Intrafollicular Hormonal Environment of Equine Follicles Aspirated for In Vitro Embryo Production

During transvaginal oocyte aspiration (TVA), follicle selection is based primarily on ultrasonographic diameter, although follicles of similar size may differ in their physiological and endocrine status.

This study aimed to characterize the intrafollicular endocrine environment of equine follicles at aspiration and evaluate associations with oocyte developmental outcomes.

Follicular fluid was collected from antral follicles ranging from 5 to 40 mm in diameter prior to routine oocyte aspiration, and concentrations of luteinizing hormone (LH), anti-Müllerian hormone (AMH), and inhibins were quantified. Recovered cumulus-oocyte complexes were subjected to in vitro maturation, intracytoplasmic sperm injection, and embryo culture to assess maturation and blastocyst development.

AMH concentrations were highest in 15 mm follicles and declined in larger follicles. Inhibin B was positively correlated with AMH, indicating coordinated granulosa cell endocrine activity. Inhibin A concentrations were significantly higher in follicles from which oocytes were not recovered. No consistent associations were detected between follicular fluid hormone concentrations and oocyte maturation, and blastocyst formation could not be robustly evaluated because of the limited number of embryos produced. Substantial variation in hormone concentrations was observed, particularly within the 10-25 mm range, which is commonly aspirated during TVA.

These findings indicate that intrafollicular AMH and inhibins primarily reflect granulosa cell endocrine status, and their role as predictors of oocyte competence remains to be determined in future studies. The marked endocrine variability among follicles of similar diameter suggests that follicle size alone does not reliably represent functional follicular stage in the mare.