Comparative patterns of follicle development and selection in ruminants.

Expanding technological capabilities, particularly in ultrasonography and molecular endocrinology, have bridged the gap between form and function of the ovary, and have been a catalyst for intense research activity in this area during the last decade. However, the study of follicular dynamics is still in its infancy in ruminant species other than cattle, and controversy persists regarding the pattern of follicular growth and the existence of follicular dominance. The bovine model of ovarian function is presented as a foundation for concepts surrounding the control of follicular development in ruminants, and to place in context the results of recent studies in sheep, goats, muskoxen, cervids and camelids. This comparative approach is used to determine important generalities that appear to be applicable, as fundamental physiological phenomena, to all ruminant species. Although clear differences in follicular dynamics are evident, differences appear to be specific rather than general, and the following conclusions are consistent with results reported in ruminant species to date: (1) follicles grow in a wave-like fashion; (2) periodic surges in circulating FSH are associated with follicular wave emergence; (3) selection of a dominant follicle involves a decline in FSH and acquisition of LH responsiveness; (4) periodic anovulatory follicular waves continue to emerge until occurrence of an LH surge (that is, at the time of luteolysis during the ovulatory season or during transition from the anovulatory season); (5) within species, there is a positive relationship between the duration of the oestrous cycle and the number of follicular waves; (6) progesterone suppresses LH secretion and growth of the dominant follicle; (7) the duration of the interwave interval is a function of follicular dominance, and is negatively correlated with circulating FSH; (8) follicular dominance in all species is more pronounced during the first and last follicular waves of the oestrous cycle; and (9) pregnancy, the prepubertal period and seasonal anoestrus are characterized by regular, periodic surges in FSH and emergence of anovulatory follicular waves.