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 F2 alpha and a synthetic progestagen (3 mg Norgestomet) for 2 or 10 days on the first day of dominance of the second 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 follicle at ovariectomy 18 h after implant removal and before the predicted gonadotrophin surge. Ultrasound scanning determined emergence of a new wave of follicles in five N10 heifers the day before (n = 1) 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) than those from N2 heifers (13.6 +/- 0.4 mm). The oestradiol:progesterone ratio of follicular fluid from N10-Dom heifers was reduced (21.7 +/- 3.1 versus 34.1 +/- 4.4; P < 0.05), while inhibin A (as measured by immunoradiometric assay) was increased (12.7 +/- 1.0 versus 9.0 +/- 0.7 micrograms ml-1; P < 0.05) compared with N2 heifers. Eleven of twelve N2 oocytes demonstrated nuclear 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). In contrast to N2 oocytes, N10-Dom oocytes showed a larger perivitelline space containing more cumulus cell process endings, vacuoles, irregular vesicles, and more mitochrondia and lipid droplets throughout the ooplasm, yet the degree of cumulus cell expansion and atresia was similar. Thus, final oocyte maturation leading to metaphase I is initiated in most dominant follicles with a dominance period of > 9 days before the gonadotrophin surge and is associated with a reduction in dominant follicle health. However, ovulatory ability is maintained and will lead to the ovulation of aged oocytes, markedly reducing subsequent pregnancy rates.
University College Dublin ->