Effect of dominant follicle persistence on follicular fluid oestradiol andinhibin and on oocyte maturation in heifers

The aim of the present study was to characterize in detail the cytoplasmic and nuclear morphology of cattle oocytes recovered from follicles that are dominant for more than 9 days (with low fertility after ovulation), and to relate morphological changes to intrafollicular markers of follicle health. Beef heifers received prostaglandin F-2 alpha and a synthetic progestagen (3 mg Norgestomet) for 2 or 10 days on the first day of dominance of the se cond dominant follicle (DF2) of the oestrous cycle, to give a 4 day (n = 19 ; N2) or 12 day (n = 21; N10) duration of dominance of the dominant follicl e at ovariectomy 18 h after implant removal and before the predicted gonado trophin surge. Ultrasound scanning determined emergence of a new wave of fo llicles in five N10 heifers the day before (n = Ij or day of ovariectomy (n = 4) (N10-NonDom). Dominant follicles from the remaining N10 heifers (N10- Dom) were larger (P < 0.05) on the day of ovariectomy (17.8 +/- 0.6 mm) tha n those from N2 heifers (13.6 +/- 0.4 mm). The oestradiol:progesterone rati o of follicular fluid from N10-Dom heifers was reduced (21.7 +/- 3.1 versus 34.1 +/- 4.4; P < 0.05), while inhibin A las measured by immunoradiometric assay) was increased (12.7 +/- 1.0 versus 9.0 +/- 0.7 mu g ml(-1); P < 0.0 5) compared with N2 heifers. Eleven of twelve N2 oocytes demonstrated nucle ar activation without germinal vesicle breakdown, while seven of eight N10- Dom oocytes had undergone germinal vesicle breakdown and had progressed to metaphase I (6/8) or II (1/8). Ln contrast to N2 oocytes, N10-Dom oocytes s howed a larger perivitelline space containing more cumulus cell process end ings, vacuoles, irregular vesicles, and more mitochrondia and lipid droplet s throughout the ooplasm, yet the degree of cumulus cell expansion and atre sia was similar. Thus, final oocyte maturation leading to metaphase I is in itiated in most dominant follicles with a dominance period of > 9 days befo re the gonadotrophin surge and is associated with a reduction in dominant f ollicle health. However, ovulatory ability is maintained and will lead to t he ovulation of aged oocytes, markedly reducing subsequent pregnancy rates.