Temporal relationships among LH, estradiol, and follicle vascularization preceding the first compared with later ovulations during the year in mares

Diameter of the preovulatory follicle, plasma concentrations of LH and estradiol, and vascularization of the follicle wall, based on color-Doppler signals, were characterized in 40 pony mares for 6 days preceding ovulation (Days −6 to −1; preovulatory period). Comparisons between the preovulatory periods preceding the first compared with a later ovulation during the year were used to study the relationships between LH and estradiol and between vascularization and estradiol. Diameter of the preovulatory follicle was greater (P < 0.02) and concentration of LH was less (P < 0.02) during the first preovulatory period, whereas concentration of estradiol was not different between the first and second preovulatory periods. Vascularized area (cm2) of the follicle wall increased at a reduced rate during the first preovulatory period, as indicated by an interaction (P < 0.03) between day and group. Vascularized area was similar between the preovulatory groups on Day −6, and a reduced rate of increase resulted in a lesser (P < 0.001) area on Day −1 before the first ovulation (1.4 ± 0.1 cm2) than before a later ovulation (2.2 ± 0.2 cm2). Results demonstrated that follicle vascularization and the LH surge were attenuated preceding the first ovulation of the year with no indication that estradiol was involved in the differences between the first and later ovulations.