116 THE CHARACTERISTICS OF CORPUS LUTEUM SIZE, BLOOD FLOW, AND PLASMA PROGESTERONE CONCENTRATION AFTER OVULATION OF THE FIRST AND SECOND WAVE DOMINANT FOLLICLE

Abstract

The characteristics of the corpus luteum (CL) are greatly affected by the characteristics of the preceeding preovulatory follicle, such as follicle size and function. A previous study reported that the concentration of oestradiol in follicular fluid and production of androstenedione and progesterone (P4) by cultured theca cells are higher in the first follicular wave dominant follicle than in the second follicular wave dominant follicle. In addition, blood flow in the wall of the preovulatory follicle is higher in the first follicular wave than in the second follicular wave. These results suggest that the characteristics of the dominant follicle in the first and the second follicular wave may be different. The objective of this study was to compare CL characteristics, such as diameter, cross-sectional area, blood flow area, and plasma P4 concentration after ovulation of the dominant follicle between the first and the second follicular waves. Preovulatory follicles and CL formed after ovulation of the first follicular wave (W1; n = 5) and second follicular wave (W2; n = 6) were examined in non-lactating Holstein cows. In W1, PGF2α and GnRH were administrated on D7 and D9 of the oestrus cycle (D0 = oestrus), respectively, to induce follicular maturation. In W2, GnRH was administrated on D6 to induce a new follicular wave; subsequently, PGF2α and GnRH were administrated on D14 and D16, respectively. Diameter and percentage of follicular circumference with blood flow of preovulatory follicle on D10 in W1 and D17 in W2 were measured by transrectal colour Doppler ultrasonography. Diameter, cross-sectional area, and blood flow area of CL formed after ovulation in W1 (W1CL) and W2 (W2CL) were also examined on Day 3, 6, and 9 after ovulation (Day 1 = ovulation day). Blood samples were collected from Day 1 to 9 for P4 measurement. Quantitative end-points for diameter and percentage of follicular circumference with blood flow of the preovulatory follicle were analysed between the groups by using the unpaired Student's t-test. Diameter, cross-sectional area, blood flow area of CL, and plasma P4 concentration were analysed by repeated-measures ANOVA followed by Scheffe's F-test as a multiple comparison test. Larger diameter and higher percentage of follicular circumference with blood flow of the preovulatory follicle were observed in W1 compared with those in W2 (P < 0.01). Diameter of CL was larger in W1CL than in W2CL regardless of day (P < 0.001). Cross-sectional area of CL was larger in W1CL than in W2CL on Day 6 and 9 but not on Day 3. Blood flow area of CL was larger in W1CL than in W2CL on Day 3 and 6 but not in Day 9. Plasma P4 concentrations were higher in W1CL than in W2CL on Day5 and 7 (P < 0.05). In conclusion, larger size and higher percentage of follicular circumference with blood flow of preovulatory follicle in W1 lead to a larger size and a blood flow area of CL as well as higher plasma P4 concentration. These results suggest that preovulatory blood flow status affects the morphology and function of CL.