Figure 1. Metaphase II oocyte (1)

Infertility affects millions of couples all around the world. In spite of the solutions to their problems reproductive technology can achieve, the efficacy is eventually limited by the number and the quality of the oocytes available from the woman. In actuality, such efficiency is determined by the ovarian reserve, the oocyte quality and the maternal age, among the most important factors (2).

DIMINISHED OVARIAN RESERVE (DOR)

Since ovarian reserve defines the quantity and quality of the primordial follicle pool, diminished ovarian reserve (DOR) indicates a reduction in quantity in women of reproductive age. Consequently, it represents important cause of infertility in many couples. Moreover, DOR may be associated with low pregnancy rates and high pregnancy loss regardless of age, but further research is needed in order to fully understand its implications (3).

ADVANCED MATERNAL AGE

It is well known that women’s fertility declines sharply after age 35 due to several factors, which include specific issues of reproductive organs (uterus and oviducts), general health and decreasing number and quality of oocytes over time. The oocyte pool starts to decline during foetal life and continues within the reproductive life of women. Oocyte quality also decreases as a consequence of the increased rate of aneuploidies observed with age: 74% at the age of 41–42, and up to 93% after the age of 42 (5). Advanced age is too associated with a reduction in the quality of the oocyte cytoplasm (ooplasm), which directly affects oocyte maturation (3).

What are the main reasons for this reduction in ooplasm quality? Mitochondria are one of the most important organelles, which are affected in different ways (6,7):

– Morphological and functional abnormalities
– Mitochondrial swelling
– Alterations in mitochondria’s cristae
– Vacuolization
– Alterations of the membrane potential
– Alterations of the metabolic pathways in cummulus cells, which may result in impaired mitochondria biogenesis during oogenesis

These effects are due to the higher ratio of mutation consequence of the proximity of these organelles to the respiratory chain, the inefficient repair mechanism and the exposure of histories. How these changes affect oocyte quality (8)? First of all, negative effects on chromosome segregation have been observed as a result of a decreasing ATP concentration (9,10). Additionally, defects have been found in different signalling pathways such as Ca2+ signalling, which affects fertilization and the subsequent embryo development (11).

Implications MitochondriaFigure 2. Overview of the multiple implications of mitochondria in ovarian ageing and their deleterious consequences on reproductive function (12)

Nevertheless, different mitochondrial haplogroups should be taken into consideration. These have different bioenergetic functions, including production of reactive oxygen species (ROS) and mitochondrial coupling efficiency, aspects that might affect the oocyte longevity (13). Consequently, new techniques are being developed in order to increase the reproductive options in women with oocyte problems. Recently, one of these techniques that have been highly treated in the media is the development of additional viable oocytes from polar body genomes (2).

HOW DOES TRANSFER OF POLAR BODY GENOME WORK?

Originally, the transfer of polar body has been applied to cases of infertility with a genetic cause, such as the presence of mitochondrial diseases. These cases can be treated with the use of donor oocytes in clinical practice. Additionally, another application is the formation of human metaphase II (MII) oocytes, which increases the number of available oocytes for an assisted reproduction cycle (2).

Two specific combined steps are needed. First, the donor oocyte spindle is removed, which requires the utilization of polarized light. Once located, it will be biopsied, obtaining an enucleated oocyte (14,15,16). Secondly, the patient polar body is biopsied, provided elimination of the spindle apparatus has been confirmed.

Once both processes have been performed, the last step is the introduction of the polar body genome inside the enucleated oocyte (17).

Polar body genome transfer technique

Figure 3. Polar body genome transfer technique (2)

FUNCTIONAL HUMAN OOCYTES GENERATED BY TRANSFER OF POLAR BODY GENOMES

Hong Ma and his group have tried to test the efficiency and possible limitations of this technique (ref). The main objective to be achieved was the formation of spindles resembling those typical of MII oocytes, including the appropriate chromosome dosage.

HOW EFFICIENT IS THIS TECHNIQUE?

Although DAPI staining demonstrated that all polar body nuclear transfer (PBNT)-oocytes contained spindle-chromosome complexes, only two of five experimental oocytes formed metaphase spindles similar to intact MII oocytes. This low number may be due to residual meiotic activity in enucleated human MII oocytes, which is sometimes not enough to induce formation of normal MII-like spindles.

For a different cohort of oocytes, the rate of successful fertilization was 76%, still slightly lower than control oocytes. Furthermore, 42% of embryos reached blastocyst stage, indicating that most of the PBNT-oocytes were capable of completing the second meiotic division. Short tandem repeat (STR) analysis revealed that two sampled PBNT-blastocysts contained normal diploid chromosomes, determining that these embryos were completely viable.

WHAT CAN BE CONCLUDED?

•    Polar body genome transfer seems to be a significant technique for the improvement of assisted reproductive technology (ART) outcomes and pregnancy rates, particularly for women with decreased ovarian reserve and low response to stimulation.

•    The cytoplasm from young donor oocytes may reduce incidences of low cytoplasmic oocyte quality.

•    It could provide an additional technique to support mitochondrial replacement therapy.

Nevertheless, this technique is not suitable for women who cannot produce mature oocytes, typical profile of ART patients. Additionally, incidences of aneuploidy resulting from errors in mitosis or in the second meiotic division may still occur because of women advanced age. Larger datasets from this technique are needed to confirm its efficacy and safety. Also, improving preimplantation genetic screening (PGS) is critical before eventual clinical application. 

REFERENCES

  1. Available from: https://bigpictureeducation.com/sperm-and-egg-cells-images. [Cited 8 December 2016].
  2. Ma H, O’Neil RC, Marti Gutierrez N, Hariharan M, Zhang ZZ, He Y, et al. Functional Human Oocytes Generated by Transfer of Polar Body Genomes. Cell Stem Cell. 2016; 20, 1-8.
  3. Jirge P. Poor ovarian reserve. Journal of Human Reproductive Sciences. 2016;9(2):63.
  4. Cohen J, Chabbert-Buffet N, Darai E. Diminished ovarian reserve, premature ovarian failure, poor ovarian responder—a plea for universal definitions. Journal of Assisted Reproduction and Genetics. 2015;32(12):1709-1712.
  5. Meczekalski B, Czyzyk A, Kunicki M, Podfigurna-Stopa A, Plociennik L, Jakiel G et al. Fertility in women of late reproductive age: the role of serum anti-Müllerian hormone (AMH) levels in its assessment. Journal of Endocrinological Investigation. 2016;39(11):1259-1265.
  6. Kushnir VA, Ludaway T, Russ RB, Fields EJ, Koczor C, Lewis W. Reproductive aging is associated with decreased mitochondrial abundance and altered structure in murine oocytes. J Assist Reprod Genet. 2012 Jul;29(7):637–42.
  7. Simsek-Duran F, Li F, Ford W, Swanson RJ, Jones HW, Castora FJ. Age-associated metabolic and morphologic changes in mitochondria of individual mouse and hamster oocytes. PloS One. 2013;8(5):e64955.
  8. Lynch M, Koskella B, Schaack S. Mutation pressure and the evolution of organelle genomic architecture. Science. 2006 Mar 24;311(5768):1727–30.
  9. Schon EA, Kim SH, Ferreira JC, Magalhães P, Grace M, Warburton D, et al. Chromosomal non-disjunction in human oocytes: is there a mitochondrial connection? Hum Reprod Oxf Engl. 2000 Jul;15 Suppl 2:160–72.
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  13. May-Panloup P, Boucret L, Chao de la Barca J-M, Desquiret-Dumas V, Ferré-L’Hotellier V, Morinière C, et al. Ovarian ageing: the role of mitochondria in oocytes and follicles. Hum Reprod Update. 2016 Nov;22(6):725–43.
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  16. Cimadomo, D., Capalbo, A., Ubaldi, F.M., Scarica, C., Palagiano, A., Canipari, R., and Rienzi, L. (2016). The impact of biopsy on human embryo developmental potential during preimplantation genetic diagnosis. BioMed Res. Int.2016, 7193075
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