Follicle selection in domestic ruminants.

Ultrasound imaging has shown that cattle exhibit 2 or 3 waves of follicular development during an oestrous cycle. The waves consist of the contemporaneous appearance, about every 7 days, of a group of follicles > or = 5 mm in diameter. One follicle gradually becomes larger than the rest (i.e. dominant). There are several lines of evidence suggesting that the waves occur regularly under conditions of basal LH and FSH. (1) Cycles with 3 waves of follicular development are longer and have longer luteal phases than do cycles with 2 waves, indicating that the number of waves in a cycle is determined by the time of luteal regression. (2) Cycles with 4 or 5 waves of follicular development can occur when the luteal phase is artificially prolonged with exogenous progesterone. (3) Waves of follicular development occur during pregnancy. However, the secondary surge of FSH may be important in initiating new follicular recruitment after ovulation, since suppression of the secondary surge delays the first wave of follicular development. Follicles are functionally dominant (capable of ovulating after luteal regression) while they are still growing and early during their plateau in growth. Functional dominance is lost some time between the early and late plateau phases, while the follicle is still morphologically dominant (i.e. the largest follicle). The factors that lead to dominance of one follicle and the mechanisms that suppress the growth of subordinate follicles are not well understood. When the luteal phase is artificially extended with low doses of exogenous progesterone, the normal pattern of follicular development is altered and the ovulatory follicle grows for a prolonged period of time. This finding indicates that subtle changes in the hormonal milieu can dramatically alter follicular dynamics and that the experimental model of prolonged dominance may be useful in studying the mechanisms of follicular dominance. In contrast, patterns of follicular development in sheep must be assessed in more indirect ways, but sheep offer the advantage of breeds that differ in ovulation rate. Correlation of the endocrine environment with ovulation rate in this species provides a valuable approach to understanding the mechanisms controlling follicle selection and ovulation rate. It has been suggested that in some species a high ovulation rate is achieved by increased recruitment, whereas in others there is increased selection. There is evidence that oestradiol is involved in regulating the number of dominant follicles in sheep. Follicular recruitment requires the presence of gonadotrophins, particularly FSH. In general, the mechanisms that regulate follicular selection and dominance in domestic ruminants are not well understood. Further experiments may determine the relative roles of paracrine factors and ovarian-pituitary-hypothalamic interactions in regulation of follicular selection and dominance in cattle and sheep.