Spontaneous activation of ovulated rat oocytes during in vitro culture

Ovulated rat oocytes were observed to activate spontaneously during in vitro culture. The possible mechanisms involved in this activation were studied by culturing oocytes at various times (0–6 hr) after ovulation, in different media, and for different incubation periods (0–5 hr). Activated oocytes extruded the second polar body within 60 to 90 min of culture. Following 3 to 4 hr of culture chromosomes were scattered throughout the cytoplasm; however, no pronuclear formation was observed. Neither time after ovulation nor incubation in different media affected the rate of activation. The length of time the oviducts remained in the animal after cervical dislocation, however, significantly (P < 0.01) affected the rate of activation. Oocytes obtained as rapidly as possible had an activation rate of 21% during in vitro culture, whereas oocytes obtained from oviducts which remained in the animal for 5 min after cervical dislocation had an activation rate of 94%. Therefore, exposure to changing oviductal conditions following cervical dislocation appears to be the critical factor influencing spontaneous activation of metaphase II rat oocytes during in vitro culture. Our studies demonstrate that rat oocytes can spontaneously activate during in vitro culture, a factor which may affect the fertilizability of the oocyte.