Can the Presence of Ovarian Corpus Luteum Modify the Hormonal Composition of Follicular Fluid in Mares?

SIMPLE SUMMARY: The presence of corpus luteum (CL) in the ovary could exert a local differential effect on the hormonal composition of follicular fluid (FF), and it could also indirectly influence follicular development and oocyte quality. Using FF samples of follicles of different sizes in mares, we evaluated the differences between the composition of steroids (oestrogen, androgen and progesterone) in relation to the presence or absence of CL in the ovary. We suggest that FF concentrations of steroid hormones are not only related to follicular size but also to the presence or absence of CL in the ovary. ABSTRACT: The hypothesis of this study was to investigate if the presence of corpus luteum (CL) in one ovary could modify the hormonal content of follicular fluid (FF) in the follicles. Sixty ovaries were taken after the slaughter of 30 clinically healthy mares. In relation to the sizes, the follicles were classified into three different categories, as small (20–30 mm), medium (31–40 mm) and large (≥41 mm). Blood samples were collected from the jugular vein of mares before their slaughter, and then the FF samplings were extracted from each single follicle. The ovaries that were collected were classified into two groups, according to the presence (CL-bearing) or absence (non-CL-bearing) of CL. The serum and FF samples were analysed for progesterone (P(4)), oestradiol-17β (E(2)), testosterone (T), androstenedione (A(4)) and dehydroepiandrosterone (DHEA). Intrafollicular P(4) concentrations in large follicles of CL-bearing groups were lower than for non-CL-bearing ones. Intrafollicular E(2) concentrations increased with the increase of the follicle diameter in both groups, CL-bearing and non-CL-bearing. However, in the FF with a large and medium follicle size, E(2) concentrations were significantly higher in non-CL-bearing groups than in CL-bearing groups. T and A(4) significantly increased in the large and medium follicle sizes when compared to the small follicle sizes in both groups, but higher concentrations in the non-CL-bearing group were obtained. Intrafollicular DHEA significantly decreased with the increase of the follicular diameter in both groups. Steroid hormones in FF dynamically changed, according to the presence or not of CL in the ovary. This study brings new knowledge on the role of the CL in the follicular hormonal composition in mares.