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
Selection is a general term used for monovular species to indicate that usually only one follicle of a follicular wave reaches dominant status, especially as indicated by
ovulation. More broadly, the term could be used for multiovular species when the number of follicles of a wave exceeds the number of ovulations. This does not imply
that the mechanism of selection is similar in both monovular and multiovular species. However, the occasional occurrence of multiovulations in monovular species and the
frequent occurrence in other species could be useful for studying the nature of selection
Ž .
of one follicle Wiltbank et al., 2000 . There is no consensus on use of the term selection for a specific point during follicle growth, and the term is confusing when used to
Ž discuss time of occurrence. In the earlier ultrasound studies in cattle Ginther et al.,
. Ž
. 1989 and mares Bergfelt and Ginther, 1993 , means for the future dominant and largest
subordinate follicles gradually diverged in diameter beginning on the day of follicle emergence. This gave the impression that the dissociation into dominant and subordinate
follicles was a gradual event beginning at emergence. Later inspection of individual follicle-diameter profiles indicated that dissociation was often an abrupt event and was
Ž .
Ž .
termed deviation in cattle Ginther et al., 1996a, 1997a and mares Gastal et al., 1997 . The gradual mean divergence was attributable to the occurrence of deviation at different
Ž .
times among waves Fig. 1 . Deviation is characterized by continued growth of the largest follicle to become the
dominant follicle and a reduction or cessation of growth by the remaining follicles to become subordinate follicles. Diameter deviation is retrospectively judged to have begun
at the ultrasound examination preceding the first examination with an apparent change in diameter difference between the two largest follicles. For some waves, the time of
deviation may not be obvious, especially because the subordinate follicles may continue to grow for 1 day, but at a reduced rate. Averaged over several reports, the mean
diameters of the two largest follicles in cattle at the beginning of deviation were 8.5 and 7.7 mm with deviation beginning a mean of 2.5 days after emergence of the largest
Ž .
follicle at 4 mm Ginther et al., 1997a, 1998, 1999; Kulick et al., 1999 . The
corresponding values in mares were 22 and 19 mm and 6.2 days after emergence of a Ž
. 6-mm follicle Gastal et al., 1997, 1999a,c,d .
The future dominant follicle emerges earlier, on average, than the other follicles of Ž
the wave. In cattle, the future dominant follicle emerged at 3 mm a mean of 6 h Ginther .
Ž .
et al., 1997a or at 4 mm a mean of 7 h Kulick et al., 1999 earlier than the future Ž
largest subordinate follicle. The follicles grow in parallel, on average Ginther et al., .
1997a; Kulick et al., 1999 , so that the largest follicle maintains about a 0.5-mm-diame-
Ž .
ter advantage until deviation Kulick et al., 1999 . Similarly in mares, on average, the future dominant follicle reached 6 mm before the future largest subordinate follicle and
Ž maintained a mean diameter advantage of 3 mm until the beginning of deviation Gastal
. et al., 1997 .
Although none of several experiments have statistically detected a mean difference in diameter growth rates between the two largest follicles from emergence to deviation,
considerable variation occurs among individual waves, presumably reflecting biologic variation as well as measuring error. Error can be considerable, especially when
distortion of the ultrasound image of the follicle occurs because the face of the transducer is not optimally oriented relative to the surface of the follicle or the walls of
Ž .
the follicle are obscured by artifacts Ginther, 1995 . In heifers, the future dominant Ž
follicle was the largest follicle 2 days after wave emergence at 4 mm Bodensteiner et .
Ž .
al., 1996a or 4–5 mm Evans and Fortune, 1997 in 82 and 79 of follicular waves, respectively. Examples of the occasional occurrence of the largest follicle reaching a
diameter equivalent to the expected diameter at deviation and then ceasing to grow have Ž
. Ž
. been published for cattle Ginther et al., 1996b and mares Gastal et al., 1997 .
Because of the diameter advantage of the largest follicle, it could be concluded that selection occurs or begins before ultrasonic detection of emerging follicles. With this
reasoning, however, the beginning of selection would not be definable, since other factors may determine which of the underlying follicles becomes sensitive to an FSH
surge; the largest follicle in a range of responsive diameters may be designated by chance. A further consideration against use of the term selection before deviation is that
the future nondominant follicles retain their capacity to become dominant until after deviation; that is, they have not been selected against. In cattle, many of the growing
follicles are capable of becoming dominant; a randomly selected 5-mm follicle can be
Ž directed toward dominance by destroying other 5-mm follicles as they appear Gibbons
. Ž
. Ž
. et al., 1997 . In both heifers Ginther et al., 1999 and mares Gastal et al., 1999c , the
second-largest follicle becomes dominant when the largest follicle is ablated at the expected beginning of deviation. Furthermore, initiation of FSH treatment early in a
Ž wave resulted in a delay in the apparent equivalent of deviation in cattle Adams et al.,
. 1993; Mihm et al., 1997 and development of dominance by several follicles in horses
Ž .
Squires et al., 1986; Rosas et al., 1998 , demonstrating the pre-deviation capabilities of follicles. It appears that the terms deviation and selection can be used synonymously, but
to minimize confusion, deviation will be used in the remainder of this report to assure focusing on the relatively narrow time span where the future dominant follicle and
future largest subordinate follicle begin to differ in growth rates in individual follicular waves.
5. Control of deviation
