In humans, oocytes are continuously arrested at the immature phase of first meiosis. This arrest continues until puberty, at which time fully grown oocytes resume meiosis at each ovulation cycle . Each ovary has more than 500,000 follicles at birth , and the oocytes that are not released in the ovulation cycle can remain arrested in the female for up to fifty years . Maternal age is a critical factor in women’s fertility - as many as half of the oocytes from IVF patients over the age of 38 years contain errors in chromosome numbers [61, 62]. Aneuploidy is reported to increase to nearly 60% of oocytes from women over 43 years of age . Data from the New Hope Fertility Center shows that the rate of blastocyst formation is around 70% for women younger than 35, but this rate decreases with 10% when women turn 38 and continues to decrease by an additional 10% for every two years of further maternal aging.
Regardless of age, the frequency of chromosome segregation errors is much higher in oocytes compared to spermatocytes, and this rate increases with age . Previous research on human oocytes involved the use of fluorescence in situ hybridization (FISH), and more recently the more precise array comparative genomic hybridization (aCGH) and next-generation sequencing (NGS). These methods have been widely used on human polar bodies (PBs) for the identification of oocyte aneuploidy [61, 62, 64-66]. Such research has suggested that segregation errors occur at similar rates between MI and MII [57, 61, 62, 66]. Aneuploidy is caused when chromosomes do not segregate appropriately during MI and MII .
The loss of chromosome cohesion is the leading cause for chromosome missegregation . The distance between sister chromatids increases significantly with maternal age due to the gradual loss of cohesion between them [68-70]. Homologous chromosomes and sister chromatids are held together along the chromosome arms and at the centromere by ring-like cohesive ties, called the cohesin complex. Current evidence from model organisms suggests that after chromosome cohesion is established in premiotic S phase, aging oocytes are unable to reload the cohesin complex components during the prolonged prophase arrest [71-74]. Loss of cohesin complex components may be further facilitated by the age-related degradation of crucial cytoplasmic factors involved in protecting the cohesin complex from degradation . Accumulated oxidative damage through prolonged aging may negatively impact oocyte quality [75, 76]; aged oocytes also have a high intracellular pH , which may damage binding interactions between the cohesin subunits and enhance the loss of cohesion. Hence, new research and development is needed to uncover methods of replacing cytoplasm of older eggs and producing fertilizable oocytes for older women.
Although the human species is successfully outbred with highly variable combinations of nuclear and mitochondrial haplotypes from across the globe, recent mitochondrial replacement research has implied that these haplotypes may need to be carefully matched when conducting nuclear transfer for Mito Disease prevention [47, 108]. Namely, healthy mtDNA from the donor oocyte may need to be matched for haplotype/haplogroup compatibility with the patient’s nuclear haplotype/haplogroup because the nuclear transfer procedure inevitably involves a small amount of maternal mtDNA carryover which can be potentially selected for amplification and/or segregated in the growing fetus tissues [30, 47]. A specific sequence on the mitochondrial DNA which is responsible for mtDNA replication can contain polymorphisms that render a replication advantage preferentially for the maternal mtDNA to the donor mtDNA when in proximity to the maternal nucleus in the reconstituted oocyte. In order to prevent the possibility of pathogenic mtDNA reversal  or the need for haplotype/haplogroup matching, development of new protocols is required for the complete elimination of pathogenic carryover in nuclear transfer for Mito Disease prevention.