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
When two spherical follicles of different diameters increase in diameter at the same rate, the rate of change in surface area over the same time span is greater for the larger
Ž .
follicle Ginther, 1998 ; surface area of a sphere is calculated exponentially from radius. This consideration indicates that the surface area of the largest follicle gains an
increasing advantage over the next-largest follicle, even though the difference in diameter between the two follicles is constant. It is not known if the increasing
advantage in surface area for the largest follicle plays a role in deviation. Surface area changes seem more representative of functional changes, because function involves the
cells that line the follicle. Nevertheless, this review will use diameters because of tradition.
Concentrations of FSH decline for a few days after the peak of the FSH surge in Ž
. Ž
. cattle Adams et al., 1992 and mares Bergfelt and Ginther, 1992 . The decline,
therefore, encompasses deviation. More specifically, low FSH concentrations are tempo- Ž
rally associated with deviation in cattle Ginther et al., 1997a, 1998, 1999; Kulick et al., .
Ž .
1999 and mares Gastal et al., 1997 . The concentrations continue to decline for several Ž
. days after deviation in mares Fig. 1 , whereas the decline ends within hours after
Ž .
Ž .
expected deviation in cattle Fig. 2 . In a recent study in cattle Ginther et al., 1999 , attainment of a diameter of G 8.5 mm by the largest follicle was used as a reference for
Ž .
the expected beginning of deviation Hour 0 ; FSH concentrations were determined for Hours y16, y8, and 0, every hour between Hours 0 and 16, and then every 8 h.
Concentrations decreased between Hours y16 and 0, continued to decrease until Hour Ž
. 10, and then increased after Hour 16 Fig. 2 . The results indicated a close association
between the attainment of a mean diameter of G 8.5 mm and a continued decrease in FSH concentrations for a short time thereafter. As noted earlier, a role for the low
concentrations of FSH in the deviation mechanism in both species is consistent with the findings of a delay or prevention of deviation following administration of FSH.
Ž .
Ž .
We postulated in cattle Ginther et al., 1996b; 1997a and mares Gastal et al., 1997 that the events underlying the beginning of diameter deviation are abrupt. Even when the
intervals between scanning in cattle was 8 h, the beginning of deviation was readily Ž
assigned to a specific examination in six of eight profiles of follicular waves Kulick et
Ž .
Ž . Ž .
Fig. 2. Mean SEM circulating concentrations of FSH at 8-h a and 1-h b intervals normalized to the Ž
. expected time of deviation largest follicle at G8.5 mm; Hour 0 . In the ablation group, the G8.5-mm follicle
Ž .
was ablated. The stars indicate changes P - 0.05 between hours for the indicated groups, and the pound Ž
. marks indicate a difference P - 0.05 between groups within hours. The data indicate that FSH concentra-
tions continued to decrease for 8 h after expected deviation and that FSH concentrations increased between 5 Ž
. and 8 h after ablation of the largest follicle. Adapted from Ginther et al. 1999 .
. al., 1999 . Apparently, when the largest follicle reaches a decisive developmental stage,
rapid activation of the deviation mechanism blocks the second largest follicle before it reaches a similar decisive diameter. Thus, the difference in diameter between the two
largest follicles indicates that the smaller follicle must be inhibited in - 8 h in cattle Ž
. Ž
equivalent to a diameter difference of 0.5 mm and in - 1 day in mares equivalent to a .
difference of 3 mm . Diameter deviation is likely preceded by biochemical or functional deviation. In this regard, echotextural changes were detected in the wall of the largest
Ž follicle on the day before the beginning of diameter deviation in mares Gastal et al.,
. 1999b . Thus, diameter deviation was preceded by echotextural or structural deviation.
The depression of FSH concentrations could be the pivotal event in deviation if the FSH is depressed below the quantities required by the smaller follicles, but not the
largest follicle, and the changes in FSH concentrations and follicle development are closely coupled. This last stipulation is to satisfy the postulate that deviation must occur
rapidly before the next largest follicle reaches a diameter similar to the diameter of the largest follicle at the beginning of deviation. The closeness of coupling between the two
events was investigated in heifers by ablating the largest follicle when it reached G 8.5
Ž .
mm expected beginning of deviation and, in another experiment, administering a Ž
. known FSH depressant Ginther et al., 1999 . Ablation of the largest follicle when it
Ž .
reached G 8.5 mm Hour 0
resulted in increased circulating FSH concentrations Ž
. between Hours 5 and 8 Fig. 2 . Growth rate of the retained second-largest follicle
between Hours 0 and 8 was greater in the ablation group than in the controls. A single Ž
injection of a minimal dose of an FSH depressant 4.4 ml of a near steroid-free follicular .
fluid at the expected time of deviation resulted in decreased FSH concentrations by Hour 6. Reduced growth rate of the largest follicle occurred within 6 h after the
suppression in FSH concentrations. These studies demonstrated that a close temporal coupling between a change in FSH concentrations and the follicle response could
establish the deviation mechanism in - 8 h before the second-largest follicle reaches a similar critical diameter. In conclusion, the largest follicle affected FSH concentrations
and FSH affected the follicles within the time represented by the difference in diameters between the two largest follicles at deviation.
Following administration of a near steroid-free fraction of follicular fluid, the FSH concentrations and diameters of the largest follicle were lower than the corresponding
Ž .
values in controls at the expected time of deviation Ginther et al., 1999 . These results indicated that the largest follicle in controls required the basal FSH concentrations. The
requirement for the basal FSH concentrations for continued growth of the largest follicle has also been demonstrated by administration of a single minimal dose of estradiol
Ž
. Ginther et al., 2000; Fig. 3 . Although regression of the smaller follicles at the time of
deviation can be attributed to inadequate FSH, these results indicate that the low concentrations of FSH are required for continued growth of the largest follicle. Thus,
deviation involves a change in the largest follicle so that it is sensitive to a concentration of FSH that is inadequate for the smaller follicles.
Ž .
Ablation or retention Hour 0 of the largest follicle was done at G 7.5 mm vs. G 8.5 Ž
. mm Ginther et al., 2000 . The mean FSH concentrations for the 8.5-mm groups were
greater for the ablation group than for the control group at Hours 8 and 12, but there was no difference between the 7.5-mm groups at any hour. These results supported the
Ž .
Ž . Ž .
Fig. 3. Mean SEM circulating concentrations of FSH a and diameters of the largest follicle b when Ž
estradiol injections were given when the largest follicle reached G8.5 mm expected time of deviation; Hour .
0 . There was a significant interaction between estradiol-treated and control groups for both end points. The data indicate that the low concentrations of FSH after deviation were required for continued growth of the
Ž .
dominant follicle. Adapted from Ginther et al. 2000 .
hypothesis that by the time the largest follicle reaches the expected beginning of deviation it has developed a greater capacity for suppressing FSH. In a similar ablation
Ž .
study in mares, a two-follicle model was used Gastal et al., 1997 . When the larger Ž
. follicle reached G 20 mm actual diameter, 21.2 mm; expected day of deviation; Day 0
follicle ablation was done so that both follicles, only the larger follicle, or only the smaller follicle was retained. The decline in FSH concentrations of the wave-stimulating
FSH surge continued for several days after Day 0 in the groups with both follicles or
Ž .
only the larger follicle retained Fig. 4 . In the group with only the smaller follicle retained, the FSH concentrations and diameter of the smaller follicle increased between
Days 0 and 1. The continued decrease in FSH at the expected beginning of deviation
Ž .
Ž . Ž .
Fig. 4. Mean SEM circulating concentrations of FSH a and estradiol b in mares in which two follicles Ž
were studied; the remaining follicles were ablated. When the larger follicle reached G 20 mm expected day of .
deviation , one of the follicles was ablated, resulting in groups with retention of the smaller, larger, or both follicles. In both groups with the larger follicle present, FSH continued to decrease and estradiol increased.
Ž .
None of these effects were attributable to the smaller follicle. Adapted from Gastal et al. 1999c .
was attributable to the larger follicle; there was no indication that the smaller follicle was involved. Thus, the continued FSH suppression at the expected beginning of
deviation was a function of the larger follicle. These findings support the hypothesis, in mares as in heifers, that when the largest follicle reaches a critical diameter the FSH
depression continues and thereby the next largest follicle is depressed before it reaches a similar critical diameter. Thus, two abilities develop in the future dominant follicle by
Ž . the beginning of diameter deviation: 1 ability to suppress circulating FSH to below the
Ž . concentrations required by other follicles and
2 ability to utilize the low FSH
concentrations in its further growth and development.
Fig. 5. Schematic model of the proposed functional coupling between circulating FSH concentrations and diameters of the two largest follicles during development of a follicular wave. The following numbered
Ž . statements refer to the circled numbers in the figure. 1 Emergence of a follicular wave is stimulated by an
FSH surge that reaches its peak approximately when the largest follicle of the wave is 4.0 mm. The declining Ž
. Ž . FSH concentrations continue to exert a required positive effect on the follicles solid arrows . 2 By the time
the follicles reach 5.0 mm, they develop an FSH-suppressing ability. All of the growing follicles G 5.0 mm Ž
. Ž . contribute to a decline in the FSH surge for approximately the next 2 days broken arrows . 3 By the time the
largest follicle reaches a diameter of 8.5 mm, it plays a major role in the continued FSH decline. Follicle deviation begins when the FSH concentrations are suppressed and temporarily maintained below the
Ž . concentrations required by the smaller follicles. Hence, the smaller follicles become subordinate follicles. 4
The suppressed FSH concentrations are adequate and required for at least the initial continued growth of the Ž
. Ž .
largest follicle dominant follicle after the beginning of deviation. 5 At an unknown point relative to deviation, LH begins to play a role in the continued growth of the dominant follicle. From Ginther et al.
Ž .
2000 .
It has been proposed that regression of the smaller follicles involves a direct effect of Ž
follicle inhibitors secreted by the larger follicle for review see Armstrong and Webb, .
Ž .
1997 , but the evidence for such an effect is not convincing Fortune, 1994 . There have been no reports that suggest that such inhibitors can be produced specifically by the
largest follicle at the beginning of deviation and enter the vascular system to affect other follicles. Taken together, the ablation studies in cattle and horses indicate that inhibition
of the smaller follicles during follicle deviation is attributable to continued suppression of FSH concentrations below the concentrations required by the smaller follicles, but not
the largest follicle. It does not seem necessary to invoke a follicle-to-follicle inhibitory mechanism, unless indicated by further studies. It is postulated, instead, that the essence
of the selection of a dominant follicle is a close two-way functional coupling between
Ž .
changing FSH concentrations and follicular growth and development Fig. 5 .
6. Follicular systemic inhibitors of FSH
