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Ovarian function and response in prepubertal ewes through to puberty

Sumbung, Frederick Patta (1985) Ovarian function and response in prepubertal ewes through to puberty. PhD thesis, Murdoch University.

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The objectives of this study were to assess ovarian function and responses to various reproductive stimuli in prepubertal ewe lambs, and to determine the hormonal patterns that accompany follicular activity through to puberty. Hormonal changes and occurence of oestrus were monitored in a group of young ewe lambs to establish at what age and bodyweight puberty occurred. A number of stimuli were then used to provoke ovarian and reproductive activity in groups of prepubertal ewe lambs.

Groups of ewe lambs were either placed with a ram, fed monensin supplement, primed with progesterone or challenged with FSH-P. Blood samples were collected to measure hormone levels and the lambs were ovariectomized to determine ovarian morphology and function. Follicles were dissected from the ovaries, antral fluid collected and the follicles were incubated in culture medium to measure steroid production. Gonadotrophin binding site present in the granulosa cells of the stimulated follicles was also quantified.

The natural onset of puberty in Corriedale ewe lambs was attained at the age of 40-42 weeks with a threshold bodyweight of 34-36 kg in the breeding season. Response of ewe lambs to the introduction of a ram seemed to depend on their age. ewe lambs aged 20-24 weeks showed no response whereas ewe lambs aged 34-36 weeks responded with increased LH pulse frequency and an LH surge which occured within 24 hours. The LH surge was followed by a transient increase in peripheral progesterone levels which lasted for 2-3 days, suggesting that ovulation had occurred but the resultant corpora lutea were short lived.

Monensin supplementation stimulated ovarian function with an increase in the number of small follicles and an increase in the antral fluid oestradiol content of large follicles. Progesterone priming caused an increase in oestradiol secretion into culture medium by large follicles. Withdrawal of progesterone treatment resulted in an increase LH pulse frequency. Neither of these treatments stimulated follicular maturation nor an LH surge.

After FSH stimulation, ewe lambs in control, monensin supplemented and progesterone-primed group showed a sharp increase in plasma LH levels. In control lambs the peak LH levels occured at intervals ranging from 12 h to 36 h after the initiation of FSH treatment, whereas in the progesterone-primed lambs the LH peak was later with tighter synchronization (36h to 42h). The monensin supplemented lambs showed an intermediate increase in LH levels with less synchrony (24h to 36h) than the progesterone-primed lambs following this FSH treatment. Both monensin supplemented and progesterone-primed lambs had a higher LH peak than control ewe lambs.

Ovariectomy after the LH surge but before ovulation showed that ovarian weight in progesterone-primed ewes was greater than in either control or monensin supplemented lambs. FSH treatment stimulated an increase in the development of large follicles in monensin supplemented and progesterone-primed ewe lambs. Follicles > 7mm in diameter secreted high levels of progesterone into culture medium, indicating that there was a shift in the steroidogenic capacity of the preovulatory follicles. Antral fluid from follicles of control lambs contained high levels of testosterone. Monensin supplementation and progesterone priming stimulated an increase gonadotrophin binding sites in granulosa cells after FSH in treatment.

Studies on ovaries removed after ovulation showed that monensin supplemented and progesterone-primed ewe lambs had more ovulations after FSH treatment and had significantly more functional corpora lutea 12 days after FSH treatment. Increased ovulations and increased numbers of functional corpora lutea were both associated with increased gonadotrophin binding sites. These result indicate a possible mechanism for the known influence of progesterone priming on the persistence of corpora lutea in ewes.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Veterinary Studies
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Williamson, Peter
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