Effects of Follicular Aspiration on the Bovine Oestrous Cycle

Increasing numbers of reports in the past eight years have described oocyte recovery from cattle. Most of these studies focused on the oocyte recovery rate and the quality of the harvested cumulus oocyte complexes. The present thesis deals with the impact of the ultrasound guided follicular aspiration on the hormonal profiles of the cow as well as with the effects of the applied technique on the follicular dynamics and the fate of the aspirated follicles. Once weekly follicular aspiration did not extend the interoestrous intervals of the cows, provided the last aspiration was not performed after day 18 of the oestrous cycle. In the present study the overall oocyte recovery rate (30.5%) was comparable with the average found in the literature. An increased number of follicles emerged after aspiration regardless of the stage of the cycle. Plasma oestradiol concentration decreased dramatically soon after aspiration and was maintained at low levels dependent on the stage of the oestrous cycle. Ablation of all ultrasonically visible follicles resulted in a small increase in plasma LH concentration regardless of the stage of the oestrous cycle, but this increase was not statistically significant from either the pre-aspiration concentrations or from that measured in the control animals. Selective aspiration of the dominant follicle resulted in a significant decrease in plasma oestradiol concentration, and a significant increase in plasma FSH concentration at all three stages of the luteal phase. However, ablation of the dominant follicle did not appear to affect LH secretion very significantly. Repeated aspirations during the early lineal phase resulted in the formation of an accessory luteal structure, within seven days of aspiration. The accessory structure had the macroscopical and histological appearance typical of a corpus luteum and contained higher amounts of oestradiol and lower amounts of progesterone than the cyclic corpus luteum. Furthermore, aromatase activity and FSH receptors were detectable only in the accessory structure. The methodology used in the present research offers an alternative model for study of a: the interactions between ovarian steroids and the pituitary gonadotrophins, and b: the role of the dominant follicle at different stages of the oestrous cycle. Appearance of an accessory luteal structure, although of very low incidence, indicates that under a certain hormonal milieu follicular rupture is sufficient to induce luteinisation without an LH peak