OBJECTIVE: To determine whether oxygen (O2) tension (20% vs. 5%) has an impact on oocyte maturation rates and morphology during in vitro maturation (IVM).
METHODS: A prospective, observational, monocentric, sibling-oocyte study.
METHODS: University Hospital.
METHODS: A total of 143 patients who underwent IVM for fertility preservation purposes from November 2016 to April 2021 were analyzed. Patients were included when ≥2 cumulus-oocyte complexes (COCs) were retrieved. The cohort of COCs obtained for each patient was randomly split into two groups: group 20% O2 and group 5% O2.
METHODS: Cumulus-oocyte complexes were incubated for 48 hours either under 5% O2 or 20% O2. After 24 and 48 hours of culture, every oocyte was assessed for maturity and morphology, to estimate oocyte quality. Morphology was evaluated considering six parameters (shape, size, ooplasm, perivitelline space, zona pellucida, and polar body characteristics), giving a total oocyte score ranging from -6 to +6.
METHODS: Maturation rates and total oocyte scores were compared using paired-sample analysis between group 20% O2 and group 5% O2.
RESULTS: Patient median age was 31.4 [28.1-35.2] years-old. The mean serum antimüllerian hormone levels and antral follicle count were 3.2 ± 2.3 ng/mL and 27.2 ± 16.0 follicles, respectively. A mean of 10.7 COCs per cycle were retrieved, leading to 6.1 ± 2.4 metaphase II oocytes vitrified (total maturation rate = 57.3%; 991 metaphase II oocytes/1,728 COCs). A total of 864 COCs were included in each group. Oocyte maturation rates were not different between the two groups (group 20% O2: 56.82% vs. group 5% O2: 57.87%, respectively). Regarding oocyte morphology, the mean total oocyte score was significantly higher in group 5% O2 compared with group 20% O2 (3.44 ± 1.26 vs. 3.16 ± 1.32, P=.014).
CONCLUSIONS: As culture under low O2 tension (5% O2) improves oocyte morphology IVM, our results suggest that culture under hypoxia should be standardized. Additional studies are warranted to assess the impact of O2 tension on oocyte maturation and the benefit of IVM under low O2 tension for embryo culture after utilization of frozen material.

Oocyte in vitro maturation (IVM) is an assisted reproductive technology with a long and sometimes checked history. It is a minimally invasive technique involving the deliberate collection of immature oocytes from patients that have received no or minimal ovarian stimulation and the culture of oocytes to maturity in vitro, before standard procedures thereafter. Now, IVM is classified as nonexperimental and is primarily indicated for patients with a high antral follicle count, especially patients with polycystic ovaries or polycystic ovary syndrome, as well as for fertility preservation in cancer patients. In the recent past, IVM practice has had a confusing array of clinical protocols and has been slow to adapt to new scientific insights; however, recently, significant advances have been made in IVM culture methods based on new knowledge from animal studies, combined with defining a simple patient treatment protocol. These improvements have led to significant recent progress in IVM practice to the extent that IVM is now routinely practiced in a growing number of centers with specialized expertise around the world.

Cryopreservation of ovarian tissue to preserve the fertility of girls and young women at high risk of sterility is now widely practiced. Pieces of cryopreserved ovarian cortex can be thawed and autografted to restore fertility, but because of the risks of reintroduction of the cancer, transplantation may not be possible for girls and women with blood-borne leukemias or cancers with a high risk of ovarian metastasis. Cryopreserved ovarian tissue contains mainly primordial follicles but also provides access to immature oocytes from small antral follicles, which may be matured in vitro to provide an additional source of mature oocytes. So in cases in which transplantation is contraindicated, fertility restoration could be safely achieved in the laboratory either by in vitro maturation (IVM) of oocytes aspirated from growing follicles or by the complete in vitro growth (IVG) and maturation (IVM) of primordial follicles to produce fertile metaphase II (MII) oocytes. The development of IVM and IVG methods to support all stages of oocytes available within ovarian tissue will maximize the potential for all patients undergoing fertility preservation.

Storing female reproductive potential can offer enhanced prospects for future conception in women whose fertility is threatened by cytotoxic therapies. Human female reproductive potential can be cryopreserved and stored at very low temperatures as embryos or gametes. Gamete (oocyte) cryopreservation circumvents potential issues associated with ownership when future use is being considered and may, therefore, be more generally acceptable as an approach. Advances in the technology, in particular the clinical application of vitrification, have significantly improved the outcomes from mature oocyte cryopreservation, which are now comparable to those from embryo cryopreservation. In cases where mature oocyte cryopreservation is not feasible, ovarian cortex containing primordial follicles can be cryopreserved, and over 100 births have now been reported following grafting of stored ovarian tissue. Ovarian tissue cryopreservation is now an established approach to preserve future fertility for young women; however, the efficiency is difficult to determine particularly for the prepubertal tissue with a scarcity of data.