This study investigated the capacity of healthy oocytes derived from follicles of different size to undergo normal fertilization and early embryonic development in vitro and full-term development in vivo. Ovaries were collected from a local abattoir and dissected and classified as follows: group A, > 4-8 mm (large); group B, > 2-4 mm (medium); and group C, > 1-2 mm (small). Oocytes were isolated by puncturing the follicular wall and pressing of the follicle. Only healthy-looking cumulus-oocyte complexes (COC) were used for in vitro maturation. Oocytes were fertilized in vitro by frozen/thawed semen from one bull. Approximately one-fourth of all oocytes was fixed and stained 15-20 h after fertilization, to determine penetration rates. The remaining eggs were transferred to culture medium and were cultivated for up to 9.5 days. Cleavage was observed 65 h and 7 days after fertilization. Expanded, hatching, and hatched blastocysts were fixed and stained after 9.5 days of culture. A total of 86 blastocysts derived from group A and B oocytes was nonsurgically transferred to synchronized recipients 7-8 days after onset of culture. A total of 6.624 follicles were dissected from 265 ovaries, and 1,485 oocytes were isolated from 1,671 group A follicles, 3,509 oocytes from 3,862 group B follicles, and 965 oocytes from 1,091 group C follicles. The fertilization rate, rate of normal fertilization, rate of polyspermy, and rate of other abnormal fertilization features were as follows: group A, 84.9%, 43.2%, 34.1%, 7.6%; group B, 83.6%, 44.8%, 31.1%, 7.8%; and group C, 61.7%, 13.1%, 33.7%, 19.1 %. The cleavage rate and the percentage of embryos with 5-8 cells after 65 h amounted in group A to 68.9% and 36.9%; in group B, 64.3% and 32.7%; and in group C, 19.7% and 0.4%, respectively. The percentage of blastocysts on days 7 and 9.5 as well as the rate of hatched blastocysts were 27.7%, 24.8%, and 21.7%, respectively, in group A; 20.9%, 19.2%, and 16.8%, respectively, in group B; and zero in group C. Transfer of 38 blastocysts of group A to 19 recipients led to 10 pregnancies; transfer of 48 blastocysts to 24 recipients of group B resulted in 12 pregnancies. It is concluded that blastocysts from follicles ranging from > 2 to 8 mm in size have a similar developmental potential. Oocytes from 1- to 2-mm follicles, however, have a significantly lower competence to undergo in vitro maturation and fertilization and, under the conditions of our experiments, completely lack the capability to cleave beyond the 8-cell stage.
