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 ooc ytes (group A) and 51 non-activated oocytes (group B) were injected with hu man sperm and checked for fertilization after 16-18 hours. Then, 20 hamster oocytes were cultured in vitro in medium with or without calcium and exami ned 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 o f meiotic spindle near the first polar body, protrusion of oolemma, and com plete extrusion of second polar body with pronucleus formation. After cultu re for 1.5 hours in vitro, 30%-40% of the oocytes demonstrated meiotic spin dle; but after 9 hours, 50% showed complete activation. Culture in medium w ithout calcium showed a little delay in the process of activation. The earl iest sign of spontaneous activation in hamster oocytes was the appearance o f 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 thei r removal. It is important to recognize the earliest sign of spontaneous ac tivation and perform intracytoplasmic sperm injection (ICSI) before it happ ens.