Ultrastructure of human oocytes after in vitro maturation

Study hypothesis: How does the ultrastructure of human oocytes matured in vitro compare with oocytes collected from women after full hormonal stimulation?

Study finding: The ultrastructure of human oocytes matured in vitro is largely, but not entirely, similar to those matured in vivo.

What is known already: Embryos derived from in vitro-matured oocytes often have limited developmental potential, possibly as an effect of inappropriate in vitro maturation (IVM) conditions. Transmission electron microscopy (TEM) is a valuable research tool to compare in vivo and in vitro matured oocytes. However, previous studies on the ultrastructure of human IVM oocytes were done with inadequate material or inappropriate IVM conditions, and have limited significance.

Study design, samples/materials, methods: Immature cumulus cell-enclosed oocytes, retrieved from mid-sized antral follicles of women requiring IVM treatment, were matured in vitro for 30 h. No leftover germinal vesicle-stage oocytes collected from fully stimulated cycles were used. Control in vivo matured oocytes were obtained from age-matched women undergoing full ovarian stimulation. In vitro and in vivo matured oocytes were analysed by TEM and compared according to previously established morphometric criteria of oocyte quality.

Main results and the role of chance: All oocytes had normal ooplasm showing uniform distribution of organelles. Mitochondrial morphology appeared similar between the maturation conditions. Cortical granules were found typically stratified in a single, mostly continuous row just beneath the ooplasm in all oocytes. Microvilli were well preserved after IVM. Vacuoles were only occasionally found in all oocytes and, if present, they were frequently associated with lysosomes. Mitochondria-smooth endoplasmic reticulum (M-SER) aggregates and mitochondria-vesicles (MV) complexes were commonly found in in vivo matured oocytes. However, large MV complexes partially replaced M-SER aggregates in IVM oocytes.

Limitations, reasons for caution: As a note of caution it should be noticed that, being laborious and technically demanding, TEM cannot be applied to a large number of samples in a single investigation. Therefore, our data require further independent confirmation.

Wider implications of the findings: Our data suggests the notion that TEM remains a valuable research tool that can also offer quantitative data if associated with morphometric criteria of evaluation. Therefore, it can be adopted to test pre-clinically the performance of novel in vitro systems that are demanded to make oocytes IVM more successful in the human.

Large scale data: Not applicable.

Study funding and competing interests: This study was independently funded by Biogenesi Reproductive Medicine Centre, Monza, Italy. All authors declare that their participation in the study did not involve factual or potential conflicts of interests.