Formation of male pronuclei in partitioned human oocytes

Ninety-five metaphase II human oocytes, aged in vitro for either one day or for two days, and five fresh immature oocytes with no visible germinal vesicle nucleus were partitioned into small cytoplasts after removal of the zona pellucida and exposure to cytochalasin B. Seventy-one metaphase II and four immature oocytes were used as intact zona-free controls. The cytoplasts derived from each partitioned oocyte and all the zona-free whole oocytes were exposed to normal or subfertile donor sperm and later assessed for signs of male pronucleus development. A total of 711 fragments (an average of 8 fragments per partitioned egg) with a mean diameter of about 50 microns were produced from the 100 partitioned oocytes. When exposed to normal sperm, 76% of the 1-day-old metaphase II fragments, 67% of the 2-day-old metaphase II fragments, and 83% of the immature oocyte fragments were fertilized. The mean number of decondensing nuclei per partitioned oocyte was 11.9 for the 1-day-old metaphase II oocytes, 6.4 for the 2-day-old metaphase II oocytes, and 13 for the immature oocytes. The mean number of decondensing nuclei per a whole zona-free oocyte was 5.6 for the 1-day-old metaphase II oocytes (p < 0.05), 6.7 for the 2-day-old metaphase II oocytes (NS), and 13 for the immature oocytes. When exposed to subfertile sperm, 54% of the 1-day-old metaphase II fragments and 28% of the 2-day-old metaphase II fragments were fertilized.(ABSTRACT TRUNCATED AT 250 WORDS)