PARENT SESSION
Follicular Development

CHARACTERISTICS OF EARLY PROLONGED DOMINANT FOLLICLES IN CATTLE.

Bridges, P¹, Fortune, J¹, ¹

ABSTRACT- Exposure of bovine dominant follicles to sub-luteal concentrations of progesterone extends follicular growth and dominance. Follicles become persistent, oocyte quality is impaired and fertility is compromised. Prolonged dominance is accompanied by increased LH pulse frequency, elevated plasma estradiol and premature luteinization (BOR 58:1241-49, 1998). We hypothesized that premature luteinization occurs only after extended follicular persistence and that follicles in initial stages of prolonged dominance resemble follicular phase (FP) follicles. Early prolonged dominant (PD) follicles were induced by providing sub-luteal concentrations of exogenous progesterone after luteal regression initiated on Day 6 (pm) of the estrous cycle. Luteal phase (LP), FP (luteolysis initiated on Day 6, pm) and PD follicles were collected by ovariectomy (n = 4/group) on Day 8 (am). Size and growth rate of follicles from LP, FP and PD groups did not differ. Plasma progesterone in PD heifers was intermediate between LP and FP heifers (P < 0.01) by Day 7 (am), confirming induction of early PD follicles. LH pulse frequency was assessed in jugular samples collected every 12 min for 6 h on Day 7 (pm) and was greater (P < 0.05) in FP and PD than LP groups. Estradiol (P < 0.01) and androstenedione (P < 0.01) were higher in follicular fluid from FP and PD than LP follicles (estradiol = 1310 ± 128, 911 ± 186 and 74 ± 9; androstenedione = 186 ± 53, 89 ± 38 and 3 ± 0.3 ng/ml in FP, PD and LP follicles, respectively); whereas progesterone did not differ among groups. Isolated granulosa and theca cells were cultured for 3 h and steroidogenic response to gonadotropins assessed. Secretion of estradiol by granulosa cells was not affected by treatment with gonadotropins and was greater in FP and PD than LP groups (P < 0.05), indicating increased aromatase activity. Secretion of progesterone by granulosa cells in response to LH or FSH (128 ng/ml) was similar among groups. Secretion of androstenedione by theca cells was similar among groups in the presence or absence of LH; progesterone was not detectable. In conclusion, sub-luteal progesterone rapidly increased LH pulse frequency and estradiol production. Prospective PD follicles resembled follicular phase dominant follicles, and premature luteinization was not evident at this early stage of prolonged dominance.

KEY WORDS: follicular development, gonadal function, gonadal steroids, fertility