In order to determine the optimum condition and timing for in vitro maturation of oocytes to metaphase of meiosis II (M II), the immatured follicular oocytes were recovered by puncturing the large(1.0~1.5 mm in diameter) and small(<1.0 mm in diameter) follicles in the ovaries of rabbits treated intramuscularly with a single dose of 100 TU PMSG 68 hours previously. The follicular oocytes were classified into three grades by the attachment of cumulus cells. The Grade I and II follicular oocytes from large follicles were cultured in BO-DM medium with 10% FCS, 35 g /nl of FSH, 10 g /ml of LH and 1 g /ml of estradiol-17 at 39t in a 5% incubator for 11 to 23 hours. In 3 hours interval during the culture period, the oocytes were harvested and their cumulus cells were removed with hyaluronidase. The denuded oocytes were stained with Hoechst 33342 dye and their meiotic status and extrusion of the first polar body (PB) were examined under a fluorescence microscope. Also the fragmentation of the first PB and the distance between the first PB and nucleus were examined. The results obtained were as follows: 1. The mean recovery rate of follicular oocytes from the large and small follicles was 59. 9 and 31.3%, respectively. The mean number of oocytes recovered per rabbit and the Grade I percentage were 14.6 and 94.4% in large follicles, but 2.1 and 61.1% in small follicles, respectively. All the parameters examined were different significantly (p<0.05) between both the folliclular size. 2. Most of the follicular oocytes(86.8%) were matured in vitro to M II phase in 14 hours in Grade I oocytes, but the significantly(p<0.05) less oocytes(45.5%) were matured in Grade II oocytes. 3. The first PB was extruded in most of the oocytes(94.7%) in 14 hours of culture with the fragmentation rate of 29.6%, but the fragmentation rate of the first PB increased significantly (p<0.05) as the culture period for maturation was longer to 20 hours(63.5%). 4. The distance between the first PB and nucleus was increased linearly (p<0.05) as the maturation time passed from 14(7.1rn) to 23 hours(58.4m). 5. From the above results it was concluded that the optimum time for in vitro maturation culture might be 14 hours in the follicular oocytes from rabbit primed with PMSG for 68 hours, expecially when these follicular oocytes were used for recipient cytoplasms in embryo cloning.

The nudear changes of bovine oocytes during 24 hrs. of culture for mejotic maturation were examined. Bovine oocytes were collected from small(<2 mm), medium(2~6 mm) and large(>6mm) follicles and classified into three grades by their morphological characteristics. A total of 242 oocytes collected were obtained:from 184 small, 157 medium and 1 large follicles, respectively and were classified into 95 grade I, 155 grade H and 92 grade III oocytes. All the bovine oocytes collected and graded were washed with a basal medium and incubated in groups of 10 for 24 hrs in 5% and 39. The basal medium used was composed of TCM-199 supplemented with sodium bicarbonate, sodium pyruvate, streptomycin, penicillin G and 10% FCS. The oocytes were cultured in drops of 50,l basal medium supplemented with 35g /ml FSH, 10g /ml LH and 1g /ml estradiol-17. The oocytes were fixed and examined on their chromosomal status by 1% acetorcein staining in the interval of 3 hrs. Most of the grade I oocytes developed to germinal vesicule stage at 0 to 3 hrs., germinal vesicle breakdown at 6 hrs., metaphase I at 9 to 15 hrs., anaphase I and telophase I at 18 hrs., and metaphase II and the first polar body at 24 hrs. after culture for meiotic maturation. However, it was found that compared to grade I oocytes, grade H and W oocytes reached earlier to germinal vesicle breakdown and most of them developed earlier to M II stage at 21 hrs. after culture.