Relationships Between Follicular Size and Antral Fluid Steroid Concentrations at Three Stages of the Estrous Cycle in the Ewe

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Abstract
The relationship between follicular size and antral fluid (AF) concentrations of estradiol [E2], testosterone, and progesterone at 3 stages of the sheep estrous cycle was studied. Ewes were ovariectomized (OVX) during the luteal phase (group 1), and either prior to (group 2), or after the peak (group 3) of the preovulatory luteinizing hormone surge. In experiment 1, with slaughterhouse material used as a random source of follicles at all stages of the estrous cycle, follicular diameter and AF volume were highly significantly correlated; in experiment 2, follicular size was estimated by measuring AF volume. Follicles from OVX ewes were grouped based on follicular size: small follicles (< 5.3 mm in diameter) contained < 40 .mu.l of AF, and large follicles (> 5.3 mm in diameter) contained > 40 .mu.l of AF. AF concentrations of progesterone and E2 were significantly correlated with AF volume in large follicles in groups 2 and 3; no correlation was seen in small follicles. AF testosterone concentrations were inversely related to AF volume in small follicles in group 1 and in large follicles in group 2. AF progesterone concentrations were low; AF testosterone concentrations were elevated in small follicles in group 2, and in large follicles AF testosterone and E2 concentrations were significantly elevated. In follicles with > 40 .mu.l AF volume, follicular size may be used as a reliable indicator of AF steroid concentrations, but only during the follicular phase of the estrous cycle. These data are indicative of rapidly changing AF concentrations of E2, testosterone, and progesterone during the follicular phase and imply that changes in follicular steroidogenesis are reflected in AF steroid concentrations. Small follicles appear to have a limited capability to synthesize progesterone in group 2 animals, and small follicles at all stages of the estrous cycle have limited aromatase activity.