Prepubertal heifers versus cows-The differences in the follicular environment.

The oocyte quality is to a large extent influenced by the sexual maturity of the donor female. Although this phenomenon has already been broadly described in domestic animals, the underlying mechanisms are poorly understood. Published data focus on oocyte ultrastructure, fertilization abnormalities, and blastocyst developmental rate. The goal of the present experiment was to characterize the follicular environment (oocyte, cumulus [CC] and granulosa (GC) cells as well as follicular fluid [FF]) in ovarian follicles of prepubertal heifers and cows. Each experimental replicate included the following set of traits within individual follicles: lipid droplets (LDs) number in oocytes, expression of seven genes involved in energy metabolism (fatty acids [FAs] metabolism-ELOVL2, ELOVL5, SCD, FADS2, glucose transport-GLUT1, GLUT3, GLUT8) in CC and GC as well as FA composition and glucose concentration in FF. According to our results, cow oocytes were larger in diameter and contained more LD than those from prepubertal heifers, both before and after IVM. The LD number was also higher in cow oocytes after IVM, when compared to immature oocytes. The FF from cow follicles had elevated glucose content similarly to the majority of the analyzed FA. Transcript analysis revealed differences for five out of seven analyzed genes (ELOVL, FADS2, SCD, GLUT3, GLUT8) in CC and GC cells. However after considering the female category, the only difference was noticed for the mRNA of SCD gene, which was more abundant in cow GC. This finding may indicate distinct roles of CC and GC in follicular energy metabolism. In conclusions, we suggest that distinct properties of follicular environment in prepubertal heifers and cows may be responsible for differences in the quality of oocytes from the two categories of donors. We hypothesize that suboptimal environment in heifer follicles (glucose and FA lower content in FF) determines reduced quality of their oocytes (lower diameter and LD number) and limited maturation potential. Besides, energy demands of heifer oocytes may be restricted due to a low LD number, exerting a negative effect on the development of the future embryo. The advantages of cow gametes (e.g., higher LD number and diameter) attributed to oocytes of superior quality may support the statement that cows donate oocytes of better quality than heifers.