Oocyte-induced haploidization

Artificial Oocyte: Development and Potential Application

Millions of people around the world suffer from infertility, with the number of infertile couples and individuals increasing every year. Assisted reproductive technologies (ART) have been widely developed in recent years; however, some patients are unable to benefit from these technologies due to their lack of functional germ cells. Therefore, the development of alternative methods seems necessary. One of these methods is to create artificial oocytes. Oocytes can be generated in vitro from the ovary, fetal gonad, germline stem cells (GSCs), ovarian stem cells, or pluripotent stem cells (PSCs). This approach has raised new hopes in both basic research and medical applications. In this article, we looked at the principle of oocyte development, the landmark studies that enhanced our understanding of the cellular and molecular mechanisms that govern oogenesis in vivo, as well as the mechanisms underlying in vitro generation of functional oocytes from different sources of mouse and human stem cells. In addition, we introduced next-generation ART using somatic cells with artificial oocytes. Finally, we provided an overview of the reproductive application of in vitro oogenesis and its use in human fertility.

Haploidy in somatic cells is induced by mature oocytes in mice

Haploidy is naturally observed in gametes; however, attempts of experimentally inducing haploidy in somatic cells have not been successful. Here, we demonstrate that the replacement of meiotic spindles in mature metaphases II (MII) arrested oocytes with nuclei of somatic cells in the G0/G1 stage of cell cycle results in the formation of de novo spindles consisting of somatic homologous chromosomes comprising of single chromatids. Fertilization of such oocytes with sperm triggers the extrusion of one set of homologous chromosomes into the pseudo-polar body (PPB), resulting in a zygote with haploid somatic and sperm pronuclei (PN). Upon culture, 18% of somatic-sperm zygotes reach the blastocyst stage, and 16% of them possess heterozygous diploid genomes consisting of somatic haploid and sperm homologs across all chromosomes. We also generate embryonic stem cells and live offspring from somatic-sperm embryos. Our finding may offer an alternative strategy for generating oocytes carrying somatic genomes.

Advances in improving fertility in women through stem cell-based clinical platforms

INTRODUCTION

Due to their regenerative ability, stem cells are looked at as a promising tool for improving infertility treatments in women. As the main limiting factor in female fertility is represented by the decrease of ovarian reserve, the main goals of stem cell-based clinical platforms would be to obtain in vitro or in vivo neo-oogenesis. Refractory endometrial factor infertility also represents an obstacle for female reproduction for which stem cells might provide novel treatment strategies. Areas covered: A systematic search of the literature was performed on MEDLINE/PubMed database to identify relevant articles using stem-cell based clinical or research platforms in the field of female infertility. Expert opinion: In vitro oogenesis has not so far developed beyond the stage of oocyte-like cells whose normal progression to mature oocytes and ability to be fertilized was not proved. Extensive epigenetic programming of gamete precursors and the complex interactions between somatic and germ cells required for human oogenesis likely represent the main obstacles in stem-cell-based neo-oogenesis. Also resuming oogenesis in vivo in adulthood still appears a distant hypothesis, as there is still a lack of consensus about the existence and functionality of adult ovarian stem cells.

Effects of ovarian disaggregation on adult murine follicle yield and viability

Follicles are isolated from ovaries for numerous reasons, including IVM, but adult murine yields are <2 follicles mg−1. The aim of the present study was to optimise ovarian disaggregation and develop methods applicable to the rapid screening of follicle viability. Ovaries from adult mice (n = 7) were halved and disaggregated mechanically, or by using collagenase IV (Col-IV; 590 U mL−1) or animal origin-free collagenase IV (AOF) at 590 or 1180 U mL−1. Isolated follicles were stained with 4′,6′-diamidino-2-phenylindole (DAPI; nuclei), chloromethyl-X-rosamine (CMXRos; mitochondria) or fluorescein isothiocyanate-conjugated anti-α-tubulin antibody. Follicle diameters and staining were measured and analysed using ImageJ, and data analysed using GraphPad Prism. Col-IV disaggregation yielded the highest number of follicles (17 ± 10 follicles mg−1 ovarian tissue). All disaggregation methods released more secondary follicles (86 ± 20 per ovary; P < 0.05) than any other size cohort. Mechanical and Col-IV disaggregation yielded similar numbers of morphologically intact follicles, whereas AOF disaggregation caused more damage (P < 0.01). As the morphological disruption increased, DAPI and CMXRos staining decreased (P < 0.05), and tubulin localisation became more heterogeneous. Col-IV disaggregation gave the best yield of morphologically intact follicles containing viable granulosa cells. In conclusion, we improved adult murine follicle yields and applied molecular markers to assess follicle morphology, cellular cytoskeleton and mitochondrial function.

In vitro development of non-enucleated rat oocytes following microinjection of a cumulus nucleus and chemical activation

The present study examined in vitro development and the cytological status of non-enucleated rat oocytes after microinjection of cumulus nuclei and chemical activation. Oocyte–cumulus complexes were collected from gonadotropin-treated prepubertal female Wistar rats 14 h after human chorionic gonadotropin (hCG) injection. Cumulus nuclei were injected into ovulated oocytes and then stimulated in the presence of 5 mM SrCl2 for 20 min at various time points (0–3.5 h) after injection. Some of the reconstituted eggs were cultured to observe the pronuclear formation, cleavage, and blastocyst formation. The incidences of eggs forming at least one pronucleus or containing two pronuclei were not significantly different among the periods (82.4–83.5% and 43.4–51.9%, respectively). Nor did the incidences of eggs cleaving (86.7–97.7%) and developing to the blastocyst stage (0–3.5%) differ depending on when, after injection, stimulation began. When some of the reconstituted eggs were observed for cytological morphology 1–1.5 h after injection, 71.7% of the eggs caused premature chromatin condensation, but only 46.2% of them formed two spindles around each of maternal and somatic chromatins. However, the morphology of the somatic spindles differed from that of the spindles, which formed around the oocyte chromatins. Only 7.5% of the eggs contained the normal chromosomal number. In many reconstituted oocytes, before activation, an abnormal spindle formation was observed in the somatic chromatins. In conclusion, these results show that non-enucleated rat oocytes injected with cumulus nuclei can form pronuclei and cleave following chemical activation, whereas blastocyst formation is very limited, probably caused by abnormalities in the spindle formation and distribution of somatic chromatids.

Butyrolactone I reversibly alters nuclear configuration, periooplasmic microtubules and development of porcine oocytes

In the present study, we investigated the effects of specific cdc2 kinase inhibitor, butyrolactone I (BL I) on the prevention of germinal vesicle breakdown, changes of microtubular structures, and development of porcine oocytes after removal of the drug. In Experiment 1, cumulus-oocyte complexes (COCs) were cultured (44 h) in NCSU-23 medium containing different concentrations of BL I. The percentages of oocytes remaining at GV stage were 0, 0, 32, 80, and 84% (P<0.05), and the maturation rates were 86, 63, 30, 0, and 0% (P<0.05) for oocytes treated with 0, 10, 20, 40, and 80 microM of BL I, respectively. When oocytes were released from BL I incubation (Experiment 2) and cultured for an additional 44 h, 79, 84, and 83% of oocytes resumed meiosis, but only 52, 38 and 17% of oocytes reached normal metaphase II (MII) stage in the groups treated with 20, 40 and 80 microM BL I, respectively. In Experiments 3-5, reversibility and development of oocytes and embryos were evaluated after removal of the inhibitor. A reduced duration of BL I incubation (22 h) at 20 microM increased the percentage of oocytes remaining at the GV stage compared to the control group (85% versus 9%, P<0.05). Blastocyst rates were lower in treatment groups than in the control (44 h) group (0-14% versus 24%; P<0.05). However, all developing blastocysts possessed similar cell numbers, regardless of the drug-treated or non-treated controls. Taken together, treatment with 20-80 microM of BL I effectively prevented the resumption of meiosis and polymerization of periooplasmic microtubules. Furthermore, reversibility of the oocytes after reduced duration of BL I treatment was satisfactory.

Artificial gametes

In vitro fertilization (IVF) has been an efficient medical treatment for infertility in the past decades. However, conventional IVF approaches may be insufficient when gametes are lacking or non-viable thus precluding a significant number of patients from treatment. Ultimately, creation of artificial gametes may provide an universal solution for all indications. Somatic cell nuclear transfer (SCNT) has provided successful cloning in different animal species indicating that a derived technology may be applicable in infertility treatment procedures. Attempts to produce functional male or female gamete through nuclear transfer have been described through the process called haploidization. Initial successes have been observed, however, significant alterations at spindle construction and chromosomal segregation were also described. Stem cell technology may provide an alternative route to obtain fully functional gametes. Both sperm cells and oocytes were obtained using specific culture conditions for embryo originated stem cell. These two mainstream approaches are presented in the current review. Both of these techniques are involving sophisticated methods and consequently both of them demonstrate technical and ethical challenges. Related questions on (mitotic/meiotic) cell division, genetic/epigenetic alterations and cell renewal are needed to be addressed before clinical application.

Applications of emerging technologies to the study and conservation of threatened and endangered species

Sustaining viable populations of all wildlife species requires the maintenance of habitat, as well as an understanding of the behaviour and physiology of individual species. Despite substantial efforts, there are thousands of species threatened by extinction, often because of complex factors related to politics, social and environmental conditions and economic needs. When species become critically endangered, ex situ recovery programmes that include reproductive scientists are the usual first line of defence. Despite the potential of reproductive technologies for rapidly increasing numbers in such small populations, there are few examples of success. This is not the result of a failure on the part of the technologies per se, but rather is due to a lack of knowledge about the fundamental biology of the species in question, information essential for allowing reproductive technologies to be effective in the production of offspring. In addition, modern conservation concepts correctly emphasise the importance of maintaining heterozygosity to sustain genetic vigour, thereby limiting the practical usefulness of some procedures (such as nuclear transfer). However, because of the goal of maintaining all extant gene diversity and because, inevitably, many species are (or will become) 'critically endangered', it is necessary to explore every avenue for a potential contributory role. There are many 'emerging technologies' emanating from the study of livestock and laboratory animals. We predict that a subset of these may have application to the rescue of valuable genes from individual endangered species and eventually to the genetic management of entire populations or species. The present paper reviews the potential candidate techniques and their potential value (and limitations) to the study and conservation of rare wildlife species.

Genetics, epigenetics and gene silencing in differentiating mammalian embryos

A highly complex pattern of differentiation involving maternal and embryonic factors characterizes the early development of mammalian embryos. These complex genetic and proteonomic patterns of early growth also involve various forms of gene silencing and tissue reprogramming. Understanding the nature of fundamental developmental events is hence essential to appreciate the significance of natural and induced forms of remodelling, damaged forms of gene expression and gene silencing during the initial stages of growth. Natural forms of remodelling include subtle genetic events involved in, for example, the changing nature of imprinting from before fertilization or the inactivation of one X chromosome in female blastocysts. Induced forms include the consequences of nuclear transfer and embryo cloning or the immediate effects of placing embryos in culture media. Animal and human studies are described in this paper, relating reprogramming to detailed embryological and clinical knowledge gained through the use of IVF, preimplantation genetic diagnosis and the establishment in vitro of stem cells. Attention concentrates on the consequences of variations in all growth stages from the formation of oocytes, through fertilization, the differentiation of blastocysts and early haemopoietic stages in mammalian species. Unique features of gene expression or gene modification are described for each developmental stage.

Construction and fertilization of reconstituted human oocytes

Construction of artificial gametes may be made possible by transferring somatic cells into enucleated oocytes and inducing chromosomal halving of their nuclei. This study examines the possibility of constructing viable human gametes, and their potential for participation in normal fertilization. Spare germinal vesicle-stage oocytes were donated by consenting patients undergoing intracytoplasmic sperm injection (ICSI). Approximately 62% of in-vitro matured oocytes survived enucleation and subsequent cumulus cell injection. Following micromanipulation and subsequent activation, about 40% of the reconstituted oocytes yielded two pronuclear-like entities. This was not accompanied by extrusion of a polar body, but resulted in the formation of two 'putative haploid' pronuclei. Therefore selective removal of a female pronucleus marker was required to restore a balanced ploidy. Male pronuclei were identified by association with sperm mitochondria. Additional pronuclei were then removed, allowing further cleavage. Zygotes derived were 'putatively haploid' in approximately 38% of cases with a limited number of chromosomes assessed. However, on karyotypic analysis, blastomeres isolated from cleaving embryos showed a chaotic distribution of chromosomes. Oocytes could induce 'putative haploidization' of transplanted somatic cell nuclei independently of donor cell gender. Fertilization of artificial oocytes was followed by embryonic cleavage despite blastocyst development and chromosomal content possibly being compromised.

Effect of Treating Induced Mitochondrial Damage on Embryonic Development and Epigenesis

Germinal vesicle transplantation (GVT) has been proposed as a possible treatment to correct age-related oocyte aneuploidy caused by dysfunctional ooplasm. How healthy ooplasm regulates normal meiosis and subsequent development has yet to be elucidated, but impaired mitochondrial metabolism may be attributable to incomplete segregation of the oocyte chromosomes. In the present study, after ooplasmic mitochondrial damage by photoirradiating chloromethyl-X-rosamine, examination of the oocyte nuclei's ability to survive after transfer into healthy ooplasts was performed. To assess their fertilizability and potential for development, GVT oocytes were fertilized by intracytoplasmic sperm injection (ICSI) and transferred to foster mice. Condition of the offspring at birth was assessed, and epigenetic analysis was performed. Photosensitization consistently inhibited oocyte maturation. However, after GVT of photosensitized nuclei into healthy ooplasts, 67.2% were reconstituted, and 76.2% of these matured normally, with an overall rate of 51.2%, much higher than that (6.0%) in the mitochondrially injured oocytes. After ICSI, 65.8% (52/79) of GVT oocytes were fertilized normally, and 21.1% (11/52) eventually reached the blastocyst stage. The transfer of 132 two-cell GVT embryos into the oviducts of pseudopregnant females resulted in 17 apparently healthy live offspring. For some key developmental genes, a high level of expression was identified in the GVT and "rescue"-derived fetal adnexa. Thus, one can induce in oocyte mitochondria a photosensitization-based type of damage, which consistently inhibits GV breakdown, meiotic spindle formation, chromosomal segregation, and polar body extrusion. Germinal vesicle transplanted and rescued oocytes were able to undergo maturation, fertilization, and embryonic cleavage and, ultimately, to develop to term. This approach may provide a model with which to study the age-related ooplasmic dysfunction seen in human oocytes.

Current advances in artificial gametes

The birth of Louise Brown, the first IVF baby, in 1978 marked a breakthrough in infertility treatment. In recent decades, several important new techniques have been introduced. One limiting factor has been the requirement to use reproductive cells (gametes) for fertilization and for embryonic development. Somatic cell nuclear transfer (cloning) has been successful in mammals, opening a potential new approach for the treatment of human infertility. In addition, nuclear transfer to achieve embryo development starting from somatic cells instead of gametes, and the creation of artificial oocytes/spermatozoa has been attempted. The present paper reviews the various alternative approaches to haploidization of somatic cells. It has been observed that chromosome segregation (of the donor somatic nucleus) may take place; however, this process is largely random, thus leading to major cytogenetic abnormalities. An alternative approach is related to stem cell technology, to be further explored in the future. Culture conditions may be adjusted so that the totipotent embryonic stem cells will differentiate to specific gametes, sperm cells or egg cells. Injecting spermatozoa produced in this manner into recipient oocytes has led to pronuclear formation and early cleavage stages in some embryos. Finally, the birth of parthenogenetic mice indicates that some of these epigenetic problems can be overcome, and that some of the embryos may survive to birth.

Cryopreservation of Isolated Mouse Germinal Vesicles

The storage of unfertilized oocytes, either immature, maturing or mature, is still unsatisfactory. Here we describe an approach in which germinal vesicles isolated as karyoplasts from immature oocytes are vitrified by open the pulled straws (OPS) method in evacuated porcine zonae pellucidae. After thawing, their survival was almost absolute. Moreover, when thawed GV-karyoplasts were fused to immature oocyte cytoplasts the maturation of reconstructed cells resulted in the production of secondary oocytes--metaphase II.

Cytogenetic analysis of human somatic cell haploidization

Despite recent interest in the derivation of female and male gametes through somatic cell nuclear transfer, there is still insufficient data on chromosomal analysis of these gametes resulting from haploidization, especially involving a human nuclear donor and recipient oocytes. The objective of this study was to investigate the fidelity of chromosomal separation during haploidization of human cumulus cells by in-vitro matured human enucleated MII oocytes. A total of 129 oocytes were tested 4-7, 8-14, or 15-21 h after nuclear transfer (NT) followed by electro-stimulation, resulting in 71.3% activation efficiency on average. Haploidization was documented by the formation of two separate groups of chromosomes, originating from either polar body/pronucleus (PB/PN), or only 2PN, which were tested by 5-colour FISH, or DNA analysis for copy number of chromosomes 13, 16, 18, 21, 22 and X. Two PN were formed more frequently than PB/PN, irrespective of incubation time. In agreement with recent reports on mouse oocytes, as many as 90.2% of the resulting haploid sets tested showed abnormal chromosome segregation, suggesting unsuitability of the resulting artificial gametes for practical application at the present time.

Epigenetic reprogramming in early embryonic development: effects of in-vitro production and somatic nuclear transfer

A considerable proportion of offspring, in particular in ruminants and mice, born from nuclear transfer (NT)-derived and in-vitro-produced (IVP) embryos is affected by multiple abnormalities of which a high birthweight and an extended gestation length are the predominant features; a phenomenon that has been called 'large offspring syndrome' (LOS). The underlying mechanisms are largely unknown at present, but alterations of epigenetic modifications of embryonic and fetal gene expression patterns, primarily caused by alterations in DNA methylation are thought to be involved in this syndrome. In mammals, DNA methylation is essential for the regulation of transcription during development and differentiation. This review summarizes results from studies in which mRNA expression patterns from IVP and NT-derived embryos were compared with those of their in-vivo counterparts. Numerous aberrations have been found ranging from suppression of expression to de-novo overexpression or more frequently to a significant up- or down-regulation of a specific gene. These observations emphasize the need for further epigenetic studies during preimplantation embryo development to gain insight into the molecular regulation correlated with an undisturbed embryonic and fetal development. Understanding molecular mechanisms will aid improvements in biotechnologies applied to early embryos in all species, including humans.

The role of mitochondria in the establishment of oocyte functional competence

Mitochondria are maternally inherited, semi-autonomous organelles with their own genomes (mtDNA), largely responsible for the generation of energy in the form of cellular ATP. However, mitochondrial replication and transcription of mtDNA do not commence until well into embryonic differentiation. This means that the oocyte needs to contain sufficient stocks of functioning mitochondria to fuel the first few days of embryonic development. In this review, I examine how qualitative and quantitative aspects of mitochondria help us define the notion of functional competence.

Nuclear and Microtubule Dynamics of G2/M Somatic Nuclei During Haploidization in Germinal Vesicle-Stage Mouse Oocytes

During the haploidization process, it is expected that diploid chromosomes of somatic cells will be reduced to haploid for the generation of artificial gametes. In the present study, we aimed to use enucleated mouse oocytes at the germinal vesicle-stage (G2/M) as recipients for somatic cells that are also synchronized to the G2/M stage for haploidization. The reconstructed oocytes were then induced to undergo meiosis in vitro and observed for their nuclear morphology and microtubule network formation at various expected stages of the meiotic division. Following in vitro maturation, more than half (62/119, 52.1%) of the reconstructed oocytes completed the first round of meiosis-like division, as evidenced by the extrusion of pseudopolar bodies (PBs). However, accelerated PB extrusion, approximately 3-4 h earlier than that by control oocytes occurred. Furthermore, abnormally large pseudo-PBs, as large as four times the normal PB sizes, were observed. During the process of in vitro maturation at both the expected stages of metaphase I (MI) and metaphase II (MII), condensed chromosomes were observed in 38.7% and 55.2% of oocytes, respectively. However, two other types of nuclear configurations were also observed: 1) uneven distribution of chromatin and 2) an interphase-like nucleus, indicating deficiencies in chromosome condensation. Following oocyte activation, more than half (21/33, 63.6%) of the reconstructed oocytes with pseudo-PBs formed separated pseudopronuclei (PN), suggesting formation of functional spindles. The formation of bipolar spindle-like microtubule network at both the expected MI and MII stages during in vitro maturation was confirmed by immunohistochemistry. In summary, this study demonstrated that a high proportion of G2/M somatic nuclei appear to undergo meiosis-like division, in two successive steps, forming a pseudo-PB and two separate pseudo-PN upon in vitro maturation and activation treatment. Moreover, the enucleated geminal vesicle cytoplast retained its capacity for meiotic division following the introduction of a somatic G2/M nucleus.

Reprogramming of epigenetic inheritance by somatic cell nuclear transfer

Somatic cloning by nuclear transfer returns a differentiated cell to a totipotent stage, a process termed nuclear reprogramming. During this de-differentiation process, genes inactivated during tissue differentiation are re-activated in a temporal and spatial special manner. It is believed that tissue differentiation occurs through epigenetic mechanisms, genetic inheritance that does not involve changes in DNA sequences. Developmental abnormalities and a high mortality rate in cloned offspring have frequently been observed and probably result from incomplete nuclear reprogramming. In this review, the reprogramming of two epigenetic mechanisms, imprinting and X chromosome inactivation, as well as recent attempts to modify pre-existing epigenetic marks in donor cells to improve nuclear transfer efficacy, are discussed.

Averting abnormal inheritance: potential of gene therapy and preimplantation diagnosis

Serious inherited disease in children can be averted by preimplantation genetic diagnosis (PGD) and potentially by gene therapy in addition to prenatal diagnosis. PGD is now well established and provides a secure, if expensive and complex form of care. Gene therapy has been practised only in animals, although its success in alleviating various conditions in adults and newborns, together with the scientific drive of the genome project, make it a highly likely approach over coming years. Pros and cons of both approaches are contrasted and compared. Newer reproductive techniques such as somatic cell hybridization promise to add new dimensions to gene therapy, and could be combined with PGD. This paper discusses the finer details of these options, their safety and the ethical issues they have raised.

Implications of cloning technique for reproductive medicine

The birth of Dolly following the transfer of mammary gland nuclei into enucleated eggs established cloning as a feasible technique in mammals, but the moral implications and high incidence of developmental abnormalities associated with cloning have induced the majority of countries to legislate against its use with human gametes. Because of such negative connotations, restrictive political reactions could jeopardize the therapeutic and scientific promise that certain types of cloning may present. For example, in addition to its proposed use as a way of generating stem cells, the basic technique of nuclear transplantation has proven useful in other ways, including its application to immature eggs as a new approach to the prevention of the aneuploidy common in older women, and for some recent advances in preimplantation genetic diagnosis. Thus, while attempts at reproductive cloning in man would seem premature and even dangerous at present, this field will require rational rather than emotional reactions as a basis for legislation if the therapeutic promise of stem cell research and the experimental potential of nuclear transplantation techniques are to be fully realized.

Introduction to Cloning by Nuclear Transplantation

Despite its long history, the cloning of animals by nuclear transplantation is going through a "renaissance" after the birth of Dolly. The amount of work and achievements obtained in the last seven years are probably greater than those obtained in half a century of research. However, the principal obstacles outlined years ago with the work on somatic cell cloning in amphybia, are all still there in mammals. The importance of somatic cell nuclear transfer is, without any doubt, beyond the scope of replicating superior animal genotypes. It is an invaluable experimental tool to address fundamental scientific issues such as nuclear potency, cell de-differentiation, chromatin structure and function, epigenetics, and genome manipulation. For these reasons the importance of cloning is not for what it can achieve but for the technical support it can provide to biomedical research and in particular to the study of epigenetics, cancer and stem cell biology, cell therapy and regenerative medicine. In this introductory paper we will summarize the intellectual and technical framework of cloning animals by nuclear transfer that still remains the only absolute way of judging the success of the procedure. Together with the achievements of the recent past we will mention the very last developments and the many questions that still remain open. Current research efforts are expected to provide some answers and certainly new questions.

Micromanipulation of the human oocyte

Intracytoplasmic sperm injection (ICSI) provides an excellent outcome in a consistent manner, and is therefore used worldwide as a routine procedure. Since its introduction, few modifications have been made to its methodology. Recently, a combination of ICSI with micro-hole drilling by laser (LA-ICSI) of the zona pellucida appeared to decrease oocyte degeneration rates and to improve embryo quality and implantation. Cytoplasmic transfer is a more recently introduced procedure where the objective is to improve the quality of patients' oocytes by transferring cytoplasm from a good quality donor oocyte, in cases where it is assumed that cytoplasm is compromised. Nuclear transfer, involving exchange of nuclei between donor and receptor oocytes, is still an experimental procedure, the objective being similar to cytoplasmic transfer in improving oocyte/embryo quality. A nuclear transfer procedure involving somatic cells for reproductive purposes should not be used in humans, for ethical and technical considerations. On the other hand, nuclear transfer for therapeutic purposes to obtain stem cells may be considered in respect of its unique potential in medicine. Finally, the most recently emerged new concept under investigation is the haploidization of somatic cells for the purpose of creating artificial gametes.

Somatic cell haploidization: an update

Oocyte donation is the only method of treating female sterility caused by complete absence of oocytes, with the loss of genetic motherhood. Genetic fatherhood of males with complete absence of spermatozoa can only be restored by assisted reproduction treatment if sperm precursor cells belonging to the male germline can still be recovered from the testis. Otherwise, sperm donation is the only available solution. Somatic nucleus haploidization after injection into previously enucleated donor oocytes (diploid-to-haploid reduction) might enable the reconstruction of new oocytes carrying the complete nuclear genome of female patients lacking their own oocytes. Such newly formed oocytes could subsequently be fertilized by spermatozoa from the patient's husband. In cases of male infertility with complete absence of the germline, the patient's somatic cell nuclei could be injected into the oocytes without previous enucleation, and somatic nucleus haploidization would occur in the presence of the original female nucleus (triploid-to-diploid reduction), hopefully leading to the formation of a diploid embryo. Both interventions differ substantially from cloning because embryos are formed by syngamy with the male and female genomes originating from the two genetic parents, as in natural fertilization. Ultrastructural remodelling of mouse somatic cell nucleoli can be achieved in enucleated metaphase II mouse oocytes. Haploidization has also been attempted with Sertoli cells and with fibroblasts, both of which are also available in male patients. Experiments are currently under way to assess the regularity of chromatid segregation during somatic nucleus haploidization.