Improved cryotolerance and developmental competence of human oocytes matured in vitro by transient hydrostatic pressure treatment prior to vitrification

At present, the metaphase II (MII) oocytes have a very special structure that leads to complex difficulties associated with its vitrification, and their efficacy still needs a large amount of study to observe. The present study was to investigate whether transient hydrostatic pressure (THP), which was utilized for oocytes before vitrification, had positive effect on the oocytes' developmental ability and reactive oxygen species, and had no damage on meiotic spindle, zona pellucida, and DNA copy number.

All the immature oocytes used in this research were collected between February 2015 and December 2015 in Shanghai Ji Ai Genetics & IVF Institute. The MII oocytes, which were originated from metaphase I (MI) oocytes, were randomly distributed into three groups: A) fresh oocytes; B) vitrification; and C) vitrification after THP treatment. The embryo developmental outcome was evaluated after intracytoplasmic sperm injection and embryo culture. Furthermore, the meiotic spindle behavior, reactive oxygen species (ROS), zona pellucida (ZP), and DNA copy number variation were evaluated and compared among the three groups.

A total of 568 MII oocytes were included in the study. Embryos from group B had fewer cells on day 3 compared with group A and C (5.01 ± 2.11 for group A, 3.89 ± 2.21 for group B, and 4.69 ± 2.05 for group C). The developmental feature of blastocyst in groups A and C were superior to those of group B. The MII oocytes were manipulated with THP before vitrification, and the equilibration time was significantly shorter in the vitrification procedure (244.9 ± 30.1 vs. 181.5 ± 10.1). The ROS, ZP of vitrified/warmed oocytes in group C were improved with THP before vitrification. THP had no influence on the meiotic spindle and DNA copy number variation of vitrified/warmed oocytes.

The results of the study indicated that sublethal THP treatment before vitrification increased the developmental competence of human in vitro matured oocytes, reduced vitrification-related changes in the ROS, which occurred during oocyte vitrification, and did not damage the meiotic spindle, ZP and DNA copy number variation.