Summary. The

The aim of the present study was to gain evidence that the level of LH secretion preceding the preovulatory LH surge is an important determinant of follicular maturation and corpus luteum function in the ewe. In addition it was hoped to establish whether the pattern of LH delivery to the ovary (pulsatile v. constant) is a critical factor in the maturation of a preovulatory follicle. To accomplish this, progesterone-primed anoestrous ewes were repeatedly injected i.v. with LH or luteinizing hormone-releasing hormone (LH-RH), or given an i.v. infusion of LH, over a 72 h period. These animals, together with the appropriate controls, were exposed to a sexually active ram so that oestrous activity could be recorded. All ewes were subjected to intensive blood sampling regimes so that the plasma levels of LH and progesterone could be determined and compared to those which occurred in the same breed of sheep during the oestrous cycle. Collectively the data suggest that the plasma levels of LH preceding the preovulatory LH surge are an important determinant of follicular maturation as judged by subsequent corpus luteum function. Moreover, they show that follicular maturation can be achieved with widely differing patterns of LH delivery to the ovary during the preovulatory period and that a strict pulsatile delivery of LH may not be an absolute requirement.

The patterns of growth and atresia of antral follicles including that of the presumptive preovulatory follicle were examined in sheep ovaries for a 24\p=n-\48-h period after the induction of luteolysis with a prostaglandin analogue, cloprostenol or cloprostenol + PMSG. Ewes were ovariectomized at various times after the initiation of the treatments. All follicles \m=ge\1 mm in diameter were dissected from the excised ovaries and the antral fluid and granulosa cells recovered. Individual follicles were classified as healthy or atretic on the basis of the number of granulosa cells recovered and then subclassified as to whether they contained intrafollicular levels of oestradiol that were \ m=ge\ or < than 100 ng/ml. In another series of similarly treated ewes, the ovarian secretion rates of oestradiol and the intrafollicular concentrations of oestradiol in all large antral follicles (\ m=ge\ 5 mm diameter) as well as the levels of progesterone in peripheral plasma were measured at different times after induction of luteolysis. The results showed that a large 'oestrogenic' follicle (\m=ge\5 mm diameter and secreting \ m=ge\ 1ng oestradiol/min) appears around 10 h after the cloprostenol injection and that this presumptive preovulatory follicle emerges before the corpus luteum has ceased to function. Moreover, the presumptive preovulatory ('oestrogenic') follicle appears to develop from the pool of small 'oestrogenic' follicles (1\p=n-\3 mm diameter) after the onset of luteolysis. The emergence of a large 'oestrogenic' follicle is accompanied by a widespread increase in atresia (> 80%) in all other classes of antral follicles (\ m=ge\ 1 mm in diameter). During the first 10 h of cloprostenol-induced luteolysis, PMSG (a) prevented the normal occurrence of atresia in the large follicle population; (b) enhanced oestrogen secretion in a greater proportion of large antral follicles compared to that in control animals; (c) temporarily 'rescued' and/or prevented small antral follicles (1\p=n-\4 mm diameter) from undergoing atresia; but (d) had little, or no, effect on the overall population of antral follicles (\ m=ge\ 1 mm diameter). After 24 h, the atresia-preventing effects of PMSG were no longer discernible and the only obvious difference noted, compared to the controls, was the number of large oestrogen-secreting follicles.

The concentrations of follicle-stimulating hormone (FSH), luteinizing hormone (LH), prolactin, progesterone, androstenedione and oestradiol were determined in the antral fluid of ovarian follicles > 1 mm in diameter as well as in ovarian venous or peripheral venous plasma, or both, from at least four different animals on each day throughout the oestrous cycle of the sheep. The individual steroid hormones in antral fluid were examined in relation to the steroid-secretion rates in ovarian venous plasma, follicle size and the hormone levels in jugular venous plasma.

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