Follicular Growth During the Luteal Phase

Early stages of ovarian follicular development are not dependent upon gonadotropins; a pulse of follicle stimulating hormone (FSH) initiates growth of a cohort of tertiary follicles[4,5] and is required for growth from approximately a diameter (mm) of 2 to 4 (ewe) or 4 to 9 (cow). Frequent pulses of LH are necessary for development beyond about 4 or 9 mm, respectively. As follicles grow, they secrete increasing amounts of estradiol-17p and inhibin. Each hormone has negative feedback effects upon secretion of FSH (estradiol limits secretion of GnRH, and inhibin reduces responsiveness of the anterior pituitary to GnRH). As FSH becomes limiting, the dominant follicle becomes more reliant on LH.[4] Pulses of LH maintain growth of dominant follicles until either ovulation or atresia. During continued progesterone dominance, increased secretion of estradiol and inhibin by the growing follicle causes a gradual decrease in FSH. Further, pulse frequencies of LH (kept low by progesterone) become limiting and the dominant follicle becomes atretic. Estradiol and inhibin then decrease, allowing a short-lived increase in FSH that recruits a new cohort of follicles.[4,5]

Waves of follicular growth occur sequentially, with one follicle of a cohort becoming largest (dominant) every seven to 10 days in the cow (Refs. [5] and [6]; Fig. 2), or one to three follicles every three to four days in most breeds of ewe or doe.[7,8] Numbers of follicular waves during the estrous cycle vary by species and breed. British and continental breeds of beef and dairy cows usually have two to three follicular waves.[5,6] However, two to

Hormone Cycle Bull

Day of the estrous cycle

Fig. 1 Hormonal patterns during the bovine estrous cycle. (Drawn by Darron L. Smith and Keith Inskeep.) (View this art in color at www.dekker.com.)

Day of the estrous cycle

Fig. 1 Hormonal patterns during the bovine estrous cycle. (Drawn by Darron L. Smith and Keith Inskeep.) (View this art in color at www.dekker.com.)

four follicular waves may be seen in Brahman cattle and approximately four may be typical in ewes and does.[7,8] Follicular waves are less well-defined in ewes and does.[7,8] Largest follicles show less dominance over smaller ones or recruitment of new ones in ewes than in cows. Thus, in ewes, ovulatory follicles may arise from different cohorts, as early as day nine or as late as day 15.

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