Patterns of Follicular Growth During the Four-Day Estrous Cycle of the Rat 1

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Abstract
Female Sprague-Dawley rats underwent laparotomy during metestrus at 70-75 days of age or remained untreated to study the effects of surgical stress on follicular growth. Groups of rats were killed on each day of a 4-day estrous cycle, serial sections of the ovaries were prepared histologically and the number and size of follicles with one or more complete layers of cuboidal granulosa cells were determined. Since no differences due to surgery were found, the data were pooled by day of the estrous cycle (17 or 18 rats/day of cycle) for characterization and comparison of size distribution of follicles on different days of the estrous cycle. Follicles were classified as atretic or healthy and divided into groups by increments of 20 .mu.m of diameter for graphing. Data were analyzed by analysis of variance and least squares means. Significant differences were found in the distribution of both healthy and atretic follicles among days of the estrous cycle. At least 21 follicles/ovary were recruited from < 260 .mu.m into > 250 .mu.m in diameter between proestrus and estrus, and the follicles for ovulation were selected by diestrus. A greater number of growing follicles of 70-100 .mu.m in diameter were present at diestrus. From the disappearance of follicles > 260 .mu.m between estrus and proestrus, it appears that atresia is a very rapid process.

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