Regulation of ovarian follicular dynamics in farm animals. Implications for manipulation of reproduction

In this review, the main features of folliculogenesis are summarized and co mpared among species. In the past few years, ultrasonography has clarified follicle growth patterns, and our understanding of follicle maturation has improved considerably. As the follicles develop towards the ovulatory stage , three features appear to be highly conserved across all species: 1) the s equence of events (recruitment, selection and dominance); 2) the sequential need for gonadotropins (FSH for recruitment, LH for dominance) and 3) the large variability of numerical parameters (number of waves per cycle, numbe r of follicles per wave) as well as temporal requirements (time of selectio n, duration of dominance). In addition, specific follicles may also have va riable gonadotropin requirements (thresholds). When patterns of follicle de velopment at different physiological states are compared across species, fo llicular waves were detected in cattle, sheep and horses and during the pre pubertal period in swine, suggesting that ovaries of all species operate on a wave basis unless they are prevented from doing so. Efficient estrus control treatments should have the ability to affect 1) th e wave pattern by preventing the development of persistent dominant follicl es containing aging oocytes, and 2) the recruitment of the future ovulatory follicle whatever the stage of the wave at the time of treatment. This wou ld allow synchronous ovulation of a growing dominant follicle. Manipulation of the luteal phase follicular waves after mating or AI may also optimize fertility. Superovulation is still an efficient technique to obtain progeny from genet ically valuable females. Administration of exogenous gonadotropins acts to reveal the underlying ovarian variability. Ovarian response of each female depends on the number of gonado-sensitive follicles present at the time whe n treatment is initiated. Identification of the number of such follicles fo r each female would improve efficacy of superovulation, by allocating poten tial nonresponders to other techniques (OPU/FIV). One of the main component s of the within female response to superovulation is the stage of the wave when gonadotropins are injected. Treatment in the absence of a dominant fol licle ensures a response close to the female's specific maximum. The develo pment of practical approaches to achieve this still requires further resear ch. (C) 2001 by Elsevier Science Inc.