Relationships of changes in B-mode echotexture and colour-Doppler signals in the wall of the preovulatory follicle to changes in systemic oestradiol concentrations and the effects of human chorionic gonadotrophin in mares.

The study investigates the effects of human chorionic gonadotropin (hCG) on changes in the wall of the preovulatory follicle in mares, and its relationship with systemic estradiol concentrations. The results suggest that ultrasonographic changes in the follicle’s wall are not temporally correlated with estradiol changes, and that the latter part of an ovulatory LH surge has a negative effect on systemic estradiol.

Study Design and Procedures

• The study utilized a duplex grey-scale and colour-Doppler ultrasound to observe changes in the wall of the preovulatory follicle in mares.
• When the follicle reached a diameter of 35mm or larger (marked as hour 0), mares were divided into control and hCG-treated groups. Each group comprised 16 mares.
• The hCG treatment was administered at hour 0. The researchers scanned the mares every 12 hours until hour 36, every hour from hours 36 to 48, and every 12 hours subsequently until ovulation.
• Blood samples were taken every 12 hours for estradiol assay, a test to measure estradiol hormone levels in the blood.

Observations and Findings

• During the 0-24 hour period after hCG administration, estradiol concentrations decreased in the hCG group while increasing in the control group.
• From 0-36 hours following treatment, the thickness and echogenicity of the granulosa (a layer of the ovarian follicle) increased in the hCG group but remained unaltered in the control group.
• Between 36 and 12 hours before ovulation, granulosa and colour-Doppler endpoints increased in both groups, while estradiol levels decreased.
• The prominence and the percentage of the follicle’s circumference with an anechoic band (indicative of fluid content) decreased, along with a reduction in the colour-Doppler signals representing arterial blood flow in the follicle wall, during the last 4 to 1 hours before ovulation.

Conclusions

• The observed ultrasonographic changes in the follicle’s wall did not coincide temporally with fluctuations in estradiol concentrations.
• Just before ovulation, there was a decrease in the prominence of an anechoic band and colour-Doppler signals.
• The study’s results support the hypothesis that the later phase of the ovulatory LH surge has an inhibitory effect on systemic estradiol, demonstrated by the immediate fall in estradiol levels following hCG injection.