A study of some endocrine aspects of folliculogenesis in sheep

Berry, Janet Edna (1976) A study of some endocrine aspects of folliculogenesis in sheep. PhD thesis, University of Glasgow.

Abstract

Some aspects of follicular development and function were studied in sheep, during the neonatal period and during the oestrous cycle. The number and appearance of large follicles (4 mm. or greater in diameter), ovarian weight and volume, uterine horn diameter and pituitary weight were recorded at laparotomy or slaughter. The preparation of histological sections allowed total ovarian oocyte counts to be carried out and uterine epithelial height to be measured. Peripheral plasma levels of total unconjugated oestrogens were determined by a sensitive radioimmunoassay. Two breeds of sheep, Finnish Landrace and Scottish Blackface, were used throughout the studies in order to investigate the physiological factors responsible for variations in litter size. Mean litter sizes of 2.37 and 1.29 were produced by the Finnish and Blackface flocks respectively. In mature cycling ewes a peak in the peripheral plasma level of oestrogens was recorded during prooestrus. the peak levels were not significantly different between the two breeds. Elevated levels of oestrogens occurred on the day before and on the day of oestrous onset in the Finnish ewes, whereas the level was elevated only on the day before the onset of oestrus in the Blackfaces. These differences may be related to the greater number of ovulating follicles and the longer duration of oestrus reported for the more fecund breed. In the neonatal lambs, of both breeds, an increase in ovarian volume between 7 and 35 days of age was accompanied by an increase in the number of small vesicular follicles (generally 2 mm or less in diameter) visible on the ovarian surface. Uterine horn diameter and uterine gland development also increased during the same period. The peripheral plasma level of oestrogens remained basal in control lambs at all ages. At 35 days of age the number of oocytes in vesicular follicles was significantly greater in the Finnish lambs than in the Blackfaces. Follicular development was stimulated in neonatal lambs by the administration of PMSG. In a proportion of lambs at 7, 21 and 35 days of age the ovaries were refractory to PMSG-treatment. Among the lambs that were stimulated, the response, in. terms of the. development of large follicles, was extremely variable. The greatest range in the response occurred in. lambs of 49 days of age and less, and with the higher doses of PMSG. Uterine horn diameter and epithelial Weight were significantly greater in the. treated lambs than in controls. PMSG-treatment was associated with a significant elevation in the peripheral plasma level of oestrogens. The concentrations increased progressively until the administration of HCG, 4 days after MSG, resulted in an immediate decline in the concentration, HCG also induced ovulation or luteinisation in a proportion of the large follicles. Spontaneous ovulation of the stimulated follicles did not occur in lambs of 49 days of age and less. The peak level of oestrogens was significantly correlated with the number of large follicles. The use of methallibure (I.C.I. 33,828) was investigated as a possible means of suppressing the endogenous secretion of gonadotrophins in the neonatal iamb. In both breeds, the parenteral administration of methallibure daily from birth had no effect on ovarian size, uterine structure and plasma levels of oestrogens at 35 days of age. However the number of oestrogens in primordial follicles was greater in methallibure-treated animals. Pituitary'' weight, as a percentage, of bodyweight, was significantly reduced in the treated Finnish lambs and significantly greater in the treated Blackface lambs, Methallibure," treatment resulted in a smaller and delayed release of LH following LH-RH stimulation, at 45 days of age, in both breeds. In both normal and meth3.lliburetreated animals the response to FMSG, in terms of follicular development and levels of oestrogens, wa.s not significantly different between the Finnish and Blackface lambs. Therefore it xjas concluded that, in lambs of 7-49 days of age, ovarian sensitivity was similar in the two breeds, This implies that extra-gonadal factors are responsible for the different ovulation rates reported for mature Finnish and Blackface ewes.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Adviser: P G Hignett
• Keywords: Animal sciences, Endocrinology
• Date of Award: 1976
• Depositing User: Enlighten Team
• Unique ID: glathesis:1976-73123
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 14 Jun 2019 08:56
• Last Modified: 14 Jun 2019 08:56
• URI: https://theses.gla.ac.uk/id/eprint/73123