Ovarian morphology and the concentration of steroids during the oestrous cycle of sheep actively immunized against androstenedione

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Abstract
Summary. Ewes were immunized against androstenedione-11α-hemisuccinyl—bovine serum albumin (androstenedione-11–BSA) or BSA alone (controls). The ovaries were examined macroscopically and ovarian venous blood was collected at laparotomy by in-situ cannulation of the ovarian vein on Day 10 of the oestrous cycle, or by needle puncture of the ovarian vein 36 h after prostaglandin treatment. Follicular fluid was also collected from follicles >5 mm diameter in some ewes. Ovaries and adrenal glands were collected and examined histologically. Oestrone, oestradiol-17β, androstenedione, testosterone and progesterone were measured by radioimmunoassays. Immunization against androstenedione-11–BSA was followed by an increase in the number of surface follicles >3 mm in diameter and in ovulation rate. These changes were accompanied by an increased production of androgens (especially androstenedione) and of oestrogens during the preovulatory period. There were no observed changes in the adrenal cortex or pituitary glands, but the ovaries were heavier in immunized ewes and luteinized follicles were present in the ovaries of 1 of 4 immunized ewes. These results show that immunization against androstenedione-11–BSA leads to increased ovulation rate and suggest two possible mechanisms: (1) interference with the negative feedback effects of oestradiol-17β and/or (2) a reduction in the incidence of follicular atresia.