Spontaneous activation of hamster oocytes in vitro

OBJECTIVES

To observe the time of spontaneous activation of hamster oocytes and to find out the early signs of hamster oocytes activation.

METHODS

First, 63 hamster oocytes including activated and non-activated oocytes (group A) and 51 non-activated oocytes (group B) were injected with human sperm and checked for fertilization after 16-18 hours. Then, 20 hamster oocytes were cultured in vitro in medium with or without calcium and examined under light microscope every half an hour up to 9 hours.

RESULTS

Under light microscope, hamster oocytes activation were observed to undergo 4 stages: metaphase II stage without meiotic spindle, appearance of meiotic spindle near the first polar body, protrusion of oolemma, and complete extrusion of second polar body with pronucleus formation. After culture for 1.5 hours in vitro, 30%-40% of the oocytes demonstrated meiotic spindle; but after 9 hours, 50% showed complete activation. Culture in medium without calcium showed a little delay in the process of activation. The earliest sign of spontaneous activation in hamster oocytes was the appearance of meiotic spindle near the first polar body. Although group A and B showed no difference, the damage rate was lower in group B (19.7% +/- 16.6%) than in group A (35.9% +/- 5.7%) (P = 0.08), whereas the fertilization rate (18.3% +/- 10.9%) was higher in group B than in group A (14.2% +/- 8.7%) (P = 0.135).

CONCLUSIONS

Hamster oocytes may be activated spontaneously early after their removal. It is important to recognize the earliest sign of spontaneous activation and perform intracytoplasmic sperm injection (ICSI) before it happens.