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W. D. Currie, R. J. Medhamurthy, S. J. Cook and N. C. Rawlings

Diurnal and seasonal fluctuations were detected in luteinizing hormone (LH) interpulse interval, but not amplitude, in ewes examined during the mid-luteal phase of an oestrous cycle at five stages of the breeding season. Daytime and night-time LH interpulse intervals were greater in the early and late than in the mid-breeding season (P < 0.05). During the early and late breeding season, LH interpulse interval was less during daylight than during darkness (P < 0.05). Toward the mid-breeding season, interpulse interval decreased during daytime earlier in the season than the night-time decrease. It was concluded that the diurnal fluctuations observed are a component of a circannual rhythm in LH secretion resulting from gradual seasonal transitions in photoperiodic drive to, or an endogenous rhythm in, the hypothalamic–pituitary axis in ewes.

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P. M. Bartlewski, A. P. Beard, S. J. Cook and N. C. Rawlings

The aim of the present study was to document ovarian antral follicle dynamics throughout seasonal anoestrus in sheep. Daily transrectal ultrasonography was performed during four 17 day scanning periods from March to July in Western White-faced crossbred ewes. Blood samples were collected each day with ultrasonographic scanning for measurement of serum concentrations of FSH, oestradiol and progesterone. Blood samples were also taken every 15 min for 6 h, mid-way through each period of ultrasonographic examination, to determine the patterns of secretion of gonadotrophic hormones. Hormonal data were then related to observed changes in follicular populations and the patterns of antral ovarian follicle turnover. Ultrasonography showed that the ovaries of anoestrous ewes remained active and that the largest ovarian antral follicles grew to a periovulatory size (≥ 5 mm in diameter) at all stages of anoestrus. The total number of all ovarian follicles ≥ 3 mm in diameter was lower during early anoestrus compared with at mid-anoestrus because of a significantly smaller number of small (3 mm) and medium (4 mm) ovarian follicles. The largest ovarian follicles (attaining ≥5 mm in diameter before regression) exhibited a wave-like pattern of growth; an average of three waves of follicular development were recorded in sheep during each of the four 17 day scanning periods in anoestrus, with follicular waves emerging approximately every 5 days. This rhythmic pattern of follicular emergence was found to be associated with the occurrence of fluctuations in serum FSH concentrations. The growth rate of the largest follicles of the wave increased significantly from early to late anoestrus in sheep. In addition, ovarian follicles not growing beyond 3 mm in diameter showed organized patterns of growth and regression; their numbers tended to be lower (P = 0.09) at 3 days before and on the day of follicular wave emergence. Some ewes were seen to maintain synthesis of progesterone throughout anoestrus. This submaximal progesterone secretion tended to occur at irregular intervals and was not coupled with changes in concentrations or patterns of gonadotrophin release, ovulations or detectable morphological luteinization of ovarian antral follicles. It was concluded that the growth of ovarian antral follicles to an ovulatory size was maintained throughout anoestrus in ewes, with a transient shift in the number of small and medium-sized follicles during mid-anoestrus, and that the periodic emergence of waves of large follicles (≥ 5 mm in diameter) occurred in synchrony with an endogenous rhythm of FSH secretion.

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D. L. Cook, J. R. Parfet, C. A. Smith, G. E. Moss, R. S. Youngquist and H. A. Garverick

Summary. Two experiments were conducted to (1) investigate developmental endocrinology of ovarian follicular cysts (cysts) in cattle and (2) evaluate effects of cysts on hypothalamic and hypophysial characteristics. Cysts were induced with oestradiol-17β (15 mg) and progesterone (37·5 mg) dissolved in alcohol and injected s.c. twice daily for 7 days. Cysts were defined as the presence of follicular structures (which may or may not have been the same structure) of 2·0 cm in diameter or greater that were present for 10 days without ovulation and corpus luteum development.

In Exp. 1, 22 non-lactating, non-pregnant Holstein cows were allocated to 3 groups. Beginning on Day 5 (oestrus = Day 0) of the oestrous cycle, 7 cows (Controls) were treated with twice daily s.c. injections of ethanol (2 ml/injection) for 7 days. Luteolysis was then induced with PGF-2α and blood samples were collected daily every 15 min for 6 h from the morning after the PGF-2α injection (Day 13) until oestrus. Steroids to induce cysts were injected as previously described into the remaining cows (N = 15). Three blood samples were collected at 15-min intervals every 12 h throughout the experimental period. Additional blood samples were collected every 15 min for 6 h on a twice weekly basis. After steroid injections, follicular and luteal structures on ovaries were not detected via rectal palpation for a period of 36 ± 4 days (static phase). Then follicles developed which ovulated within 3–7 days (non-cystic; N = 7) or increased in size with follicular structures present for 10 days (cystic; N = 8). Mean (± s.e.m.) concentrations of LH, FSH, oestradiol-17β and progesterone in serum remained low and were not different during the static phase between cows that subsequently developed cysts or ovulated. During the follicular phase, mean serum concentration of LH (ng/ml) was higher (P < 0·1) in cows with cysts (2·9 ± 0·2) than in cows without cysts (1·1 ± 0·1) or control cows (1·4 ± 0·2). In addition, LH pulse frequency (pulses/6 h) and amplitude (ng/ml) were higher (P < 0·1) in cows with cysts (3·6 ± 0·3 and 2·2 ± 0·3, respectively) than in non-cystic (2·3 ± 0·2 and 1·0 ± 0·2, respectively) and control (1·8 ± 0·1 and 1·1 ± 0·2, respectively) groups during the follicular phase.

In Exp. 2, 20 non-lactating, non-pregnant dairy cows were used: 15 cows received exogenous steroids as previously described. Hypothalamic and hypophysial tissues were collected after diagnosis of cystic structures in 11 cows (cystic group). The remaining 4 cows in the steroid-treated group ovulated and were assigned to the control group in addition to 5 non-steroid treated cows. Hypothalamic and hypophysial tissues were collected during the late-luteal phase (Days 16–18) from these control cows (N = 9). Anterior pituitary concentrations (μg/g) of LH (60·5 ± 11·0, 44·6 ± 11·7), FSH (30·2 ± 4·0, 22·1 ± 4·6) and receptors for GnRH (17·2 ± 2·2, 23·4 ± 2·6 m × 10−10/mg protein) did not differ between cows with cysts and control cows, respectively. Content of GnRH (ng) in the combined preoptic area and hypothalamus proper was higher (P < 0·05) in control cows (37·7 ± 6·6) than cows with cysts (18·6 ± 6·1). In the pituitary stalk median eminence, GnRH content (ng) tended to be higher (P ≥ 0·1) in cows with cysts (38·5 ± 9·6) compared with control (21·1 ± 15·2) cows.

Secretory patterns (mean concentration, pulse frequency and amplitude) of LH were therefore increased during the follicular phase in cows which developed cysts compared to cows which subsequently ovulated. In addition, hypothalamic GnRH content, but not pituitary characteristics, appeared to be altered in cows with cysts.

Keywords: ovary; follicular cysts; dairy cattle; gonadotrophins; hypothalamus; pituitary

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D. L. Cook, C. A. Smith, J. R. Parfet, R. S. Youngquist, E. M. Brown and H. A. Garverick

Summary. Non-lactating, multiparous dairy cows diagnosed as having cysts by palpation per rectum were used. Cysts were induced with oestradiol-17β (15 mg) and progesterone (37·5 mg) dissolved in ethanol and injected s.c. twice daily for 7 days. Following initial diagnosis of cysts, ovaries were exposed by midventral laparotomy, and the perimeter of the base of each cyst was marked with subepithelial injections of charcoal. Ovaries were removed from cows by transvaginal incision at 10 days (Group 1; N = 8), 20 days (Group 2; N = 8), or 40 days (Group 3; N = 7) after marking of cysts. Ovaries were examined for structures present and their relationship to the marked site. Corpora lutea with ovulation papilla were present in 7/23 cows (1/8, 4/8 and 2/7, for Groups 1, 2 and 3, respectively). In these 7 cows, corpora lutea were at a site different from the original structure that was marked. Marked structures persisted for the duration of the experimental period in 1 and 2 cows, in Groups 1 and 3, respectively. In the remaining 13 cows, new large follicular structures (cysts) were present at a site other than that marked with charcoal. These structures developed on the ovary contralateral to the one originally marked in 9 of 13 cows. Cysts are therefore dynamic in nature and may persist or may be replaced by others.

Keywords: ovary; follicular cysts; turnover rate; dairy cattle

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P. M. Bartlewski, A. P. Beard, S. J. Cook, R. K. Chandolia, A. Honaramooz and N. C. Rawlings

Transrectal ultrasonography of ovaries was performed each day in non-prolific Western white-faced (n = 12) and prolific Finn ewes (n = 7), during one oestrous cycle in the middle portion of the breeding season (October–December), to record the number and size of all follicles ± 3 mm in diameter. Blood samples collected once a day were analysed by radioimmunoassay for concentrations of LH, FSH and oestradiol. A cycle-detection computer program was used to identify transient increases in concentrations of FSH and oestradiol in individual ewes. Follicular and hormonal data were then analysed for associations between different stages of the lifespan of the largest follicles of follicular waves, and detected fluctuations in serum concentrations of FSH and oestradiol. A follicular wave was defined as a follicle or a group of follicles that began to grow from 3 to ± 5 mm in diameter within a 48 h period. An average of four follicular waves per ewe emerged during the interovulatory interval in both breeds of sheep studied. The last follicular wave of the oestrous cycle contained ovulatory follicles in all ewes, and the penultimate wave contained ovulatory follicles in 10% of white-faced ewes but in 57% of Finn ewes. Transient increases in serum concentrations of FSH were detected in all animals and concentrations reached peak values on days that approximated to follicle wave emergence. Follicular wave emergence was associated with the onset of transient increases in serum concentrations of oestradiol, and the end of the growth phase of the largest follicles (≥ 5 mm in diameter) was associated with peak serum concentrations of oestradiol. Serum FSH concentrations were higher in Finn than in Western white-faced ewes during the follicular phase of the cycle (P < 0.05). There were no significant differences in serum concentrations of LH between Western white-faced and Finn ewes (P > 0.05). Mean serum concentrations of oestradiol were higher in Finn compared with Western white-faced ewes (P < 0.01). It was concluded that follicular waves (follicles growing from 3 to ≥5 mm in diameter) occurred in both prolific and non-prolific genotypes of ewes and were closely associated with increased secretion of FSH and oestradiol. The increased ovulation rate in prolific Finn ewes appeared to be due primarily to an extended period of ovulatory follicle recruitment.