FACTORS AFFECTING MATURATION OF BOVINE OOCYTES; HORMONAL ENVIRONMENT, FOLLICULAR AND MATERNAL AGING

Abstract

Hormonal environment in which follicle grows has been shown to affect the oocyte competence. Our objective was to identify factors that affect oocyte competence and characterize the structural and functional changes induced by these factors. Fertility was compared in cattle following alterations in levels of progesterone and length of proestrus during dominant follicle growth. We hypothesized that subluteal-phase progesterone will mitigate the effect of a shorter proestrus on pregnancy rates. A shorter duration of proestrus during a fixed-time AI protocol in cattle resulted in a smaller preovulatory follicle, smaller and less functional CL with lower progesterone secretion, and lower fertility (P<0.01). A subluteal-phase progesterone milieu during ovulatory follicle growth induced higher pregnancy rates (P<0.01) and compensated for the effect of a short proestrus on pregnancy rates. Organelle behavior was characterized in oocytes obtained from follicles at different phases of dominant follicle growth. We hypothesized that ooplasmic organelles undergo changes in population and spatial distribution in a phase-specific manner. The growing phase oocytes have least area of mitochondria in contact with lipid droplets (P=0.04) and a peripheral distribution of lipids compared to an even distribution in oocytes from other phases. The regression phase oocytes showed an increase in mitochondrial number (P=0.03) and even distribution of mitochondria compared to peripheral in other phases. Moreover, oocytes from regression phase had higher (P<0.01) lipid content per unit volume of oocyte than other phases. Effect of follicular aging on nuclear maturation rates and, size and distribution of lipid droplets and mitochondria in in vivo matured oocytes were compared. We hypothesized that follicular aging after FSH starvation will result in maturation failure with accumulation of larger lipid droplets and altered distribution of mitochondria as compared to superstimulation with continued FSH support (4-d and 7-d). A 7-d FSH protocol resulted in greater proportion (P<0.01) of mature oocytes compared to other groups. FSH starvation lead to more poor quality oocytes that had ATP contents similar to short FSH group. Further, organization of mitochondria as intense and bigger clusters (P=0.01) alongwith increased size of lipid droplet (P=0.03) within the oocytes from FSH starvation group might indicate atresia. Effect of maternal aging on mitochondrial numbers, distribution and ATP content of in vivo matured bovine oocytes was studied. We hypothesized that in vivo matured oocytes from old cows will have reduced number of mitochondria, altered distribution of mitochondria and decreased the ATP content compared to those from young cows. Maternally aged oocytes had significantly less ATP content (P=0.01) although mitochondrial population and distribution pattern did not differ.