Grand Rounds

Can We Finally Culture Follicles to Reach Complete Oocyte Development?

BACKGROUND AND PURPOSE:

Complete oocyte growth from primordial follicles has been achieved in mice
In humans, previous to this report, growth to fully grown ooctyes has been accomplished but only from the secondary/multi-laminar stage
McLaughlin et al. (Molecular Human Reproduction, 2018) developed a culture system that allows complete growth from the earliest stages of human follicle development through to Metaphase II

Ovarian cortical biopsies were obtained from women undergoing elective caesarean section
The maturation of the oocyte occurred in the following steps
- For 8 days, fragments of these cortical were cultured in 300 μl of McCoy’s 5a medium with bicarbonate supplemented with HEPES, glutamine, HAS, penicillin G, streptomycin, transferrin, selenium, human insulin, recombinant human FSH and ascorbic acid
  - Fragments were cultured at 37°C in humidified air with 5% CO2 with half the media being removed and replaced every second day
  - Follicles with a mean diameter > 40 μm were removed from the fragments so that remaining growing follicles were only primordial, transitional or primary
- Secondary/multi-laminar intact follicles (100-150 µm in diameter) were selected for further culture in serum free medium, including 100 ng/ml of human recombinant Activin A
- After 8 days, cumulus oocyte complexes (complete cumulus and adherent granulosa cells) were selected and cultured in Activin A and FSH on membranes
- Complexes containing >100 µm diameter were selected for further in vitro maturation (IVM) in SAGE medium (75 mIU/ml of FSH and 75 mIU/ml of LH in humidified air and 5% CO2 at 37°C) and then fixed for analysis

87 secondary follicles of diameter 120 ± 6 μm (mean ± SEM) could be dissected for further culture following initial growth of growing follicles
After a further 8 days, 54 of the 87 follicles had reached the antral stage of development
At the end of Step 3, 32 complexes contained oocytes >100 μm diameter were suitable for IVM
Nine of these complexes contained polar bodies within 24 h
However, all polar bodies were abnormally large
Image analysis showed the presence of a Metaphase II spindle confirming that these oocytes had resumed meiosis but their developmental potential is unknown

This study demonstrated that a human oocyte can be developed from the earliest follicular stages
This study does not provide evidence as to the fertilization potential of oocytes developed within a complete in vitro growth system
Technique must be further optimized although this proof of concept study is promising
Main clinical application is fertility preservation, especially ovarian tissue cryopreservation for cancer patients
- May be especially helpful in prepubertal girls
- While autologous transplantation of cryopreserved ovarian tissue has been reported, there is a risk that tumor cells that may have contaminated the ovarian tissue will also be re-introduced
- Complete oocyte maturation would avoid concern regarding tumor cell contamination and delay in starting chemotherapy

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