Dynamics and regulation of ovarian antral follicular waves in sheep

The focus of the present thesis was on ultrasonographic, endocrine and molecular characterization of ovarian antral follicular waves in sheep. Transrectal ultrasonography and computer assisted image analysis were used to determine the feasibility of detecting ovulation and the forming corpus luteum (CL) and to non-invasively monitor CL differentiation and growth. High resolution transrectal ultrasonography and hormone measurements were used to assess changes in numbers of small ovarian antral follicles and their relationships to the emergence of follicular waves in cyclic ewes and to correlate pulsatile secretion of gonadotropins with follicular growth in a wave, during the mid to late-luteal phase of the ovine estrous cycle. A series of experiments were conducted, using treatment with injections of ovine follicle stimulating hormone (oFSH) and measurement of serum concentrations of FSH, in cyclic and anestrous sheep, to investigate the existence of follicular dominance. We also evaluated the characteristics of secretory patterns of FSH that are critical for follicular wave emergence, in anestrous ewes. The possible existence of an endogenous rhythm of FSH secretion, independent of ovarian antral follicular dynamics, was studied in ovariectomized ewes. Finally, ovarian antral follicles at defined stages of growth in a follicular wave (based on transrectal ultrasonographic observations) were collected from separate groups of sheep by ovariectomy, to profile the expression patterns of steroidogenic enzymes (3β-HSD, 17α-OH and aromatase) using immunohistochemistry and gray-scale densitometric analysis. The results of the present studies showed that it is possible to detect ovulation and visualize developing CL as early as 12-24 h after ovulation in the ewe. Changes in echotexture of the CL were closely associated with its morphological and functional characteristics, and we concluded that computer assisted image analysis holds promise for the noninvasive monitoring of CL differentiation and growth. Follicles reaching ovulatory diameter (≥ 5 mm) emerged and grew in a wave-like pattern in sheep, but without variation in the number of small follicles (1-3 mm in diameter), as seen in cattle. We concluded that all follicles that are recruited to grow beyond 2-3-mm in diameter, to 4-mm diameter in a wave, succeed in reaching an ovulatory diameter of ≥ 5 mm in the ewe. The emergence and growth of ovarian antral follicles in follicular waves, in sheep, do not require changes in LH secretion and may perhaps involve changes in the follicular sensitivity to LH. The largest follicle of a wave, in sheep, appears to have limited effects on other small follicles and on the time of emergence of the next follicular wave. Thus, functional dominance, as is present in cattle, may be absent in sheep. An endogenous rhythm for periodic peaks in serum FSH concentrations that is independent of ovarian follicular dynamics may exist in sheep. The expression patterns of steroidogenic enzymes, in the theca and granulosa compartments of antral follicles growing in each follicular wave in the ewe, paralleled serum estradiol concentrations, with the exception of the concentrations of 3β-HSD in granulosa cells, which increased continuously from follicles 3 mm in diameter to the preovulatory follicle after the LH surge. The largest follicle of any follicular wave, irrespective of the stage of the cycle, would appear to be mature enough to ovulate if a gonadotropin surge is provided.