Unfertilized Human Oocytes Research Articles

Calcium Ionophore A23187 treatment to rescue unfertilized oocytes: a prospective randomized analysis of sibling oocytes

Research questionCan 1-day old human unfertilized oocytes activate and blastulate after exposure to calcium ionophore (Ca.I) A23187? DesignProspective randomized trial analysis of sibling oocytes. Seventy unfertilized sibling oocytes from 24 couples were randomly split into two groups. In the treatment group, 35 oocytes were cultured with 5-µM Ca.I A23187 for 10 min, washed and cultured until day 6 of development (D+6). The remaining 35 oocytes (control group) were similarly cultured until D+6. Activation, cleavage and blastulation rates were compared between the two groups. ResultsComparable activation rates were observed in the oocytes incubated with Ca.I A23187 and in the control group (11.4% versus 17.1%; P = 0.49). The cleavage rate observed was 45.7% in both groups. None of the embryos reached blastocyst stage. ConclusionsActivation and cleavage can occur in unfertilized oocytes after the diagnosis of failure to fertilize. Unfortunately, the prevalence of activation is not affected by exposure to Ca.I A23187. Additionally, these embryos have no tangible reproductive potential as they arrest before reaching the blastocyst stage.

Cytoplasmic maturation in human oocytes: an ultrastructural study †.

Female fertility relies on successful egg development. Besides chromosome segregation, complex structural and biochemical changes in the cytoplasmic compartment are necessary to confer the female gamete the capacity to undergo normal fertilization and sustain embryonic development. Despite the profound impact on egg quality, morphological bases of cytoplasmic maturation remain largely unknown. Here, we report our findings from the ultrastructural analysis of 69 unfertilized human oocytes from 34 young and healthy egg donors. By comparison of samples fixed at three consecutive developmental stages, we explored how ooplasmic architecture changes during meiotic maturation in vitro. The morphometric image analysis supported observation that the major reorganization of cytoplasm occurs before polar body extrusion. The organelles initially concentrated around prophase nucleus were repositioned toward the periphery and evenly distributed throughout the ooplasm. As maturation progressed, distinct secretory apparatus appeared to transform into cortical granules that clustered underneath the oocyte's surface. The most prominent feature was the gradual formation of heterologous complexes composed of variable elements of endoplasmic reticulum and multiple mitochondria with primitive morphology. Based on the generated image dataset, we proposed a morphological map of cytoplasmic maturation, which may serve as a reference for future comparative studies. In conclusion, this work improves our understanding of human oocyte morphology, cytoplasmic maturation, and intracellular factors defining human egg quality. Although this analysis involved spare oocytes completing development in vitro, it provides essential insight into the enigmatic process by which human egg progenitors prepare for fertilization.

The combination of calcium ionophore A23187 and GM-CSF can safely salvage aged human unfertilized oocytes after ICSI.

Artificial oocyte activation using calcium ionophores and enhancement of embryonic developmental potential by the granulocyte-macrophage colony-stimulating factor (GM-CSF) have already been reported. In this study, we evaluated the synergistic effect of these two methods on aged human unfertilized oocytes after intracytoplasmic sperm injection (ICSI). Then, we cultured the resulting embryos to the blastocyst stage and screened them for chromosomal abnormalities, to assess the safety of this protocol. Aged human oocytes deemed unfertilized after ICSI were activated, either by briefly applying the calcium ionophore A23187 alone (group A) or by briefly applying the ionophore and then supplementing the culture medium with recombinant human GM-CSF (rhGM-CSF) (group B). Next, the development was monitored in a time-lapse incubator system, and ploidy was analyzed by array comparative genomic hybridization (aCGH), after whole embryo biopsy and whole genome amplification. Differences between oocytes and resulting embryos in both groups were evaluated statistically. Oocytes unfertilized after ICSI can be activated with the calcium ionophore A23187 to show two pronuclei and two polar bodies. Addition of rhGM-CSF in the culture medium of A23187-activated oocytes enhances their cleaving and blastulation potential and results in more euploid blastocysts compared to the culture medium alone. This study shows that activating post-ICSI aged human unfertilized oocytes with a combination of a calcium ionophore and a cytokine can produce good-morphology euploid blastocysts.

Three-day-old human unfertilized oocytes after in vitro fertilization/intracytoplasmic sperm injection can be activated by calcium ionophore a23187 or strontium chloride and develop to blastocysts.

Our objective was to observe the effectiveness of the calcium ionophore A23187 or strontium chloride on the activation and subsequent embryonic development of 3-day-old human unfertilized oocytes after in vitro fertilization (IVF) or intracytoplasmic sperm injection (ICSI). A total of 279 3-day-old unfertilized oocytes after IVF or ICSI were randomized to be activated by the calcium ionophore A23187 (n=138) or strontium chloride (n=141). The activated oocytes were cultured in vitro for 3-5 days. Activation rate, pronucleus formation, cleavage rate, and developmental potential of parthenotes during culture were evaluated. A total of 170 unfertilized oocytes were activated; 65 developed to cleavage stage, 19 developed to greater than the eight-cell stage, and five blastocysts were obtained. The activation rate of the calcium ionophore A23187 group was higher than that of the strontium chloride group (75.4% and 46.8%, respectively; p<0.05); there was significant difference between two groups (p<0.05). Among the 44 cleaved oocytes in the calcium ionophore A23187 group, eight developed to the two- to four-cell stage, 17 developed to the five- to eight-cell stage, 15 developed to greater than the eight-cell stage, and four blastocysts were obtained. Among the 21 cleaved oocytes in the strontium chloride group, six developed to the two- to four- cell stage, 10 developed to the five- to eight-cell stage, four developed to greater than the eight-cell stage, and one blastocyst was obtained. Three-day-old unfertilized human oocytes after IVF or ICSI could be activated by the calcium ionophore A23187 or strontium chloride, and a small part of parthenogenetic embryos developed into blastocysts. The treatment with the calcium ionophore A23187 was better than that of strontium chloride in respect to the activation rate of 3-day-old unfertilized human oocytes after IVF or ICSI.

Authors’ response: polarization microscopy and rescue ICSI

1 h We thank Dr Coticchio for his commentary on our article ‘Spindle examination in unfertilized eggs using the polarization microscope can assist rescue ICS’. His view on some key points in our study harmonize with ideas and issues we have discussed and debated over and over. Therefore, we are pleased to be able to highlight answers for issues he has raised. First, we know the issue of whether to perform rescue intracytoplasmic sperm injection (ICSI) or to cancel the current cycle is still under debate. The goal of assisted reproductive technology is to help infertile couples have a healthy baby, not increase the number of transferable embryos of compromised quality. The biological and clinical implications of oocyte ageing are well reported in numerous articles, including cytogenetic abnormalities in the resulting embryo following rescue ICSI (Morton et al., 1997; Pehlivan et al., 2004) as well as congenital abnormalities (Tarin et al., 2002). However, we should also consider that routine ICSI and IVF might also cause an increased risk of major congenital anomalies compared with natural conception (Wen et al., 2012). To our knowledge, there have been reports of successful births following rescue ICSI (Yuzpe et al., 2000; Shalom-PAZ et al., 2011). As previously shown from data in our centre (Shalom-PAZ et al., 2011), out of 20 pregnancies, only one was diagnosed with trisomy 21 and was terminated, while the other children were found to be

Integration of immunodeficiency virus in oocytes via intracytoplasmic injection: possible but extremely unlikely

To determine if human oocytes can be infected with HIV-1 via intracytoplasmic injection and to determine the infection threshold. Twenty-eight donated immature and unfertilized human oocytes from HIV-negative women were injected with 4 × 10(4) HIV-1 virions and 13 oocytes were used as uninjected controls. To determine the infection threshold, 543 cat oocytes were injected with 4 × 10(4), 4 × 10(2), or 40 copies of feline immunodeficiency virus (FIV) and 376 oocytes were used as controls. Academic hospital. PATIENT(S)/ANIMAL(S): Donated immature human oocytes and mature cat oocytes. Injection with HIV-1 or FIV. Viral integration as measured by fluorescent in situ hybridization with HIV-1-specific probes or by nested FIV polymerase chain reaction. We detected viral integration in three of 28 (11%) human oocytes injected with 4 × 10(4) copies of HIV-1. When injected with high dose FIV (4 × 10(4) copies) 16%-49% of cat oocytes showed viral integration. This decreased to 2%-7% and 0.6%-1.8% when an intermediate (4 × 10(2) copies) or low (40 copies) dose was injected, respectively. Human and cat oocytes can be infected with HIV-1 and FIV respectively, when injected with high amounts of virus. The probability of viral integration is extremely low when small amounts of virus particles are injected. Taking into account the small volume injected during intracytoplasmic injection, the chances of viral integration are 0.00002%.

Adenosine Triphosphate Content in Human Unfertilized Oocytes, Undivided Zygotes and Embryos Unsuitable for Transfer or Cryopreservation

To investigate the relationship between adenosine triphosphate (ATP) content and developmental potential in human oocytes, zygotes and embryos. ATP content was determined using a bioluminescence assay in 72 unfertilized oocytes, 26 undivided zygotes and 70 embryos unsuitable for transfer or cryopreservation, obtained from 52 women undergoing in vitro fertilization and embryo transfer (IVF-ET). The mean ATP content increased with development: zygotes had a significantly higher ATP content than oocytes, and embryos had a significantly higher ATP content than both zygotes and oocytes. Within the embryo group, the ATP content was significantly higher in polypronuclear embryos than in inferior embryos with substantial fragmentation. When analysed in relation to IVF-ET outcome, the ATP content of oocytes and embryos from women who became pregnant was significantly higher than in oocytes and embryos from those who did not become pregnant. Reduced ATP content may be related to fertilization failure, arrested division and abnormal embryonic development. The differences observed in oocyte and embryo ATP content between women who became pregnant and those who did not suggests that mitochondrial function is correlated with individual fertility.

Phospholipase C-ζ-induced Ca2+ oscillations cause coincident cytoplasmic movements in human oocytes that failed to fertilize after intracytoplasmic sperm injection

ObjectiveTo evaluate the imaging of cytoplasmic movements in human oocytes as a potential method to monitor the pattern of Ca2+ oscillations during activation.DesignTest of a laboratory technique.SettingUniversity medical school research laboratory.Patient(s)Donated unfertilized human oocytes from intracytoplasmic sperm injection (ICSI) cycles.Intervention(s)Microinjection of oocytes with phospholipase C (PLC) zeta (ζ) cRNA and a Ca2+-sensitive fluorescent dye.Main Outcome Measure(s)Simultaneous detection of oocyte cytoplasmic movements using particle image velocimetry (PIV) and of Ca2+ oscillations using a Ca2+-sensitive fluorescent dye.Result(s)Microinjection of PLCζ cRNA into human oocytes that had failed to fertilize after ICSI resulted in the appearance of prolonged Ca2+ oscillations. Each transient Ca2+ concentration change was accompanied by a small coordinated movement of the cytoplasm that could be detected using PIV analysis.Conclusion(s)The occurrence and frequency of cytoplasmic Ca2+ oscillations, a critical parameter in activating human zygotes, can be monitored by PIV analysis of cytoplasmic movements. This simple method provides a novel, noninvasive approach to determine in real time the occurrence and frequency of Ca2+ oscillations in human zygotes.

Comparative molecular portraits of human unfertilized oocytes and primordial germ cells at 10 weeks of gestation

Primordial germ cells (PGCs) are precursors of gametes and share several features in common with pluripotent stem cells, such as alkaline phosphatase activity and the expression of pluripotency-associated genes such as OCT4 and NANOG. PGCs are able to differentiate into oocytes and spermatogonia and establish totipotency after fertilization. However, our knowledge of human germ cell development is still fragmentary. In this study, we have carried out genome-wide comparisons of the transcriptomes and molecular portraits of human male PGCs (mPGCs), female PGCs (fPGCs) and unfertilized oocytes. We detected 9210 genes showing elevated expression in fPGCs, 9184 in mPGCs and 9207 in oocytes, with 6342 of these expressed in common. As well as known germ cell-related genes such as BLIMP1/PRDM1, PIWIL2, VASA/DDX4, DAZL, STELLA/DPPA3 and LIN28, we also identified 465 novel non-annotated genes with orthologs in the mouse. A plethora of olfactory receptor-encoding genes were detected in all samples, which would suggest their involvement not only in sperm chemotaxis, but also in the development of female germ cells and oocytes. We anticipate that our data might increase our meagre knowledge of the genes and associated signaling pathways operative during germ cell development. This in turn might aid in the development of strategies enabling better differentiation and molecular characterisation of germ cells derived from either embryonic or induced pluripotent stem cells. Ultimately, this would have a profound relevance for reproductive as well as regenerative medicine.

Separate and combined effects of caffeine and dbcAMP on olive baboon (Papio anubis) sperm

Improvement of baboon sperm capacitation is necessary for achieving high in vitro fertilization (IVF) rates in baboons. In this study, we evaluated separate and combined effects of caffeine and dbcAMP on baboon sperm capacitation. Sixteen male baboons (n = 16) were electroejaculated. Each sperm sample was divided into two aliquots: one for chemical activation and the other untreated control. Group 1: dbcAMP (n = 6); Group 2: caffeine (n = 6) and Group 3: combination of caffeine and dbcAMP (n = 4). In each aliquot, sperm motility after 30 minutes of incubation was evaluated as well as zona pellucida (ZP) binding ability after overnight incubation with 4-5 ZP from unfertilized human oocytes. Sperm motility and ZP binding ability in all chemically activated groups increased significantly as compared to their respective controls (P < 0.05). Combined and separate effects of caffeine and dbcAMP increases baboon sperm motility and ZP binding ability and may improve baboon IVF.

Chromatin Quality as a Crucial Factor for the Success of Fluorescent in Situ Hybridization Analyses of Unfertilized Oocytes, Polar Bodies and Arrested Zygotes

Chromatin Quality as a Crucial Factor for the Success of Fluorescent in Situ Hybridization Analyses of Unfertilized Oocytes, Polar Bodies and Arrested ZygotesMaterial that is supernumerary or unsuitable for in vitro fertilization (IVF) procedures is used for basic and for IVF-related research. Despite the disadvantages of such cells, they have contributed much to our understanding of the mechanisms and prevalence of different abnormalities.Fifty-four human unfertilized oocytes, 34 arrested bipronuclear zygotes and 15 polar bodies were fixed for analysis on the third day after in vitro insemination and were subjected to fluorescent in situ hybridization (FISH) with probes for chromosomes 18, 21, X and Y (centromere for 18, X, Y and locus-specific for 21). The aim of the study was the comparison of FISH efficiency in differently condensed chromatin.The success of FISH analysis was over 60% of analyzed cells and it was dependent on the chromatin changes (condensation and/or fragmentation) during the culture period before cell fixation. Chromatin ageing was the crucial factor for the reduced success of FISH in both oocyte chromosomes (60.0%) and pronuclei (61.76%). The chromatin of second polar bodies (PBII), and premature chromosome condensation (PCC) of the sperm chromatin in oocytes was more suitable for FISH analysis (FISH success 75.0% in PBII and 64.29% in PCC) with both centromere and locus-specific probes.These results revealed the significance of early signs of in vitro cell ageing for the success of FISH analysis and for the interpretation of results in case of analysis of unfertilized human ova, polar bodies and arrested zygotes.

Activation of unfertilized human oocytes after intracytoplasmic sperm injection

OBJECTIVE: To determine the effect of two different methods on artificial activation of unfertilized human oocytes following intracytoplasmic sperm injection (ICSI). DESIGN: Laboratory study with unfertilized oocytes. MATERIALS AND METHODS: Seventy-five oocytes that did not show any signs of normal fertilization (no 2 pronuclear formation and no second polar body extrusion) by 24h after ICSI were randomly allocated into 2 treatment groups: Group 1 (32 oocytes), the oocytes were exposed to 7% (v/v) ethanol for 6 minutes; and in Group 2 (43 oocytes), the oocytes were exposed to 5 μmol/L ionomycin (Sigma) for 5 minutes. The oocyte activation is defined in the present study: 1) Extrusion of the second polar body; and 2) Formation of 2 pronuclei. The activated oocytes were cultured in vitro for 3-5 days. The activation rate, normal pronuclear formation, cleavage, and the embryonic development were compared between groups. RESULTS: In Group 1, 10 (31.3%) out of 32 unfertilized oocytes were activated, and 7 of them cleaved. Among them, 7 cleaved embryos, 4 of them developed to 2-4 cell stage, 2 to of them further developed to 5-8 cell stage, and 1 morula stage produced eventually. In Group 2, 24 (55.8%) out of 43 unfertilized oocytes were activated, and 17 of them cleaved. Among them, 9 cleaved embryos developed to 2-4 cell stage, 5 of them developed to 5-8 cell stage, 3 of them developed to morula stage, and 2 of them further developed to blastocyst stage. CONCLUSIONS: These results show that the unfertilized human oocytes 24 after ICSI can be activated by ethanol or ionomycin stimulation. The activated oocytes can be developed to blastocyst stage. The results also show that ionomycin is more efficient compared with ethanol. The resulted embryos may be used for embryonic stem cell study.

Developmental reprogramming after chromosome transfer into mitotic mouse zygotes

Until now, animal cloning and the production of embryonic stem cell lines by somatic cell nuclear transfer have relied on introducing nuclei into meiotic oocytes. In contrast, attempts at somatic cell nuclear transfer into fertilized interphase zygotes have failed. As a result, it has generally been assumed that unfertilized human oocytes will be required for the generation of tailored human embryonic stem cell lines from patients by somatic cell nuclear transfer. Here we report, however, that, unlike interphase zygotes, mouse zygotes temporarily arrested in mitosis can support somatic cell reprogramming, the production of embryonic stem cell lines and the full-term development of cloned animals. Thus, human zygotes and perhaps human embryonic blastomeres may be useful supplements to human oocytes for the creation of patient-derived human embryonic stem cells.

Analysis of human unfertilized oocytes and pronuclear zygotes—Correlation between chromosome/chromatin status and patient-related factors

Objective The objective was to evaluate the relationship between ploidy and chromatin status of human unfertilized oocytes/zygotes and infertility history, female age, and stimulation regimens. Study design Two hundred and eighty-nine unfertilized oocytes and 63 zygotes were subjected to cytogenetic analysis: karyotyping for oocytes and fluorescent in situ hybridization (FISH) analysis for zygotes. Ploidy and chromosome/chromatin status were analyzed according to stimulation regimen, female age, and infertility history. The correlation coefficient was estimated and data were interpreted using a five-grade scale. Results Aneuploidy in karyotyped oocytes (19.7% hyperhaploidy, 18.8% hypohaploidy, and 6.3% haploid abnormal) was associated with chromosome fragmentation and lesions due to chromosome aging in culture. Premature chromosome condensation and cytoskeletal defects were significantly higher in unexplained infertility (34.7% and 52.9%, respectively; p < 0.05). Chromatin quality was most important for successful ploidy analysis of zygotes. FISH analysis of abnormal zygotes elucidated genetic aspects of pronuclear number aberrations and raised questions about the current selection criteria. Abnormalities were found to correlate moderately with stimulation strategy and female age and significantly with infertility history. Conclusion Genetic analysis of human oocytes and zygotes showed that poor chromatin quality and patient-related factors contribute to aneuploidy and pronuclear number aberrations.

Immunocytochemical evidence for the presence and location of the neurotrophin–Trk receptor family in adult human preovulatory ovarian follicles

This study was undertaken to evaluate the presence or absence of neurotrophins and their respective receptors within adult human preovulatory follicles. Prospective study of neurotrophins and their receptors in follicular cells and unfertilized oocytes from women undergoing aspiration for in vitro fertilization/intracytoplasmic sperm injection. Cells (mural and cumulus granulosa cells, unfertilized oocytes) were examined for immunocytochemical staining of neurotrophin and receptor proteins. Mural and cumulus granulosa cells were positive for BDNF, NT-4/5, NT-3, and NGF, as well as for Trk B, Trk C, and Trk A receptors. Unfertilized oocytes were positive for Trk B, Trk C, and Trk A receptors. Neurotrophins and their respective receptor proteins are present within the mural and cumulus granulosa cells of adult human preovulatory follicles. Neurotrophin receptors are present in human unfertilized oocytes. The location of the neurotrophins and their receptors suggest both an autocrine and paracrine function within the adult human ovarian follicle.

Assessing the quality of oocytes derived from in vitro maturation: are we looking under the lamppost?

Oocytes are complex cells comprising many cellular systems, each of which is essential for proper oocyte function. Methods to assess the diverse and critical cellular systems in oocytes derived from in vitro maturation are badly needed.

Cryopreservation of unfertilized human oocytes

Previous investigations revealed that choline-based freezing media developed in our laboratory were superior to conventional sodium-based media for storing mouse oocytes. This paper examines the ability of the choline-based medium CJ2 and a modified form of this medium, CJ3, to cryopreserve unfertilized human oocytes. Oocytes that were consented for research and matured overnight, as well as freshly collected, donor, mature metaphase II (MII) oocytes, were cryopreserved using choline-based media and an optimized slow-cooling protocol. The results showed higher survival and fertilization rates when CJ3 supplemented with 0.2 mmol/l sucrose was used as compared with CJ2 supplemented with either 0.1 mmol/l or 0.2 mmol/l sucrose. Freshly collected oocytes were more difficult to cryopreserve than those matured in vitro. Modification of the base medium proved to be one of the key factors in obtaining survival rates over 90%. Fertilization rates, embryo development, and genetic analysis of embryos resulting from control and frozen–thawed oocytes are provided. There appears to be a high correlation between chromosomal anomalies and abnormal morphology in embryos from thawed oocytes.

Combination of calcium ionophore A23187 with puromycin salvages human unfertilized oocytes after ICSI

Objective To determine whether oocyte activation using a combination of calcium ionophore A23187 (A23187) with puromycin could salvage human unfertilized oocytes after ICSI. Study Design One hundred and thirteen discarded unfertilized oocytes 20–68 h after ICSI were assigned to four groups: ICSI 20-h group, ICSI 44-h group, ICSI 68-h group and control. All unfertilized oocytes were exposed to A23187 (5 μM) for 5 min and subsequently were incubated with puromycin (10 μg/ml) for 4 h. Sex chromosomal analysis was performed by dual color fluorescence in situ hybridization (FISH). Results The combination of A23187 with puromycin could activate the unfertilized oocytes 20–68 h after ICSI. The best results were achieved in the ICSI 20-h group, which exhibited an activation rate of 91.2% (31/34), a cleavage rate of 64.7% (22/34) and 44.1% (15/34) high-quality embryos. The activation rate, cleavage rate and the number of high-quality embryos appeared to decrease with the cultured time of unfertilized oocytes after ICSI. FISH analysis showed six embryos with XX and seven embryos with XY in 16 embryos derived from 2PN2PB. Conclusions The combination of calcium ionophore A23187 with puromycin could effectively salvage unfertilized oocytes within 20 h after ICSI.

Apoptosis in human unfertilized oocytes after intracytoplasmic sperm injection

To investigate the presence of programmed cell death in unfertilized oocytes after intracytoplasmic sperm injection (ICSI), assuming that previous apoptotic events could be correlated with the fertilization failure. Comparison of the rate of DNA fragmentation in human oocytes at different stages of maturation soon after pick-up (control) and in unfertilized oocytes after ICSI treatment. In vitro fertilization (IVF) laboratory with extensive ICSI experience. Sixty-three patients undergoing assisted fertilization by ICSI. Terminal deoxynucleotidyl transferase-mediated digoxigenin-dUTP nick-end labeling (TUNEL) assay and anticaspase-3 cleaved immunoassay to detect apoptosis in control and ICSI-treated oocytes. Differences in the percentage of oocytes demonstrating DNA fragmentation between control oocytes and unfertilized ICSI treated oocytes at different stages of maturation. The DNA fragmentation, by TUNEL assay, appeared in all the immature control oocytes, but only 37% of mature oocytes showed DNA fragmentation. This DNA fragmentation was observed in 88.8% of the oocytes unfertilized after ICSI; furthermore, DNA fragmentation appeared as well in the sperm injected into the cytoplasm. The study has shown DNA fragmentation in human oocytes unfertilized after ICSI. The evidence is confirmed as well in control oocytes, free from in vitro culture or manipulation stress. Caspase-3 immunoassay suggests the presence of apoptosis. The high percentage of oocytes demonstrating DNA fragmentation in the unfertilized oocytes could be correlated with fertilization failure.

Influences on Sex Chromosomes and IGF-II Expression of Oocyte Activation With Calcium Ionophore A23187 and Puromycin on Human Unfertilized Oocytes After ICSI

The combination of intracytoplasmic sperm injection (ICSI) with oocyte activation has resulted in pregnancies and the birth of healthy babies for patients with repeated fertilization failure after ICSI. However, the introduction oocyte activation into human assisted reproductive technology faces significant risk because every mammal oocyte undergoing oocyte activation has the possibility of becoming parthenote, which is not capable of developing to term due to genomic imprinting. Some genomic imprinting genes, such as insulin-like growth factor II (IGF-II) that is expressed from the paternal allele play a key role in mammalian growth.