BIOLOGY OF REPRODUCTION 55, 1187-1194 (1996) Minireview Selection of the Dominant Follicle in Cattle' O.J. Ginther,2 3 M.C. Wiltbank, 4 P.M. Fricke,3,4 J.R. Gibbons, 3 and K. Kot 3 Animal Health and Biomedical Sciences 3 and Department of Dairy Science, 4 University of Wisconsin-Madison, Madison, Wisconsin 53706 INTRODUCTION On the basis of gross and histologic study of ovaries, it was proposed in 1960 that two waves of follicular activity occurred during bovine estrous cycles [1]. The two-wave hypothesis was not tested and languished for more than 20 years. Results of a histologic study in 1981 [2] were con- sistent with the two-wave hypothesis. However, results of a 1983 study [3], involving measurement of follicles and steroid assays of blood and follicular fluid, led to the con- clusion that there were three follicular waves, and each re- sulted in a dominant follicle. A review of follicle turnover in cattle in 1986 [4] concluded that, rather than two or three waves, progressively larger follicles developed with rapid atresia of each until an ovulatory-sized follicle appeared. The slow progress and divergent conclusions attest to the challenge inherent in studying follicle development. A technologic breakthrough was reported in 1984 [5] and has led to clarification of the nature of bovine folli- culogenesis for follicles with antral diameters of - 3 mm. Transrectal ultrasonic imaging provided a means for re- peated, direct, noninvasive monitoring and measuring of follicles regardless of their depth within the ovary. Profiles of mean numbers of follicles for various diameter groups were bimodal during the estrous cycle [6] and early preg- nancy [7], supporting the two-wave hypothesis. The power of the technology was expanded in 1988 with reports from three laboratories [8-11] on tracking or monitoring daily diameter changes of individual follicles. One laboratory [12] found mostly (81%) two-wave estrous cycles, whereas the others [10, 11] found mostly (80%) three-wave cycles. Some of the factors found to affect the number of waves per estrous cycle include dietary intake [13], parity, and lactational status [14]. Furthermore, the diameter attained by the dominant follicle is affected by stage of the estrous cycle [12] and pregnancy [15]. Varied numbers of waves and diameters of the dominant follicle and their sensitivity to a wide array of factors are challenging aspects of this research area. In addition to clarifying the number of waves per estrous cycle, the ultrasound tracking studies have characterized the Accepted August 30, 1996. Received March 19, 1996. 'Original research supported by the College of Agricultural and Life Sciences, University of Wisconsin-Madison and USDA grant No. 9401480. 2Correspondence: Animal Health and Biomedical Sciences, University of Wisconsin-Madison, 1655 Linden Drive, Madison, WI 53706. FAX: (608) 262-7420; e-mail: ojg@ahabs.wisc.edu composition of follicular waves [16]. The onset of the first wave of an estrous cycle is detected as a group of 4-mm follicles just before the day of ovulation. During the next few days, one of the follicles becomes dominant, and the others become subordinate. A second wave emerges at about 10 days postovulation and, for three-wave cycles, is followed by another wave at 16 days. The ovulatory follicle originates from the final wave. The wave phenomenon is under intensive investigation in many laboratories; reviews or original reports with large review sections are available [12, 14, 16-20]. The interest in this area is motivated by the desire to solve the long-time mystery involving the mechanisms underlying the selection of a specific follicle for ovulation in monovular species and the need for basic information for designing synchronization and superovu- lation protocols [21]. Even with ultrasound technology to track individual follicles, resolution of the selection phe- nomenon is proving to be as elusive as the earlier docu- mentation and characterization of follicular waves. This report presents a conceptual model of the current status of knowledge on the selection phenomenon in cattle. The model is limited to the period extending from the be- ginning of a follicular wave to the early growth phase of the selected dominant follicle and does not include the fac- tors controlling the diameter achieved by the dominant fol- licle or its maintenance and regression. The model will serve as a guide for the discussion of systemic and cellular aspects of the selection phenomenon. This minireview is not exhaustive, but amalgamates the contributions of many laboratories. During development of the model, voids ap- peared because of the lack of the desired information in the literature. The missing information was obtained by rean- alyses of data that had been used for previous publications from our laboratory. The previous reports did not present the data in a manner compatible with construction of the model. The reanalyzed data were from ultrasonic scanning of 28 follicular waves every 8 h [22], and scanning of fol- licles and assay of circulating FSH every 24 h [23]. EARLY DEVELOPMENT AND DEVIATION OF FOLLICLES Time of emergence of a follicular wave is defined as the last day or examination (if more than one examination per day) the future dominant follicle was 4 mm and is desig- nated by Day 0 or Examination 0 throughout this report. The follicles are depicted in the model (Fig. 1) by circles. The smallest circles represent 3-mm follicles. The depicted number and distribution of the cohort of growing 3-mm 1187