Role of Luteinising Hormone in ovarian follicle development and maturation in the mare

Luteinising hormone (LH) is a crucial regulator of ovarian follicle maturation, ovulation and luteinisation. Development of healthy follicles and fertile ovulation can only occur within a specific range of circulating LH concentrations, with differing upper and lower limits depending on the stage of the oestrous cycle. The objective of the three studies in this thesis was to investigate the effects of both physiological and non-physiological circulating LH levels on equine follicular maturity by examining ovulatory and steroidogenic capacity, gene expression profiles and miRNA expression in ovulatory-size follicles at various stages of the oestrous cycle and/or in response to supplementation with LH. The aim of the first study was to investigate the hypothesis that deficient circulating LH is a primary cause for the inability of equine follicles to ovulate during the physiological anovulatory season. A LH-rich equine pituitary fraction (eLH) given twice daily to early transitional mares did not restore steroidogenic capacity of the ovulatory-size follicle or advance the onset of the natural breeding season; however, it significantly stimulated follicular growth to a level similar to that occurring during the normal oestrous cycle. The results demonstrated that a deficiency in LH is critically involved in reduced follicle growth during the anovulatory season. The second study examined the effects of elevated circulating LH levels early during follicle development on follicle maturation and ovulatory ability in cycling mares, with the hypothesis that excessive LH would disrupt ovulation and produce haemorrhagic anovulatory follicles (HAFs). Treatment with eLH or a luteolytic dose of prostaglandin F2α (to stimulate an increase in endogenous levels of LH) did not have any effects on follicle growth or ovulation, but did impair follicular production of androstenedione and insulin-like growth factor 1 (IGF1), suggesting a deleterious effect of high LH on follicle and oocyte maturation. The third study examined the expression of different follicular factors associated with follicle maturation as well as microRNAs (miRNAs) in ovulatory-size follicles naturally developing under different LH milieus (oestrus, dioestrus and spring transitional period). Progesterone and IGF1 were significantly reduced in follicles developing in a low LH environment (dioestrus and transition). All four miRNAs measured, miR-378, miR-542, miR-202 and miR-21 were found at higher levels in subordinate follicles than in preovulatory follicles during oestrus. In addition miR- 202 and miR-21 were significantly increased in transitional follicles relative to oestrous follicles. The results of this study indicate that follicles developing during both the spring transitional and dioestrous periods are developmentally immature and suggested potential important roles of miRNAs in follicle maturation in the horse. In summary, although LH is a key factor promoting follicular growth, it is by itself not sufficient to restore steroidogenic activity in transitional follicles. Elevated LH levels during follicle development do not disrupt ovulation, but induce changes in follicular fluid factors related to follicle maturation and oocyte quality. Follicles developing under different LH milieus show altered miRNA expression, suggesting an important role of miRNAs in follicle maturation.