Effects of unilateral ovariectomy of follicular development, plasma gonadotropin, progesterone, IGF-I profiles, ovulation and pregnancy rates in cattle
Creators: name: Mohan, Mahesh
Publisher: name: University of British Columbia
Year: 2009

description: In cattle, during the estrous cycle, two to three DFs develop. Since cattle are monoovular, follicle deviation occurs particularly in an expeditious fashion resulting in the development of a single DF from a recruited pool, thereby, preventing multiple DFs in any single wave. The controlled emergence of a single DF in cattle is to a great extent made possible by the negative and positive feedback effects exerted by the various ovarian secretions (inhibin and E2) on the release of FSH from the pituitary. All growing follicles are capable of becoming dominant if adequate concentrations of FSH is available during the recruitment phase of follicular development. Initiation of FSH treatment during the emergence of a follicular wave stimulates many follicles to attain a diameter greater than 10 mm. Therefore, FSH seems to be a primary limiting factor in the attainment of follicular dominance. The focus of this thesis was to investigate follicular development, hormonal profiles, ovulation and pregnancy rates following removal of one ovary (ULO) in cattle. A bilateral ovariectomy completely eliminates the negative feedback effects of ovarian secretions resulting in a tremendous increase in the output of pituitary FSH and LH. Examination of the physiological changes associated with bilateral ovariectomy prompted a hypothesis that removal of one ovary would limit the inhibitory feedback to the pituitary resulting in increased FSH secretion sufficient for the growth, development and ovulation of more than one follicle. Non-lactating cycling cows and cycling heifers were used in a total of three experiments. The availability of sonographic procedures proved to be of immense help in tracking follicular development before and after ULO. In both cycling cows (Expt I) and cycling heifers (Expt II), ULO resulted in the development of co-dominant follicles in 6 out of 8 cycles and 4 out of 6 cycles, respectively. The co-dominant follicles had an altered size distribution with the largest follicle smaller in diameter than the intact animals and the second largest follicle larger than the subordinate follicles in intact animals. In both cows and heifers no significant increases were recorded in plasma gonadotropin (FSH and LH), and P₄ concentrations following ULO. In pubertal heifers, ULO increased pregnancy but not twinning rates. In experiment III, plasma FSH, LH and IGF-I concentrations following ULO were determined by performing serial blood samplings during the early follicular and mid-luteal phases of the estrous cycle. Even though mean concentrations after ULO were not significantly different from those before ULO, two of four heifers showed a two-fold increase in the magnitude of both FSH and L H surges after ULO. I conclude that ULO resulted in the development of co-dominant follicles and is a good method to obtain twin ovulations and possible twin pregnancies without causing any significant increases in plasma FSH, LH, P₄ and IGF-I concentrations. Therefore, subtle endocrine alterations at both the pituitary and ovarian levels might be the stimulus for the development and ovulation of twin follicles following ULO.