194 EFFECT OF CORPUS LUTEUM PRESENCE FOR THE DURATION OF FOLLICULAR GROWTH ON BOVINE OOCYTE DEVELOPMENTAL COMPETENCE

Abstract

It has been reported that follicular wave control is effective to increase bovine oocyte (OC) quality of ovum pickup (Imai et al. 2008 Reprod. Fertil. Dev. 20, 182). However, the effect on the bovine OC developmental competence to the presence of corpus luteum (CL) for the duration of follicular growth (FG) is not known. This study was conducted to examine the developmental competence of OC after stimulation of FG with and without CL. Japanese Black cows (n = 4/session) were synchronised in oestrus (Day 0; D0) with prostaglandin F_2α (PGF_2α; 0.5 mg cloprostenol). On Day 9, cows were removed follicles (FL) ≥ 8 mm in diameter and CIDR were inserted. Cows in the PGF_2α treatment group (PG) were administered PGF_2α (0.75 mg cloprostenol) for regression of the CL; the PGF_2α non-treatment group (non-PG) were not administered PGF_2α for the presence of CL. The cows received 20 mg of FSH twice a day from the evening of Day 10 to the morning of Day 14 in decreasing doses. Oocytes were collected from FL ≥ 5 mm by OPU on Day 15. Collected OC were classified as follows: Grade 1, several layers of cumulus cells (CC) and homogeneous OC cytoplasm; Grade 2, 1 to 3 layers of CC; Grade 3, denuded OC; Grade 4, expanded CC or degenerated OC. The OC that were classified as Grade 1 to 3 were matured in vitro for 20–22 h and then inseminated with 3 × 10⁶ sperm mL^–1 for 6 h. Then presumptive zygotes were cultured in 5% newborn calf serum + CR1aa for 216 h in micro-well culture dishes (Dai, Nippon, Japan). Cows were assigned to groups of 4 with each pair placed into 1 of the treatments, and groups switched at ≥40 days of interval for crossover trials. To compare the quality of OC derived from CL ipsilateral FL and CL contralateral FL, the OC were subdivided in CL ipsilateral group (ipsi-CL) and CL contralateral group (contra-CL) in the non-PG. In addition, half of the data from PG were regarded as unilateral ovary (regression CL group; reg-CL). Statistical analysis was carried by Student’s t-test, Tukey’s test, Fisher’s exact probability test, and chi-squared test. Part of data was log-transformed. At OPU, all cows in the non-PG group had an observed CL, and the average diameter of the CL was 16 ± 0.1 mm in the long axis and short; all cows in the PG were observed to not have a CL. In PG and non-PG, there were no significant differences in the number of FL ≥ 5 mm, the rate of cleavage, or the rate of blastocyst (BL) formation (17 ± 2.0 v. 20.8 ± 2.6, 82.7 ± 5.6% v. 86.1 ± 3.9% and 53.4 ± 5.9% v. 62.1 ± 4.7%, respectively). The number of BL tended to increase in non-PG cows compared with PG cows (10.0 ± 1.2 v. 7.8 ± 0.9; P = 0.078). The number of BL rated as Code 1 or 2 (IETS code) was higher in non-PG cows than in PG cows (76 v. 46; P < 0.01). In ipsi-CL, contra-CL, and reg-CL, statistical differences were observed between the rate of Grade 1 to 2 OC and groups; the rate of Grade 1 to 2 OC was higher in ipsi-CL (93.8 v. 79.0 v. 86.7; P < 0.01). These results suggested that the presence of CL for the duration of FG did not affect the rate of cleavage and BL formation but may have improved efficiency of embryo production by increasing quality of OC and BL.