Morphological and biochemical correlates of equine ovarian follicles as a function of their state of viability or atresia.

This research investigates the morphological and hormonal characteristics of growing ovarian follicles in horses during estrous. It finds that hormonal changes and physical features vary correlatively and possibly causatively as the follicles mature or decay, although the initiating factor for follicular atresia, or the decay of the follicles, remains undetermined.

Investigation of Follicular Growth and Decay

• The research focuses on antral follicles during oestrus, a stage of the reproductive cycle in female mammals equivalent to human menstruation.
• The study correlates the histological features and hormonal content of the follicular fluid during this period to distinguish the different stages of follicular maturation and decay.
• A critical stage of maturation is identified when the follicles reach a diameter of 3 cm, characterized by oestrogen and androgen concentration in the follicular fluid increasing three-fold.

Role of Oestrogens and Androgens

• The study suggests that oestrogens are likely to play an anti-atretogenic role, preventing follicular decay or atresia.
• In contrast to the hypothesis that atresia might be caused by a decrease in oestrogen levels, the research shows that decay begins when oestrogen levels are still high.
• Coinciding with the anti-atretogenic theory for some species, the research also found that higher androgen levels do not lead to spontaneous follicular atresia. This offers evidence contradicting the idea that androgens contribute to follicular decay.

Morphological Changes

• The research notes morphological changes in theca epithelioid cells, specialized cells in the ovarian follicles, during the maturation process. These changes, known as histological luteinization—transformation into a corpus luteum—occur in viable follicles nearing ovulation, and occasionally in atretic follicles as well.
• The study found increased levels of prostaglandin F, a hormone-like compound, associated with follicles in transition, either towards luteinization or atresia.

Role of Granulosa Cells

• Granulosa cells, another type of specialized ovarian cells, in viable follicles were capable of binding human chorionic gonadotropin (hCG), a hormone important in reproduction.
• The research could not determine whether the loss of this binding capacity precipitates atresia or is a result of it.
• Finally, the relationship between granulosa cell transition and follicular atresia remains unknown; further research is suggested to understand the link.