Selection of a dominant follicle, capable of ovulating, from among a cohort of similarly sized follicles is a critical transition in follicular development. The mechanisms that regulate the selection of a species-specific number of dominant follicles for ovulation are not well understood. Cattle provide a very useful animal model for studies on follicular selection and dominance. During the bovine estrous cycle, two or three sequential waves of follicular development occur, each producing a dominant follicle capable of ovulating if luteal regression occurs. Follicles are large enough to allow analysis of multiple endpoints within a single follicle, and follicular development and regression can be followed via ultrasonographic imaging. Characteristics of recruited and selected follicles, obtained at various times during the first follicular wave, have been determined in some studies, whereas dominant and subordinate follicles have been compared around the time of selection in others. As follicular recruitment proceeds, mRNA for P450 aromatase increases. By the time of morphological selection, the dominant follicle has much higher concentrations of estradiol in follicular fluid, and its granulosa cells produce more estradiol in vitro than cells from subordinate follicles. Shortly after selection, dominant follicles have higher levels of mRNAs for gonadotropin receptors and steroidogenic enzymes. It has been hypothesized that granulosa cells of the selected follicle acquire LH receptors (LHr) to allow them to increase aromatization in response to LH, as well as FSH. However, LH does not appear to stimulate estradiol production by bovine granulosa cells, and the role of LHr acquisition remains to be determined. Recent evidence suggests a key role for changes in the intrafollicular insulin-like growth factor (IGF) system in selection of the dominant follicle. When follicular fluid was sampled in vivo before morphological selection, the lowest concentration of IGF binding protein-4 (IGFBP-4) was more predictive of future dominance than size or estradiol concentration. Consistent with this finding, dominant follicles acquire an FSH-induced IGFBP-4 protease activity. Thus, a decrease in IGFBP-4, which would make more IGF available to interact with its receptors and synergize with FSH to promote follicular growth and aromatization, appears to be a critical determinant of follicular selection for dominance.

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Summary. Holstein heifers were given 5 injections (twice/day) of 10 ml charcoalextracted bovine follicular fluid (bFF; N = 6) or 10 ml saline (N = 5) beginning 12 h after the onset of oestrus. Blood samples were collected for determination of plasma concentrations of FSH, LH, progesterone and oestradiol-17\s=b\. Treatment with bFF suppressed the secondary FSH surge (P < 0\m=.\01). Cessation of bFF injections was followed by a rebound period during which FSH was elevated compared with controls (P < 0\m=.\01).Daily ultrasonographic examinations revealed that follicular growth occurred in waves, with 4 of 5 control heifers exhibiting 3 waves and the other 2 waves.In contrast, 5 of 6 bFF-treated animals exhibited 2 waves and the other 3 waves.Appearance of follicles in the first wave was delayed in bFF-treated heifers (Day 3\m=.\3\ m=+-\ 0\m=.\3 compared with Day 1\m=.\4\ m=+-\ 0\m=.\2; P < 0\m=.\0001) and appearance of the dominant follicle of the first wave was delayed (Day 4\ m=. \ 5\ m=+-\ 0\ m=. \ 3compared with Day 1\ m=. \ 8\ m=+-\ 0\ m=. \ 2; P < 0\m=.\0001). Follicles in the second wave appeared later in animals treated with bFF (Day 12\m=.\7\ m=+-\ 0\m=.\4compared with Day 10\m=.\4\ m=+-\ 0\m=.\6; P < 0\m=.\01), and the dominant follicle of this wave also appeared later (Day 13\m=.\0\ m=+-\ 0\m=.\5compared with Day 10\m=.\6\ m=+-\ 0\ m=. \ 5; P < 0\m=.\01).Oestradiol-17\s=b\ increased during the early luteal phase, but this increase occurred later in heifers treated with bFF (peak concentrations on Day 6\m=.\3\ m=+-\0\m=.\6 compared with Day 4\m=.\2\ m=+-\0\m=.\2;P < 0\m=.\05).LH, progesterone and cycle length were not affected by bFF. Delayed follicular growth associated with suppression of FSH suggests that the secondary FSH surge is important in the initiation of follicular development early in the bovine oestrous cycle, and thus may play a role in the regulation of ovarian follicular dynamics.

Steroidogenic acute regulatory protein (StAR), proposed to be involved in the transport of cholesterol to the inner mitochondrial membrane, has recently been cloned from MA-10 cells. Using reverse transcription-polymerase chain reaction, we generated a complementary DNA encoding 404 base pairs of StAR from ovine luteal tissue to perform studies regarding regulation of the messenger RNA (mRNA) encoding this protein. In Exp 1, ewes were hypophysectomized (HPX) on day 5 of the estrous cycle and administered saline or physiological regimens of LH and/or GH until collection of luteal tissue on day 12 of the estrous cycle (n = 4/group). Luteal concentrations [mean +/- SEM; femtomoles per microgram poly(A)+ RNA] of mRNA encoding StAR were lower (P < 0.05) in the HPX plus saline-treated ewes (26.4 +/- 7.3) than in day 12 pituitary-intact ewes (n = 4; 77.7 +/- 9.3). Replacement of LH (59.1 +/- 13.1), GH (59.1 +/- 12.8), or LH and GH (69.9 +/- 4.5) in HPX ewes increased (P < 0.05) concentrations of mRNA encoding StAR to values not different from those in day 12 controls. In Exp 2, ewes on day 11 or 12 of the estrous cycle were injected with prostaglandin F2 alpha (PGF2 alpha) to induce luteal regression. Corpora lutea were collected 4, 12, or 24 h after injection (n = 4-5/time point) and from untreated control ewes (n = 4) or 24 h after injection of saline (n = 4). Treatment with PGF2 alpha decreased (P < 0.05) concentrations of progesterone in serum 4, 12, and 24 h after injection. Concentrations of StAR mRNA were decreased (P < 0.01) to 47%, 19%, and 8% of control values 4, 12, and 24 h after PGF2 alpha injection, respectively. In Exp 3, ewes received ovarian arterial infusions of saline, PGF2 alpha, or phorbol 12-myristate 13-acetate (PMA), and luteal tissue was collected 0 (no infusion), 4, 12, or 24 h later (n = 3-4/group). Treatment with PGF2 alpha or PMA decreased (P < 0.05) concentrations of progesterone in serum 4, 12, and 24 h postinjection. Steady state concentrations of mRNA encoding StAR (P < 0.05) were 36% and 25% of the control value 12 and 24 h after PGF2 alpha injection. Injection of PMA decreased (P < 0.05) concentrations of StAR mRNA to 75% and 50% of control values at 4 and 12 h, but concentrations of mRNA encoding StAR were not different from control values at 24 h.(ABSTRACT TRUNCATED AT 400 WORDS)