Stimulation of waves by FSH surges

several days before the largest follicle reached maximum diameter. Functional selection against nonovulatory follicles approximately 5 days before ovulation was confirmed by Ž a reduced response to a superovulatory gonadotropin regimen Pierson and Ginther, . Ž 1988a . Ultrasonic tracking of individual follicles from day-to-day Fortune et al., 1988; Pierson and Ginther, 1988b; Savio et al., 1988; Sirois and Fortune, 1988; Knopf et al., . 1989 confirmed these interpretations and provided additional and more detailed charac- terization. In various herds, a predominance of either two-wave or three-wave estrous cycles was found, accounting for earlier reports of two versus three waves. Each wave is characterized by emergence of a group of follicles at 4 mm, growth of all follicles for a few days, and then dissociation into a large follicle that continues to grow and smaller follicles that regress. The anovulatory wave which begins as 4-mm follicles during the periovulatory period will be featured in this report because it has the most consistent characteristics and has been most extensively studied. In the earlier studies, the follicles Ž . Ž . were defined as largest F1 and second largest F2 , followed by the terminology Ž . Ž dominant Goodman and Hodgen, 1983 , dominant and nondominant Ireland and . Ž Roche, 1987; Sirois and Fortune, 1988 , or dominant and secondary Ireland and Roche, . 1987; Savio et al., 1988 . Thereafter, the terms dominant and subordinate follicles have been used most frequently. 2.2. Mares Ž The types of follicular waves that develop in mares are major waves characterized . Ž by dominant and subordinate follicles and minor waves largest follicle does not attain . the diameter of a dominant follicle . Based on transrectal palpation, a single major Ž . follicular wave was proposed initially for the equine estrous cycle Ginther, 1979 . The wave of follicles dissociated about 6 days before ovulation into a single growing preovulatory follicle and several regressing follicles. The palpation work was subse- quently substantiated by ultrasound, based on grouping of follicles into diameter Ž . categories Palmer, 1987; Pierson and Ginther, 1987a and tracking of individual Ž . follicles Sirois et al., 1989; Ginther, 1990 . There are profound breed differences in Ž . wave patterns during the estrous cycle for review see Ginther, 1992 . In some breeds Ž . quarter horses, ponies , usually only one major wave develops in late diestrus and Ž . culminates in the estrous ovulation. In other breeds thoroughbreds , a secondary major wave frequently develops in early diestrus, and the dominant follicle may be anovula- tory, as in cattle, or ovulatory. The secondary-wave phenomenon accounts for the earlier Ž . intriguing discovery Hughes et al., 1972 of diestrous ovulations. Minor follicular Ž waves have been demonstrated statistically in mares Ginther, 1993; Ginther and . Bergfelt, 1992 . The selection phenomenon of the major ovulatory wave that begins at midcycle for all breeds will be used for this report and will be compared to the selection aspects of the anovulatory wave that begins near ovulation in cattle.

3. Stimulation of waves by FSH surges

Surges of FSH stimulate the occurrence of waves during several reproductive statuses in addition to the estrous cycle and postpartum period. In cattle, FSH stimulation of Ž . Ž major waves occurs in calves Evans et al., 1994 , during much of pregnancy Ginther et . Ž . al., 1996a , and during prolonged progesterone administration Bergfelt et al., 1991a . Also, in mares, the major and minor waves develop during various status levels Ž . Bergfelt and Ginther, 1992; Ginther, 1993; Ginther and Bergfelt, 1992 . The occurrence of follicular waves during many diversified hormonal environments attests to the robustness of the follicle-selection phenomenon and is a consideration in the develop- ment of hypotheses on controlling mechanisms. Emergence of follicular waves refers to the earliest ultrasonic detection of follicles compatible with retrospective tracking. Emergence of each wave is temporally associ- Ž ated with an FSH surge for the major waves in cattle Adams et al., 1992; Sunderland et . Ž al., 1994; Gong et al., 1995; Evans et al., 1997 and horses Palmer, 1987; Ginther and . Bergfelt, 1992; Bergfelt and Ginther, 1993; Fig. 1 and for the minor waves in horses Ž . Ginther, 1993; Ginther and Bergfelt, 1992 . The FSH surge reaches a peak or plateau Ž when the largest follicle reaches about 4–5 mm in cattle Bodensteiner et al., 1996a; . Ž . Kulick et al., 1999 and about 13 mm in mares Gastal et al., 1997 . The concentrations then decline. In cattle, 3-mm follicles did not suppress FSH, but acquired this capability Ž . during their growth to 5 mm Gibbons et al., 1999 . More than one of the growing follicles contributes to the FSH decline as indicated by the rate of decline when all Ž follicles, all but one follicle, all but two follicles, or no follicles were ablated Gibbons . et al., 1997 ; FSH declined more slowly when fewer follicles were retained. Circulating FSH continues to be needed by the growing follicles in heifers, even when the FSH concentrations are decreasing during the declining portion of the FSH Ž . surge Ginther et al., 2000 . A minimal dose of estradiol was used to decrease FSH concentrations without an associated change in LH concentrations. Estradiol treatment Ž . Fig. 1. Mean SEM day-to-day diameter of follicles and circulating concentrations of FSH in 14 mares. Ž . Normalizing the data to the day of deviation b produced a sharper dissociation between follicles than when Ž . data were normalized to the day of emergence of the future dominant follicle a . A star indicates the first Ž . Ž . difference P - 0.05 in the increases or decreases in concentrations. Adapted from Gastal et al. 1997 . when the largest follicle reached G 6.0 mm resulted in depression of both FSH concentrations and diameter of the largest follicle within 8 h. The smaller follicles were also inhibited. Thus, there is a close two-way functional coupling between FSH and the follicles during the declining portion of the FSH surge. The growing follicles cause the FSH decline and, even though decreasing in concentrations, the FSH remains essential for the growing follicles.

4. Follicle selection and deviation