Chapter 3 Current Challenges in Immature Oocyte Cryopreservation

Ultrastructural Evaluation of the Human Oocyte at the Germinal Vesicle Stage during the Application of Assisted Reproductive Technologies

After its discovery in 1825 by the physiologist J.E. Purkinje, the human germinal vesicle (GV) attracted the interest of scientists. Discarded after laparotomy or laparoscopic ovum pick up from the pool of retrieved mature oocytes, the leftover GV was mainly used for research purposes. After the discovery of Assisted Reproductive Technologies (ARTs) such as in vitro maturation (IVM), in vitro fertilization and embryo transfer (IVF-ET) and intracytoplasmic sperm injection (ICSI), its developing potential was explored, and recognized as an important source of germ cells, especially in the case of scarce availability of mature oocytes for pathological/clinical conditions or in the case of previous recurrent implantation failure. We here review the ultrastructural data available on GV-stage human oocytes and their application to ARTs.

Dynamic changes in the global transcriptome of bovine germinal vesicle oocytes after vitrification followed by in vitro maturation

This study was conducted to investigate the effect of vitrification on the dynamics of the global transcriptome in bovine germinal vesicle (GV) oocytes and their in vitro-derived metaphase II (MII) oocytes. The GV oocytes were vitrified using the open-pulled straw method. After warming, GV oocytes and the resulting MII-stage oocytes were cultured in vitro for 2h and 24h respectively and were then collected. The fresh GV oocytes and their in vitro-derived MII oocytes were used as controls. Then, each pool (fresh GV, n=3; vitrified GV, n=4; fresh MII, n=1 and MII derived from vitrified GV, n=2) from the different stages was used for mRNA transcriptome sequencing. The results showed that the in vitro maturation rates of GV oocytes were significantly decreased (32.36% vs 53.14%) after vitrification. Bovine GV oocyte vitrification leads to 12 significantly upregulated and 19 downregulated genes. After culturing in vitro, the vitrification-derived MII oocytes showed 47 significantly upregulated and six downregulated genes when compared with those from fresh GV oocytes. Based on molecular function-gene ontology terms analysis and the Kyoto encyclopaedia of genes (KEGG) pathway database, the differentially expressed genes were associated with the pathways of cell differentiation and mitosis, transcription regulation, regulation of actin cytoskeleton, apoptosis and so on, which potentially result in the lower in vitro development of GV bovine oocytes.

Autophagy inhibition of immature oocytes during vitrification-warming and in vitro mature activates apoptosis via caspase-9 and -12 pathway

OBJECTIVE

The aim of this study is to determine the role of autophagy in the immature oocytes during vitrification-warming and in vitro maturation (IVM); the correlations among autophagy, apoptosis, and the activities of caspase in the immature oocytes during vitrification-warming and IVM were also explored.

STUDY DESIGN

Immature oocytes from mice were vitrified-warmed and IVM. An autophagy inhibitor (3-methyladenine) was supplemented in cryopreservation solutions and warming solutions. The expression of beclin-1 (an autophagy marker), caspase-3, -8, -9, and -12 were measured. Moreover, the viability of vitrified-warmed immature oocytes and their subsequent developmental competence were measured.

RESULTS

The levels of beclin-1 expression in both mRNA and protein in oocytes experienced vitrification-warming and IVM were significantly higher than that in fresh immature oocytes experienced IVM. The levels of caspase-3, -9, -12 expression in both mRNA and protein in oocytes vitrified with 3-methyladenine were significantly higher than that vitrified without 3-methyladenine. However, the differences in the caspase-8 expression in both mRNA and protein between the oocytes vitrified with 3-methyladenine and that vitrified without 3-methyladenine were not significant.

CONCLUSION

Immature oocyte cryopreservation exhibits autophagic activation. Autophagy inhibition of the immature oocytes during vitrification-warming and IVM activates apoptosis via caspase-9 and -12 pathway.