Follicular Phase

As luteolysis proceeds, decreasing progesterone reduces negative feedback on pulsatile secretion of LH by the anterior pituitary. Increased frequency of pulses of LH promotes final maturation of the dominant follicle. During the follicular phase, LH reaches higher concentrations than in the luteal phase, because of removal of negative feedback from progesterone on the hypothalamus.

Dominant follicles, as they grow, secrete increasing concentrations of estradiol-17p and inhibin, so that FSH declines during the follicular phase, but estradiol does not limit pulse frequency of LH. Behavioral estrus (seeking out and standing to be mounted by a male) and a preovulatory surge of LH and FSH occur when the dominant follicle(s) reach adequate size and secretion of estradiol-17p reaches threshold. Follicular diameters of approximately 10 mm in the cow and 4 mm in the ewe are required for ovulation. Cows show homosexual mounting behavior, but this is not seen in ewes and seldom in does. At the preovulatory surge of LH, concentrations of estradiol-17p abruptly decline, and a secondary surge of FSH occurs on the day of ovulation (recruiting a new cohort of follicles into the growing pool).

6 8 10 12 14 16 18 20 22 24 Days of estrous cycle

6 8 10 12 14 16 18 20 22 24 Days of estrous cycle

6 8 10 12 14 16 18 20 22 24 Days of estrous cycle

Fig. 2 Observed patterns of development for the largest follicle during the bovine estrous cycle. Shown are the average diameters of the first (O O), second (□ □), and third (A A) sequentially largest follicles in cows that had (a) two or (b) three waves of follicular development, respectively. (From Ref. [6].) (View this art in color at www.dekker.com.)

6 8 10 12 14 16 18 20 22 24 Days of estrous cycle

Fig. 2 Observed patterns of development for the largest follicle during the bovine estrous cycle. Shown are the average diameters of the first (O O), second (□ □), and third (A A) sequentially largest follicles in cows that had (a) two or (b) three waves of follicular development, respectively. (From Ref. [6].) (View this art in color at www.dekker.com.)

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