Follicular dynamics throughout the oestrous cycle in sheep. A review

Whatever the stage of the cycle, follicles of various sizes are apparent on the surface of the ovary of sheep. No evidence of waves of follicular growth at fixed-time intervals has been found during the luteal phase, and growth probably occurs at random until the follicle attains a size of 4 to 7 mm. This shows that follicles can enlarge under a wide range of hormonal levels. The recruitment of a crop of follicles, including the one which will ovulate, occurs at variable times around luteolysis due to the interaction of endocrine and follicular factors (FSH priming, sensitizing the follicle to increased LH pulsatility). All healthy follicles greater than 2 mm in diameter are recruited. Selection of the follicle due to ovulate can be defined by morphological criteria (size) or by its "killing" ability. In all cases, the timing of follicle selection is highly variable, and it is due to this variability that the mechanisms of selection are not fully known. They are presumably intraovarian, one of the follicles actively inhibiting the growth of the others. The dominant follicle is probably maintained because of its high oestradiol content, while the others undergo atresia. The main feature of terminal follicular development is its flexibility in terms of hormonal requirements, follicle size and the timing of the main events.