Echotextural changes in the follicular wall during follicle deviation in mares

The echotextural changes occurring in the follicular wall in association with deviation in diameters were studied in 8 pony mares. Echotextural changes could be useful as a reference point for future studies of the follicle-selection phenomenon. Follicles were examined daily by transrectal ultrasound from 3 d before to 3 d after the beginning of deviation (Day 0). The following echotextural end points were recorded based on a scoring or percentage system: 1) thickness of granulosa, 2) echogenicity of granulosa, 3) prominence of an anechoic layer located beneath the granulosa, and 4) extent of involvement of the circumference of the follicle by the anechoic layer. Significant main effects indicated increasing values over Days -3 to 3 and a higher mean value for the dominant-follicle status for each end point. The interaction approached significance for thickness of granulosa and was significant for each of the other 3 end points. The interaction seemed due primarily to lower values in the subordinate follicle on Days 2 and 3. The first day of an overall echotextural difference between the 2 largest follicles (15.9 +/- 0.5 d after ovulation) or between days within the largest follicle (16.1 +/- 0.5 d) was about 1 d earlier (P < 0.05) than the beginning of diameter deviation (17.4 +/- 0.8). The effects of follicle diameter versus future follicle status were compared for Days -3 to 0 by grouping follicles for each status into 2 diameter categories (16.0 to 18.9 mm and 19.0 to 22.5 mm). Increasing echotextural values for the granulosa (thickness and echogenicity) were more attributable to follicle growth, whereas the increasing values for the anechoic layer were more attributable to status as a future dominant follicle. Results indicated that development of an anechoic layer beneath the granulosa distinguished the future dominant follicle from the largest future subordinate follicle before the beginning of diameter deviation.