES4, isolated from an aneuploid embryo. Derivation of this cell line was achieved by isolation of the inner cell mass (ICM) by mechanical method. Karyotype analysis showed that the hESC line is euploid having 46 chromosomes, contrary to our expectations. The undifferentiated cells exhibited long-term proliferation capacity and expressed markers typical for hESC, such as OCT4, NANOG, and SSEA4. A comparative microarray study was carried out to analyze the transcription profile of Relicell(®)hES4 along with three other normal hESC line generated earlier in our lab. Relicell(®)hES4 manifested pluripotent differentiation potential both in vivo and in vitro. The cells were also induced to form neurons, cardiomyocytes, and pancreatic β islets. The generation of a normal hESC line from an abnormal embryo points to the fact that even such embryos can be considered for deriving new hESC lines instead of discarding them. The data represented here are the first detailed report on characterization and differentiation of an Indian hESC line generated from a PGD analyzed embryo.

Vitrified blastocysts from Preimplantation Genetic Diagnosis (PGD) as a source for human Embryonic Stem Cell (hESC) derivation

Embryos diagnosed as abnormal in Preimplantation Genetic Diagnosis (PGD) cycles are useful for the establishment of human Embryonic Stem Cells (hESC) lines with genetic disorders. These lines can be helpful for drug screening and for the development of new treatments. Vitrification has proved to be an efficient method to preserve human blastocysts. One hundred and three abnormal or undiagnosed vitrified blastocysts from the PGD programme at Institut Universitari Dexeus were donated for human embryonic stem cell derivation. The overall survival rate after warming was 70.6 %. Our results showed better survival rates when blastocysts have not started the hatching process (initial/expanded 87.8 %, hatching 68.3 % and hatched 27.3 %). Thirty-five blastocysts and 12 partially surviving embryos were seeded. One hESC line with the multiple exostoses type 2 paternal mutation was obtained.

Triploid and diploid embryonic stem cell lines derived from tripronuclear human zygotes

PURPOSE

Human embryonic stem cells (hESCs) are self-renewing, pluripotent cells that are valuable research tools and hold promise for use in regenerative medicine. The need for new hESC lines motivated our attempts to find a new resource for the derivation of hESC lines. The aim of this work was to establish more hESC lines from abnormal fertilized zygotes and to meet the emerging requirements for their use in cell replacement therapies, disease modeling, and basic research.

METHODS

A total of 130 tripronuclear human zygotes was collected 18-20 h post-insemination and cultured in a modified culture medium. The inner cell mass of 12 blastocysts were isolated by a mechanical method in order to establish embryonic stem cell lines.

RESULTS

We established four hESC lines derived from 130 trinuclear zygotes, one of which was triploid and the others were diploid. The efficiency of deriving hESC lines is 3.08 %. The ratio of deriving triploid and diploid hESC lines is 1:3. All of these hESC lines exhibited similar markers of undifferentiated hESCs and had the typical morphology of hESCs, a capacity for long-term proliferation, and pluripotent differentiation potential both in vivo and in vitro.

CONCLUSIONS

These abnormal zygotes, which otherwise would have been discarded, can serve as an alternative source for normal euploid hESC lines.

Derivation of novel genetically diverse human embryonic stem cell lines

Human embryonic stem cells (hESCs) have the potential to revolutionize many biomedical fields ranging from basic research to disease modeling, regenerative medicine, drug discovery, and toxicity testing. A multitude of hESC lines have been derived worldwide since the first 5 lines by Thomson et al. 13 years ago, but many of these are poorly characterized, unavailable, or do not represent desired traits, thus making them unsuitable for application purposes. In order to provide the scientific community with better options, we have derived 12 new hESC lines at New York University from discarded genetically normal and abnormal embryos using the latest techniques. We examined the genetic status of the NYUES lines in detail as well as their molecular and cellular features and DNA fingerprinting profile. Furthermore, we differentiated our hESCs into the tissues most affected by a specific condition or into clinically desired cell types. To our knowledge, a number of characteristics of our hESCs have not been previously reported, for example, mutation for alpha thalassemia X-linked mental retardation syndrome, linkage to conditions with a genetic component such as asthma or poor sperm morphology, and novel combinations of ethnic backgrounds. Importantly, all of our undifferentiated euploid female lines tested to date did not show X chromosome inactivation, believed to result in superior potency. We continue to derive new hESC lines and add them to the NIH registry and other registries. This should facilitate the use of our hESCs and lead to advancements for patient-benefitting applications.

Mutated human embryonic stem cells for the study of human genetic disorders

Human embryonic stem cells (HESCs) are of great interest in biology and medicine due to their ability to grow indefinitely in culture while maintaining their ability to differentiate into all different cell types in the human body. In addition, HESCs can be used for better understanding the key developmental processes and can, therefore, serve for studying genetic disorders for which no good research model exists. Preimplantation genetic diagnosis of in vitro derived embryos results in affected-spare blastocysts with specific known inherited mutations.These affected blastocysts can be used for the derivation of disease-bearing HESCs, which would serve for studying the molecular and pathophysiological mechanisms underlying the genetic disease for which they were diagnosed. This chapter describes the methods to derive HESCs carrying mutations for inherited disorders.

Aneuploid human embryonic stem cells: origins and potential for modeling chromosomal disorders

Chromosomal aneuploidies are widely recognized genetic disorders in humans that often lead to spontaneous abortion. Aneuploid fetuses that survive to term commonly exhibit impaired developmental growth and mental retardation in addition to multiple congenital malformations. Preimplantation genetic screening is used to detect chromosomal aneuploidies in early embryos. Human embryonic stem cell (ESC) cell lines generated from aneuploid embryos created a unique repository of cell lines. The spectrum of aneuploidies in these ESC lines reflects the range of common embryonic chromosomal aberrations and significantly differs from the spectrum of aneuploid human ESC lines generated by cell adaptation in culture. The aneuploid human ESC lines represent an excellent model to study human chromosomal abnormalities especially in the early stages of development.

Female sex bias in human embryonic stem cell lines

The factors limiting the rather inefficient derivation of human embryonic stem cells (HESCs) are not fully understood. The aim of this study was to analyze the sex ratio in our 42 preimplantation genetic diagnosis (PGD)-HESC lines, in an attempt to verify its affect on the establishment of HESC lines. The ratio between male and female PGD-derived cell lines was compared. We found a significant increase in female cell lines (76%). This finding was further confirmed by a meta-analysis for combining the results of all PGD-derived HESC lines published to date (148) and all normal karyotyped HESC lines derived from spare in vitro fertilization embryos worldwide (397). Further, gender determination of embryos demonstrated that this difference originates from the actual derivation process rather than from unequal representation of male and female embryos. It can therefore be concluded that the clear-cut tendency for female preponderance is attributed to suboptimal culture conditions rather than from a true gender imbalance in embryos used for derivation of HESC lines. We propose a mechanism in which aberrant X chromosome inactivation and/or overexpression of critical metabolic X-linked genes might explain this sex dimorphism.

Generation of human embryonic stem cells

This unit describes generation of human embryonic stem cell lines from early human embryos. The focus is on actual handling of embryos and early embryonic outgrowths, omitting steps required for actual generation, freezing, and thawing of embryos, as well as further culture and characterization of newly derived stem cells. Hence, the initial culture of embryos to a blastocyst stage is described, followed by removal of the protective zona pellucida layer, isolation of the inner cell mass (ICM), subsequent plating of ICM or whole embryo and, finally, the first few passages of an early embryonic outgrowth. A few alternative procedures for some steps such as zona removal and inner cell mass isolation are described, to allow procedures to be modified according to circumstances.

The Janus face of pluripotent stem cells--connection between pluripotency and tumourigenicity

Pluripotent stem cells have gained special attraction because of their almost unlimited proliferation and differentiation capacity in vitro. These properties substantiate the potential of pluripotent stem cells in basic research and regenerative medicine. Here three types of in vitro cultured pluripotent stem cells (embryonic carcinoma, embryonic stem and induced pluripotent stem cells) are compared in their historical context with respect to their different origin and properties. It became evident that tumourigenicity is an inherent property of pluripotent cells based on p53 down-regulation, expression of tumour-related genes and high telomerase activity that allow unlimited proliferation. In addition, culture-adapted genetic and epigenetic changes may induce tumourigenicity of pluripotent cells. The use of stem cells in regenerative medicine, however, requires non-malignant cell types and strategies that circumvent stages of malignancy.Reprogramming strategies of adult somatic cells that avoid the tumourigenic state of pluripotency may offer alternatives for future biomedical application.

Biochemical and morphological effects of hypoxic environment on human embryonic stem cells in long-term culture and differentiating embryoid bodies

The mammalian reproductive tract is known to contain 1.5-5.3% oxygen (O(2)), but human embryonic stem cells (hESCs) derived from preimplantation embryos are typically cultured under 21% O(2) tension. The aim of this study was to investigate the effects of O(2) tension on the long-term culture of hESCs and on cell-fate determination during early differentiation. hESCs and embryoid bodies (EBs) were grown under different O(2) tensions (3, 12, and 21% O(2)). The expression of markers associated with pluripotency, embryonic germ layers, and hypoxia was analyzed using RTPCR, immunostaining, and Western blotting. Proliferation, apoptosis, and chromosomal aberrations were examined using BrdU incorporation, caspase-3 immunostaining, and karyotype analysis, respectively. Structural and morphological changes of EBs under different O(2) tensions were comparatively examined using azan- and hematoxylineosin staining, and scanning and transmission electron microscopy. Mild hypoxia (12% O(2)) increased the number of cells expressing Oct4/Nanog and reduced BrdU incorporation and aneuploidy. The percentage of cells positive for active caspase-3, which was high during normoxia (21% O(2)), gradually decreased when hESCs were continuously cultured under mild hypoxia. EBs subjected to hypoxia (3% O(2)) exhibited well-differentiated microvilli on their surface, secreted high levels of collagen, and showed enhanced differentiation into primitive endoderm. These changes were associated with increased expression of Foxa2, Sox17, AFP, and GATA4 on the EB periphery. Our data suggest that mild hypoxia facilitates the slow mitotic division of hESCs in long-term culture and reduces the frequency of chromosomal abnormalities and apoptosis. In addition, hypoxia promotes the differentiation of EBs into extraembryonic endoderm.

Present state and future perspectives of using pluripotent stem cells in toxicology research

The use of novel drugs and chemicals requires reliable data on their potential toxic effects on humans. Current test systems are mainly based on animals or in vitro–cultured animal-derived cells and do not or not sufficiently mirror the situation in humans. Therefore, in vitro models based on human pluripotent stem cells (hPSCs) have become an attractive alternative. The article summarizes the characteristics of pluripotent stem cells, including embryonic carcinoma and embryonic germ cells, and discusses the potential of pluripotent stem cells for safety pharmacology and toxicology. Special attention is directed to the potential application of embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) for the assessment of developmental toxicology as well as cardio- and hepatotoxicology. With respect to embryotoxicology, recent achievements of the embryonic stem cell test (EST) are described and current limitations as well as prospects of embryotoxicity studies using pluripotent stem cells are discussed. Furthermore, recent efforts to establish hPSC-based cell models for testing cardio- and hepatotoxicity are presented. In this context, methods for differentiation and selection of cardiac and hepatic cells from hPSCs are summarized, requirements and implications with respect to the use of these cells in safety pharmacology and toxicology are presented, and future challenges and perspectives of using hPSCs are discussed.

Effect of karyotype on successful human embryonic stem cell derivation

The success rate of human embryonic stem cell (hESC) derivation depends on both culture conditions and embryo quality and is routinely determined by morphological criteria. However, high incidence of chromosomal abnormality even in high-grade cleavage embryos from in vitro fertilization (IVF) patients suggests that the morphological grade of supernumerary embryos obtained from IVF clinics may not be a good prediction factor for successful hESC derivation. We show here that from one donor under identical derivation conditions 12 karyotypically abnormal post-bioptic embryos did not yield hESC lines, whereas two out of four normal embryos did. This suggests that the capacity of embryos to give rise to hESC line is likely to be influenced by their genetic status.

Derivation of three new human embryonic stem cell lines

Human embryonic stem cells are pluripotent cells capable of extensive self-renewal and differentiation to all cells of the embryo proper. Here, we describe the derivation and characterization of three Sydney IVF human embryonic stem cell lines not already reported elsewhere, designated SIVF001, SIVF002, and SIVF014. The cell lines display typical compact colony morphology of embryonic stem cells, have stable growth rates over more than 40 passages and are cytogenetically normal. Furthermore, the cell lines express pluripotency markers including Nanog, Oct4, SSEA3 and Tra-1-81, and are capable of generating teratoma cells derived from each of the three germ layers in immunodeficient mice. These experiments show that the cell lines constitute pluripotent stem cell lines.

Derivation of human embryonic stem cell lines from vitrified human embryos

Human embryonic stem cell lines are usually derived from human embryos that have become excess to clinical needs in assisted reproduction programs, whether because the couple in question has completed their family or because the embryo was found to be clinically unsuitable for transfer due to severe genetic condition (in case of pre-implantation genetic diagnosis, PGD). Culturing embryos to a blastocyst stage (5-6 days after IVF) before embryo transfer or cryopreservation instead of earlier commonly used 8-cell stage (3 days after IVF) calls for new methods for embryo cryopreservation and allows higher efficiencies for the actual stem cell derivation. Despite the vast advances in other fields of embryonic stem cell research, methods for derivation of new lines have not changed much over the years, mainly due to scarcity of embryos limiting experimentation. We describe here methods required to derive new embryonic stem cell lines starting from the initial cryopreservation of an embryo and finishing with a new cell line. We cover embryo cryopreservation and warming using a highly efficient vitrification method, the production of feeder cells and feeder plates, as well as embryo handling, plating and critical early passages, including earliest possible cryopreservation of putative stem cells using vitrification.

Preimplantation genetic diagnosis as a source of human embryonic stem cells for disease research and drug discovery

Embryos surplus to therapeutic requirements following preimplantation genetic diagnosis can be used to derive human embryonic stem cell (hESC) lines carrying mutations significant to human disease. These cells provide a powerful in vitro tool for modelling disease progression in a number of cell types as well as having the potential to revolutionise drug discovery. Robust and reproducible directed differentiation protocols are needed to maximise the potential of these cells. In this review, we explore the current use of hESC and induced pluripotent stem cells in disease-specific research and discuss the use of stem cell technology in drug discovery and toxicity testing.

[Human embryonic stem cells within the context of international research activity]

Research involving pluripotent human embryonic stem cells (hESCs) is a rapidly growing field of science. Since hESCs originate from early human embryos, alternative methods for producing pluripotent cells have been developed. This article introduces some of those strategies and, in addition, covers international efforts to establish consistent international standards for cultivation, characterization and preservation of hESCs. Furthermore, global trends to form networks in the field of stem cell research as well as endeavors to harmonize ethical standards for hESC research are presented. Finally, potential applications of hESCs in the field of pharmacology/toxicology are discussed as well as recent results of animal studies using hESCs.

Parthenogenesis as an approach to pluripotency: advantages and limitations involved

Embryonic stem cells (ESCs) are invaluable cells derived from the inner cell mass of the mammalian blastocyst. They have nearly indefinite self-renewal, retain their developmental potential after prolonged periods in culture and display great plasticity that allow them to differentiate into all cell types of the body. They provide exciting opportunities to develop unique models for developmental research and hold great potential for cell and tissue replacement therapy. However, these unique cells cannot be obtained without destroying an embryo and, despite the potential therapeutic usefulness, their derivation in the human raises substantial ethical as well as legal and political concerns because it unavoidably involves the destruction of viable embryos. In the recent years a number of scientific proposals that do not require the generation and subsequent destruction of human embryos have been put forward in an attempt to fill the gap between ethical questions and potential scientific and medical benefits. In this review we briefly summarize data obtained from the literature related to these different alternative approaches and focus in more details on our experience in the derivation of parthenothes, as a possible alternative source for pluripotent cells, discussing the advantages as well as the limits of these cell lines.