P 4 is a crucial factor to negatively regulate pulsatile luteinizing hormone (LH) release from the pituitary gland and inhibits maturation and ovulation of the dominant follicle during the bovine oestrous cycle ( Kinder et al.
Increasing evidence suggests that progesterone (P 4) secreted from the corpus luteum (CL) has a regulatory effect on follicular selection and on the number of dominant follicles in cattle. Although the detailed mechanisms of follicular selection have not been completely clarified, recent studies indicate that specific changes of the intrafollicular insulin-like growth factor (IGF) system and the enhanced capacity of oestradiol (E 2) production in the future dominant follicle play a critical role in the selection of a dominant follicle accompanied by a decline of circulating follicle-stimulating hormone (FSH) towards the nadir level ( Austin et al. In cattle, deviation occurs when the largest follicle reaches an average of 8.5 mm in diameter ( Ginther et al.
Follicular selection is characterized by the appearance of differences in growth rate between a future dominant follicle and future subordinate follicles in the same cohort, termed follicular deviation ( Ginther et al. Monovular species including cattle have a restricted ovulation quota therefore, selection of the dominant follicle is the most drastic event during follicular development to decide its fertility. Our results also suggest that circulating levels of P 4 and E 2 affect pulsatile GH secretion in cattle. These results suggest that the absence of CL accompanying nadir circulating P 4 induces an enhancement of LH pulses, which involves the growth of the co-dominant follicles. After follicular deviation, the CL-absent cows showed a greater basal concentration, AUC and pulse amplitude of growth hormone (GH) than the CL-present cows. The CL-absent cows had a higher basal concentration, area under the curve (AUC), pulse amplitude and pulse frequency of LH than the CL-present cows. Oestradiol (E 2) concentrations were greater in the CL-absent cows than in the CL-present cows at day −1, day 1 and day 2 from follicular deviation. The CL-absent cattle maintained nadir circulating P 4 throughout the experimental period and showed a similar diameter between the largest and second largest follicle, resulting in co-dominant follicles. Ultrasound scanning to observe the diameter of each follicle and blood collection was performed from the day of follicular aspiration or ovulation and continued for 6 days. Luteolysis was induced in heifers and cows and they were assigned either to have the dominant follicle aspirated (CL-absent) or ovulation induced (CL-present). Using this model, the present study aimed to determine the profile of follicular development and the endocrinological environment in the absence of CL with variable nadir circulating progesterone (P 4) concentrations during the oestrous cycle in cattle.
We previously established a bovine experimental model showing that the corpus luteum (CL) does not appear following aspiration of the preovulatory follicle before the onset of LH surge.
0 kommentar(er)
