Ž follicles of different diameters in cattle at different reproductive stages Martin et al.,
. 1991; Ireland et al., 1994; Sunderland et al., 1996; Mihm et al., 1997 . The results are
complex, resulting from the subunit make-up and the wide array of forms with different molecular weights and the manner in which the subunits converge to form structurally
similar, but functionally different, compounds. Administration of an inhibin antiserum
Ž .
increases plasma FSH Glencross et al., 1994 and increases the number of 8 mm or G 10 mm follicles during a follicular wave and the number of ovulations in cattle
Ž .
Kaneko et al., 1993; Glencross et al., 1994; Hillard et al., 1995 and the number of Ž
. ovulations in mares McKinnon et al., 1992; McCue et al., 1993; Nambo et al., 1998 .
These results with anti-inhibin may reflect an interference with follicle deviation, indicating that inhibin is needed for the deviation process; however, this has not been
studied directly. Immunoreactive inhibin concentrations increase in the circulation at the
Ž time FSH concentrations decline in mares Bergfelt et al., 1991b; Nagamine et al.,
. 1998 . This occurs at the reported time of development of the ovulatory follicular wave.
These results with inhibin assays and inhibin antisera suggest that inhibin or other proteinaceous follicular factors may be involved in the deviation mechanism through
depression of circulating FSH.
It has been proposed that the decline in FSH plays a role in stimulating the follicle to Ž
. synthesize inhibin, as well as estradiol and growth factors Mihm et al., 1997 . It appears
that the declining concentrations of FSH and the developmental stage reached by the largest follicle interact to initiate deviation before other follicles reach the critical
diameter. The initiation involves a further release of FSH suppressants, which further depresses FSH below the requirements of the smaller follicles. As noted earlier, a
treatment at the time of expected deviation with a minimal dose of either estradiol or a near estrogen-free fraction of follicular fluid reduced the FSH concentrations to a level
below the concentrations found in controls. These findings and the temporal relation- ships, discussed above indicate that both estradiol and inhibin are candidates for the role
of FSH suppressants during diameter deviation.
7. Role of LH
Several studies have suggested that LH is utilized as a gonadotropin stimulant by the Ž
selected dominant follicle for reviews see Goodman and Hodgen, 1983; Gong et al., .
1996; Ginther et al., 1996b . In cattle, a change in emphasis to LH dependency so that continued development of the dominant follicle is driven at least partly by LH is
Ž . consistent with the following results: 1 Follicles did not grow beyond 7–9 mm when
Ž . Ž .
LH was suppressed Gong et al., 1995, 1996 . 2 The growth phase of the dominant follicle was associated with higher-frequency LH pulses than for the plateau phase
Ž . Ž .
Rhodes et al., 1995 . 3 The life span of the dominant follicle can be extended by Ž
. Ž . increasing the LH-pulse frequency Fortune et al., 1991; Savio et al., 1993 . 4 The
largest follicle acquires LH receptors or gene expression for LH receptors between 2 and Ž
4 days after wave emergence Xu et al., 1995; Bodensteiner et al., 1996b; Evans and .
Ž . Fortune, 1997 , which encompasses the reported day of the beginning of deviation. 5
Lactating cows on a low-energy diet had a lower LH-pulse frequency, and the diameter
Ž . Ž .
of the largest follicle was less than in controls Grimard et al., 1995 . 6 After the 90th day of pregnancy, there was both a transitional decrease in diameter of the largest
Ž .
follicle of successive waves Ginther et al., 1996a
and a decrease in LH-pulse Ž
. frequency Schallenberger et al., 1985 , indicating the two events are temporally related.
Ž . Ž
7 LH concentrations increased to a plateau before deviation, remained elevated no .
Ž significant changes until after deviation, and then decreased Ginther et al., 1998, 1999;
. Kulick et al., 1999 , indicating that elevated LH as well as reduced FSH concentrations
encompass the time of deviation. However, whether the dominant follicle utilizes LH in cattle in association with diameter deviation is not known.
In mares, LH receptors were higher in the theca when the mean diameter of the Ž
. largest follicle was 29 mm Fay and Douglas, 1987 ; this would presumably be after
deviation. Recently, values for LH receptors, a-inhibin, and aromatase were reported to Ž
be lower in the granulosa of follicles 5–9 mm than in larger follicles Goudet et al., .
1999 , but this would be well before deviation. The status of LH receptors and other factors in follicles before, during, and after deviation needs study in mares. An increase
Ž .
in circulating LH concentrations occurs before deviation Gastal et al., 1997, 1999c . In some mares, the increase continued as part of the prolonged ovulatory LH surge in this
species, but in others, a plateau encompassed deviation or a transient decrease occurred between deviation and ovulation. Thus, in mares, as well as cattle, elevated circulating
Ž .
LH is available during deviation. In recent studies in ponies Gastal et al., 1999a,e , LH concentrations were manipulated by daily treatment with various doses of progesterone
beginning before the emergence of the follicular wave. A dose that did not alter FSH concentrations did affect LH. After an LH increase in both treated and control mares, a
decrease occurred in the treated group 1 or 2 days before deviation in the controls; the reduced levels were similar to pretreatment levels. The decrease in the progesterone
group was associated to reduced diameter of the largest follicle within 2 days after deviation in the controls. However, the reduced LH did not affect the second-largest
follicle. Thus, the onset of deviation, as assessed by the second-largest follicle, was not delayed by the reduced LH, but the post-deviation growth of the largest follicle was
reduced. Although not conclusive, these results indicate that LH was not involved in the
Ž .
initiation of follicle deviation inhibition of the other follicles through FSH depression in mares, but was required for continued growth of the largest follicle after the
beginning of deviation.
8. Intrafollicular facilitators of gonadotropins
