Atresia of the dominant ovarian follicle in cattle

• Main Author: Manikkam, Mohan
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/4793

During each bovine estrous cycle, two to three dominant follicles (DFs) develop. The DF that develops at the time of corpus luteum (CL) regression ovulates. The first or second DF reaches an ovulatory size, and is capable of ovulation during the luteal phase, but undergoes morphological and functional atresia, probably due to a lack of high frequency luteinizing hormone (LH) pulses which are regulated by high plasma progesterone (P₄) concentrations. The focus of this thesis was to further investigate some of the causes and mechanisms of atresia of the DF in cattle. A Norgestomet-maintained proestrus DF was chosen as an experimental model to study atresia. It was possible to monitor this DF daily, through ultrasonography in a noninvasive manner, without interference from other follicles. In heifers, P₄ , estradiol-17β (E₂) and testosterone (T) when administered, caused regression of the DF, which was associated with a reduction in systemic LH. The role of gonadotropins in the prevention of atresia of the DF was examined by their deprivation, using bovine follicular fluid (BFF) and luteinizing hormone releasing hormone antagonist (LHRHa). Both BFF and LHRHa caused regression of the DF, but there was no FSH suppression by BFF; L H was significantly reduced by LHRHa treatment. The role of FSH on the maintenance and ovulation of the DF was studied using BFF. The DF regressed when BFF was given during maintenance phase, although there was no decrease in FSH. Regression of the DF occurred in one subgroup during the ovulatory phase, and in the other subgroup ovulation of the DF ensued. BFF injections abolished FSH and L H surges after implant withdrawal. The mechanism of atresia of the DF was investigated in cows after P₄ treatment. The DF showed a decline in insulin-like growth factors-I and -II (IGF-I and II), increase in low molecular weight IGF-binding proteins on ligand blots, a drastic reduction in aromatase activity (AA), reduced E₂ content and increased apoptotic index. High concentrations of steroids and(or) low concentrations of gonadotropins cause atresia of the DF, and the mechanism of P₄-induced atresia of the DF involves changes in IGF system, and a reduction AA.