PARENT SESSION
OVARY - B

Tuesday, August 3, 2004
10:30 AM–12:30 PM
Buchanan Courtyard

(469) THE ABSENCE OF THE CORPUS LUTEUM INDUCES CO-DOMINANT FOLLICLE DURING THE FIRST FOLLICULAR WAVE IN THE COW.

Matsui, Motozumi¹, Hayashi, Ken-Go¹, Acosta, Tomas¹, Jayawardana, Barana¹, Sato, Ayako¹, Ohtani, Masayuki¹, Miyamoto, Akio¹, ¹ Department of Agricultural and Life Science, Obihiro, Japan

ABSTRACT- Follicle deviation is the essential event in follicle selection of monovular species. FSH and LH play central roles in follicle deviation, and progesterone (P) secreted from the corpus luteum (CL) regulates the levels of FSH and LH. This study aimed to investigate the effect of the presence of CL on follicle selection in the cow. Normally cycling Holstein cows and heifers were used. A dominant follicle was aspirated at 1) before LH surge (pre-LH), at 42 h (cow, n = 5) or 36 h (heifer, n = 5) after PGF2a injection at mid luteal stage and 2) after LH surge (post-LH), at 24 h after GnRH injection following PGF2a treatment (cow, n = 4). The new follicular wave appeared after follicle aspiration. The ovaries were examined by ultrasonography on Day 3, 6 and 9 during the first follicular wave initiated after follicle aspiration in treated group (Day 0 = follicle aspiration) and after ovulation in intact group (Day 0 = ovulation, cow; n = 5, heifer; n = 3). Blood samples were collected for P, estradiol (E) and FSH determinations. CL formation did not occur in pre-LH group, resulting in plasma P level remains low (< 1 ng/ml) up to Day 8. There was no difference in diameter between first largest follicle (F1) and second largest follicle (F2) on Day 3 but also on Day 6 in pre-LH group. Only in pre-LH group, co-dominant follicle, which showed very small difference in diameter between F1 and F2 (< 2 mm), were highly observed (75%) on Day 6. Double ovulations of F1 and F2 also occurred (60%) only in pre-LH group from Day 6 to 9. Plasma levels of E and FSH in pre-LH group were not different from that in intact group. In post-LH group, CL was formed at the site of aspirated follicle. Increase in plasma P level with CL development in post-LH group was comparable to the P level after spontaneous CL formation in intact group. In intact group and post-LH group, the size of F1 was bigger (> 4.7 mm) than that of F2 on Day 6. Our results demonstrate that the absence of CL during follicular growth induces co-dominant follicle. Hormonal analyses indicate that FSH and E levels in plasma may not relate to the co-dominance of follicles. The present study suggests that the basal level of P changes the mechanism of follicle selection resulting in high frequency of co-dominance and double ovulation during the first follicular wave. Supported by JSPS and the 21^st Century COE Program (A-1).

KEY WORDS: co-dominant, cow, follicle selection, corpus luteum