Live birth rate following undisturbed embryo culture at low oxygen in a time-lapse incubator compared to a high-quality benchtop incubator.

Time-lapse (TL) incubators are increasingly used in in vitro fertilization (IVF) laboratories but there have been few studies of their effectiveness in comparison to other incubator types. Moreover, the design of most studies has been limited by the quality of the control incubator. We have therefore performed a one-year pseudo-randomized prospective study of IVF cycles using a TL incubator (EmbryoScope™) (n = 243) or a conventional incubator (K-System G-185 Flatbed) (n = 203). The two groups were well matched in terms of clinical parameters: IVF cycle attempt number, IVF/ICSI, age, number and day (3 or 5) of embryo transfer. Embryos were selected for transfer using conventional (non-TL) morphological grading. The EmbryoScope group had an increased chance of a live birth (43.2% vs. 34.5%; OR = 1.43 [95%CI: 0.96-2.13]) with significantly reduced early pregnancy loss (5.8% vs. 13.8%; OR = 0.37 [0.19-0.74]) compared to the K-System incubator. There was a higher proportion of 4-cell embryos on day 2 and 8-cell embryos on day 3 in the EmbryoScope, compared to the K-Systems. The use of TL incubators is appropriate in ART by virtue of their high specification, facility for low oxygen culture and provision of minimally disturbed culture conditions which limit exposure of human embryos to environmental stress.

Towards clinical application of pronuclear transfer to prevent mitochondrial DNA disease.

Mitochondrial DNA (mtDNA) mutations are maternally inherited and are associated with a broad range of debilitating and fatal diseases. Reproductive technologies designed to uncouple the inheritance of mtDNA from nuclear DNA may enable affected women to have a genetically related child with a greatly reduced risk of mtDNA disease. Here we report the first preclinical studies on pronuclear transplantation (PNT). Surprisingly, techniques used in proof-of-concept studies involving abnormally fertilized human zygotes were not well tolerated by normally fertilized zygotes. We have therefore developed an alternative approach based on transplanting pronuclei shortly after completion of meiosis rather than shortly before the first mitotic division. This promotes efficient development to the blastocyst stage with no detectable effect on aneuploidy or gene expression. After optimization, mtDNA carryover was reduced to <2% in the majority (79%) of PNT blastocysts. The importance of reducing carryover to the lowest possible levels is highlighted by a progressive increase in heteroplasmy in a stem cell line derived from a PNT blastocyst with 4% mtDNA carryover. We conclude that PNT has the potential to reduce the risk of mtDNA disease, but it may not guarantee prevention.

Meiosis and maternal aging: insights from aneuploid oocytes and trisomy births.

In most organisms, genome haploidization requires reciprocal DNA exchanges (crossovers) between replicated parental homologs to form bivalent chromosomes. These are resolved to their four constituent chromatids during two meiotic divisions. In female mammals, bivalents are formed during fetal life and remain intact until shortly before ovulation. Extending this period beyond ∼35 years greatly increases the risk of aneuploidy in human oocytes, resulting in a dramatic increase in infertility, miscarriage, and birth defects, most notably trisomy 21. Bivalent chromosomes are stabilized by cohesion between sister chromatids, which is mediated by the cohesin complex. In mouse oocytes, cohesin becomes depleted from chromosomes during female aging. Consistent with this, premature loss of centromeric cohesion is a major source of aneuploidy in oocytes from older women. Here, we propose a mechanistic framework to reconcile data from genetic studies on human trisomy and oocytes with recent advances in our understanding of the molecular mechanisms of chromosome segregation during meiosis in model organisms.

Age-related meiotic segregation errors in mammalian oocytes are preceded by depletion of cohesin and Sgo2.

BACKGROUND: The growing trend for women to postpone childbearing has resulted in a dramatic increase in the incidence of trisomic pregnancies. Maternal age-related miscarriage and birth defects are predominantly a consequence of chromosome segregation errors during the first meiotic division (MI), which involves the segregation of replicated recombined homologous chromosomes. Despite the importance to human reproductive health, the events precipitating female age-related meiotic errors are poorly understood. RESULTS: Here we use a long-lived wild-type mouse strain to show that the ability to segregate chromosomes synchronously during anaphase of MI declines dramatically during female aging. This is preceded by depletion of chromosome-associated cohesin in association with destabilization of chiasmata, the physical linkages between homologous chromosomes, and loss of the tight association between sister centromeres. Loss of cohesin is not due to an age-related decline in the ability of the spindle checkpoint to delay separase-mediated cleavage of cohesin until entry into anaphase I. However, we find that reduced cohesin is accompanied by depletion of Sgo2, which protects centromeric cohesin during MI. CONCLUSIONS: The data indicate that cohesin declines gradually during the long prophase arrest that precedes MI in female mammals. In aged oocytes, cohesin levels fall below the level required to stabilize chiasmata and to hold sister centromeres tightly together, leading to chromosome missegregation during MI. Cohesin loss may be amplified by a concomitant decline in the levels of the centromeric cohesin protector Sgo2. These findings indicate that cohesin is a key molecular link between female aging and chromosome missegregation during MI.