Investigations of Formation and Function of Normal and Abnormal Luteal Tissue in the Bovine

The work included in this thesis describes studies of the cyclic bovine corpus luteum. Both normal and abnormal luteal structures were investigated using various approaches. Initially, the formation, maintenance and lysis of the corpus luteum were investigated in a field trial. Heifers were subjected to two different treatments for the synchronization of oestrus and their response monitored using ultrasonic examination of the ovaries and plasma progesterone concentration. Although no difference was found in the pregnancy rate between the progestogen and the prostaglandin treatments, close examination of the results indicated that in a proportion of animals the response to both treatments was not as expected. In particular it was found that the use of a double injection of prostaglandins, given ten days apart, results in a significantly better pregnancy rate to the synchronized oestrus when the first injection is administered during a non-luteal phase. The results obtained led to detailed studies on small groups of cows in which both the follicular aspect and the Luteinizing Hormone surge were given special attention during and following similar treatments. The variation in the timing and amplitude of the surge of Luteinizing Hormone did not hinder conception. However there were indications that follicular status at the end of progestogen treatments may be involved in delaying the surge of Luteinizing Hormone. Subsequently investigations of the structure and function of the corpus luteum were undertaken using samples collected at a local abbatoir. Gross macroscopic morphology of the corpus luteum was used as a means of evaluating the stage of the cycle of the animals. The methods utilized included histology, histochemistry, electron microscopy and cell culture. The information obtained was compared to results of studies of corpora lutea formed following an injection of a prostaglandin analogue during either early or late dioestrus. In addition the injection of a GnRH analogue during mid-dioestrus was used in two cows to monitor the formation and function of accessory corpora lutea. Although no difference in response and formation of luteal tissue could be found between groups treated differently, there was indications that corpora lutea induced by an injection of either a prostaglandin F2alpha analogue or a GnRH analogue have a shorter life-span than spontaneous corpora lutea. Regressive changes were observed in both histology and electron microscopy sections from all induced corpora lutea. This was further supported by the functional insufficiency which was recognized in these structures when challenged in vitro with hormonal agents. Histochemistry did not prove very useful in determining changes in steroidogenic enzyme activity. However it did show the presence of these enzymes in both the normal and the abnormal corpus luteum. It was concluded that further work was needed in this area in order to unveil the molecular mechanisms controlling formation, function and regression in both the normal and the abnormal corpus luteum