Egg activation induced by osmotic pressure change and the effects of amiloride on the cryopreservation of mouse oocytes

Euthanasia via CO2 inhalation causes premature cortical granule exocytosis in mouse oocytes and influences in vitro fertilization and embryo development

Generation of high quality mouse metaphase II oocytes is an integral part for efficient in vitro fertilization (IVF), and subsequent embryo production for reproductive studies and genome banking. The main objectives of this study were to investigate the impact of various euthanasia methods on IVF, embryo development, and subcellular structures of MII mouse oocytes. Following superovulation regimen, female mice were euthanized by high flow CO₂ (H CO₂ ), low flow CO₂ (L CO₂ ), or cervical dislocation (CD). The MII oocytes obtained from these mice were evaluated for subcellular integrity by assessing their cortical granules and F-actin. Furthermore, fertilization and subsequent embryonic development competence up to blastocyst stage were also evaluated in vitro. The oocytes collected from females euthanized by CD resulted in significantly higher two-cell development rates (p = 0.028) and subsequently lead to in higher embryo development rates (p = 0.027) compared with oocytes from females euthanized by L CO₂ . The cortical granule integrity analysis revealed significantly higher rate of premature cortical granules exocytosis (PCGE) for L CO₂ group compared with CD and H CO₂ groups (p < 0.001). These data collectively suggest that CO₂ associated PCGE during euthanasia procedure is the main cause of decreased IVF rates and CD is the optimal euthanasia method for the purpose of obtaining good quality MII oocytes for mouse IVF and other reproductive studies.

Human oocyte cryopreservation and the fate of cortical granules

OBJECTIVE

To examine the effect of the commonly used oocyte cryopreservation protocol on the cortical granules (CGs) of human immature germinal vesicle (GV) and mature metaphase II (MII) oocytes.

DESIGN

Laboratory study.

SETTING

IVF unit.

INTERVENTION(S)

Unfertilized, intracytoplasmic sperm injected (ICSI) oocytes, and immature oocytes were cryopreserved using a slow freezing-rapid thawing program with 1,2-propanediol (PROH) as a cryoprotectant.

MAIN OUTCOME MEASURE(S)

Cortical granule exocytosis (CGE) was assessed by either confocal microscopy or transmission electron microscopy (TEM).

RESULT(S)

The survival rates of frozen-thawed oocytes (mature and immature) were significantly lower compared with zygotes. Both mature and immature oocytes exhibited increased fluorescence after cryopreservation, indicating the occurrence of CGE. Mere exposure of oocytes to cryoprotectants induced CGE of 70% the value of control zygotes. The TEM revealed a drastic reduction in the amount of CGs at the cortex of frozen-thawed GV and MII oocytes, as well as appearance of vesicles in the ooplasm.

CONCLUSION(S)

The commonly used PROH freezing protocol for human oocytes resulted in extensive CGE. This finding explains why ICSI is needed to achieve fertilization of frozen-thawed human oocytes.

Effect of KN-62, a selective inhibitor of calmodulin-dependent kinase II, on mouse oocyte activation

PURPOSE

Our purpose was to determine the association of calmodulin-dependent protein kinase II (CaMKII) with oocyte activation and to explore the network of protein kinases during mammalian fertilization.

METHODS

Mouse M-II oocytes were collected after superovulation induced by PMSG-hCG injection. The oocytes were inseminated or artificially activated by Ca ionophore (A23187) or 12-O-tetradecanoyl phorbol 13-acetate (TPA). The effects of KN-62, a specific and selective inhibitor of calmodulin-dependent protein kinase II, on second polar body emission (2PBE), pronuclear formation (PF), and cortical granule exocytosis (CGE) during fertilization or after artificial oocyte activation were investigated.

RESULTS

KN-62 inhibited 2PBE and PF after sperm or Ca ionophore inducing activation. Additionally, PF was inhibited by KN-62 after TPA activation, whereas KN-62 did not inhibit CGE in any case. KN-04, an inactive form of KN-62, did not inhibit significantly 2PBE, CGE, or PF. When oocytes were exposed to KN-62 after Ca ionophore or TPA activation, no inhibitory effects on 2PBE or PF were observed.

CONCLUSIONS

The CaMKII activation that occurs after fertilization or artificial activation of mouse oocytes is presumably secondary to increases in the intracellular free calcium concentration. As determined by the use of inhibitor, CaMKII activity is associated with 2PBE and PF but not with CGE.