PATTERNS OF FOLLICULAR DEVELOPMENT DURING THE ESTROUS-CYCLE IN MONOVULAR MERINO-DEL-PAIS EWES

Growth and regression of ovarian follicles with antral diameters great er than or equal to 2 mm were characterized during 15 estrous cycles b y daily transrectal ultrasonography (7.5 MHz probe) in 9 ewes of Merin o del Pais, a consistently monovular Spanish breed. Mean interovulator y interval was 17.5 +/- 0.5 days and ovulation rate was 1 in all ewes; of 60 to 116 follicles, greater than or equal to 2 mm observed during the entire estrous cycle, 13.0 +/- 1.2 reached a maximum diameter gre ater than or equal to 4 mm and 7.9 +/- 0.6 different follicles became the largest follicle in the animal at some point during the cycle. An average of 4.5 new follicles per ewe were detected each day, with no s ignificant effect of day of cycle. Appearance of new follicles that gr ew to greater than or equal to 4 mm tended to differ during the first 8 days of the cycle, being highest on day 3 and lowest on day 6 (P < 0 .10), but did not vary significantly during the last 6 days. Growth of new follicles from the day of-detection to the next day differed betw een, but not within, periods, averaging 1.4 +/- ,0.3 mm of the first 8 days of the cycle and 1.8 +/- 0.5 mm from day -6 through -1 (P < 0.05 ). Total number of follicles greater than or equal to 2 mm per ovary o n days 1 through 8 varied with the interaction of ovary by day, being more variable in the non-CL ovary. During the last 7 days, a linear de cline in total follicles was coupled with a linear increase in number of large follicles (P < 0.05). Differences in the size between the lar gest and second largest follicles were greater on days 5 through 8 tha n on days 1 through 4, did not differ with day of cycle on days -6 thr ough -1, then increased on the last day from 1.5 mm to 2.9 mm (P < 0.0 01). In conclusion, the monovular Merino del Pais ewe showed a more ra pid growth and turnover of ovarian follicles than other breeds studied , but identified 3-mm follicles did not emerge in other than a random distribution. There was little evidence of dominance until the ovulato ry follicle had been identified. (C) 1997 Elsevier Science B.V.