Fertilization and developmental competence of bovine oocytes derived from different categories of antral follicles

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 cumulusoocyte 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.