The effect of nutrition and metabolic hormones on follicular development in cattle
Gutierrez-Aguilar, Carlos Guillermo
Nutrition has profound effects on reproductive performance in cattle. Although these effects have been documented, the underlying mechanisms are not yet fully understood. The aim of this project was to investigate the role of metabolic hormones in mediating these nutritional effects by studying bovine folliculogenesis. The interaction of metabolic hormones and gonadotrophins on granulosa cell function in vitro was also investigated.To determine the effect of nutrition on ovarian follicular development, twenty -eight heifers were allocated to different dietary treatments. Small follicle number (< 4 mm) was increased by 37% on days 1 and 2 of the oestrous cycle in heifers consuming twice maintenance requirements (2M). Insulin concentrations were higher in 2M than in control or feed -deprived heifers. The increase in small follicle number was independent of changes in peripheral FSH and insulin -like growth factor -I (IGF -I), negatively associated with growth hormone (GH) but positively associated with circulating insulin. Numbers of medium -sized (4 to 8 mm) and large (> 8mm) follicles and FSH concentrations were not different among treatments.The relationship between postpartum follicular development and endocrine and metabolic changes in lines selected for high or low predicted breeding value for milk yield (PBV) was investigated. PBV was strongly correlated with milk production during a 305 day -lactation. This was associated with higher peripheral GH and ß- hydroxybutyrate, and lower insulin and glucose concentrations in high genetic merit cows. Although no differences were seen in follicular development between selection lines, changes in body weight influenced follicle number, and day to first ovulation postpartum was delayed in cows selected for high milk production.To gain an insight into the mechanisms underlying the nutritional effect on folliculogenesis, a serum -free bovine granulosa cell culture system was developed. In the presence of FSH, granulosa cells from small follicles differentiated in vitro and oestradiol (E,) secretion increased with time. Cells from medium -sized and large follicles secreted E2 throughout the culture period. Insulin and FSH promoted proliferation and E, production in a dose -responsive manner. The inclusion of IGF -I enhanced proliferation and E, production, even in the absence of FSH. Furthermore, cultured granulosa cells formed clumps of spherical cells with ultrastructural characteristics that resembled those of granulosa cells in vivo. In contrast, granulosa cells growing either in the base of the clump, or as monolayers on serum- coated wells, possessed morphological characteristics suggestive of early luteinisation.IGFs, as mediators of steroidogenesis and proliferation of granulosa cells, can act through endocrine, autocrine and/or paracrine mechanisms. Neither insulin nor FSH induced detectable levels of IGF -I production in granulosa cells, but IGF- binding proteins were secreted. IGF -I mRNA expression was found in theca cells, but not in granulosa cells either before or after culture. In contrast, IGF -II was expressed in both theca and granulosa cells.The interaction of IGF -I and insulin on granulosa cell proliferation and E, production in vitro was determined in cells from follicles from cattle without or with a dominant follicle in vivo, i.e. ovaries were collected either on day 3 or day 7 of the oestrous cycle respectively. E, production and proliferative capacity of granulosa cells from medium -sized follicles in vitro was inhibited by the presence of a dominant follicle during their development in vivo. However, after long -term culture, E, production did not differ between granulosa cells either from day 3 or from day 7 follicles, but there were differences in proliferative responses to FSH and IGF -I according to the size of the follicle and the stage of the follicular wave.The effects of FSH, EGF and IGF -I on early follicular development were studied by culturing preantral follicles. Follicle and oocyte diameter increased with time in culture. FSH, IGF -I and EGF stimulated follicle growth, but not oocyte growth rate. Most follicles maintained their morphology throughout culture and antra developed after 10 to 28 days of culture. EGF, IGF -I and FSH increased the probability of antrum developmentThese results demonstrate that insulin and IGF -I, but not GH, may mediate the direct effects of nutrition on follicular development. IGFs may also have an autocrine action in the follicle, being tightly regulated by the production of IGF -BPs within the follicle.