AbstractsBiology & Animal Science

In the ewe, one to three antral follicles emerge or grow from a pool of small antral follicles (1 to 3 mm in diameter) every 3 to 5 days and reach diameters of ¡Ý5 mm before regression or ovulation. Each follicular wave is triggered by a peak in serum concentrations of FSH. It is not clear what characteristics of an FSH peak cause follicular wave emergence and what aspects of development of a follicular wave are regulated by its preceding FSH peak. In Experiment 1, we found that the amplitude of FSH peaks decreased, while basal serum FSH concentrations increased across the inter-ovulatory interval (P < 0.05). However, there were no associated changes in the growth, static or regression phases of follicular waves or the number and size of follicles in a wave. In Experiment 2, using computer-assisted quantitative echotextural analysis, we found that the numerical pixel value (NPV) for the wall of anovulatory follicles emerging in the third wave of the cycle was significantly higher than for waves 1 and 2 at the time of wave emergence but it decreased as follicles reached maximum follicular diameter (P < 0.05). A tendency for a similar pattern for the wall of follicles in the last wave of the cycle was also observed (P = 0.07). In Experiment 3, treatment with ovine FSH (oFSH) increased the amplitude of an FSH peak by 5 to 6 fold. This treatment increased estradiol production (P < 0.05) but had little effect on other characteristics of the subsequent follicular wave. Daily injections of oFSH (Experiment 4) for four days, resulted in the occurrence of 4 discrete peaks in serum FSH concentrations. Each injection of oFSH resulted in the emergence of a new follicular wave. In Experiment 5, six cyclic ewes received oFSH (0.1 ¦Ìg/kg, sc) every 6 h for 42 h, to try to give a gradual increase in the leading slope of an FSH peak. Serum FSH concentrations increased in oFSH treated ewes (P < 0.05) resulting in an additional peak between two endogenously driven FSH peaks and therefore, did not give the planned gradual leading slope to an FSH peak. Ovine FSH treatment occurred in the early growth phase of wave 1 of the inter-ovulatory interval and increased the growth rate of growing follicles in that wave, compared to control ewes (P < 0.05). This apparently induced dominance in follicles in wave 1, causing them to suppress wave emergence in response to the injected FSH. In Experiment 6, oFSH was infused constantly (1.98 ¦Ìg/ewe/h, iv, n = 6) for 60 h. Infusion of oFSH maintained serum FSH concentrations at a level similar to the zenith of a peak. This resulted in a superstimulatory effect with a peak in the mean number of large follicles on Day 2 after the start of FSH infusion (P < 0.001). A hormonal milieu similar to low serum progesterone concentrations was created by treatment of ewes with prostaglandin and medroxyprogesterone acetate (MAP) sponges (Experiment 7). This treatment delayed regression of the penultimate follicular wave of a cycle. However, the delayed follicular atresia was accompanied by a greater degree of…