Ovarian antral follicular dynamics and regulation in sheep

Abstract

The main themes of the present thesis was the regulation of ovarian antral follicle growth, the manipulation of follicular dynamics and ovulation rate, as well as the characterization of the ovine corpus luteum (CL). Two treatments with ovine follicle stimulating hormone (oFSH) were used to assess the responsiveness of small antral follicles during different times in a follicular wave. Follicular dynamics were monitored by transrectal ultrasonography and serum FSH concentrations were measured. Two experiments were performed on anestrous Western White Face (WWF) ewes to independently examine whether or not the ovulations during treatment with a medroxyprogesterone acetate (MAP)-containing sponge and prostaglandin F₂ᵅ (PGF₂ᵅ), were due to the direct effects of PGF₂ᵅ on the ovary or the effects of a rapid decline in progesterone at PGF₂ᵅ-induced luteolysis. Non prolific Suffolk ewes were used to assess the effectiveness of treatment with medroxyprogesterone acetate (MAP)-containing sponge and prostaglandin F₂ᵅ (PGF₂ᵅ) to increase lambing rate. Re-introduction of rams to pre-isolated, mid-anestrous, WWF ewes was used to look at the effect of increased pulsatile secretion of LH on ovarian antral follicular dynamics at different stages of follicular wave development. We also used ovarian transrectal ultrasonography and computer assisted image analysis as non-invasive techniques to investigate whether or not there were correlations between ultrasound image attributes of the ovine CL and changing progesterone concentrations over time, in prolific and non prolific ewes. The results of the present studies showed that, in the ewe, small antral follicles can respond to the injection of FSH to yield a follicular wave more frequently than seen in a normal cycle and in the presence of a large growing antral follicle. Non induced waves can emerge during the growth phase of a wave induced by injection of oFSH. These results bring into question the presence of functional follicular dominance in the ewe. Ovulations occurred after PGF₂ᵅ injection but during continuous treatment with MAP, but those ewes experiencing a decline in serum progesterone concentrations in the presence of MAP did not ovulate any follicles. We concluded that ovulations occurring after PGF₂ᵅ injection, in the presence of a MAP sponge could be due to a direct effect of PGF₂ᵅ at the ovarian level rather than a sudden decline in circulating progesterone concentrations. Treatment of Suffolk ewes with MAP-containing sponges and injection of PGF₂ᵅ did not increase lambing rate, perhaps due to asynchrony of ovulations. Re-introduction of rams to previously isolated ewes resulted in a subtle increase in LH pulse frequency on the day of ram introduction in ewes in the static phase of a follicular wave. However, there were no consistent changes in follicular dynamics or estradiol secretion in response to this increase in LH pulse frequency. We concluded that changes in LH pulse frequency do not dramatically change ovarian antral follicular dynamics in the anestrous ewe. Both total luteal area and mean spot pixel values for the CL were correlated with the pattern of serum concentrations of progesterone from day 3 to day 15 after ovulation in WWF ewes and from day 3 to day 14 in Finn ewes. There were no significant correlations between progesterone concentrations and spot pixel heterogeneity for either WWF or Finn ewes. We concluded that pixel heterogeneity is a poor indicator of progesterone secretory ability of the CL when compared to mean pixel values. However, luteal area and mean spot pixel values are better but not strong indicators of the functional status of the CL in cyclic ewes.