Pre-ovulatory follicles and ipsilateral ovarian stroma are cooler than non-dominant antral follicles and stroma on the contralateral ovary of the mare.

Overview

• This study investigated whether pre-ovulatory follicles (POFs) in mares are cooler than other parts of the ovary, similar to findings in other species, and how temperature varies between dominant and non-dominant follicles and surrounding ovarian tissue.
• Findings showed that POFs and the ovarian stroma on the same side of the ovary are cooler compared to non-dominant follicles on the opposite ovary, suggesting a species-specific pattern of ovarian cooling related to ovulation in mares.

Background and Importance

• In several animal species, pre-ovulatory follicles (POFs) have been observed to have cooler temperatures than surrounding ovarian tissue.
• This cooling is thought to be important for fertility, influencing processes like oocyte maturation and ovulation.
• The equine ovary is structurally unique compared to other species, which might affect temperature distribution and follicular environment.
• Understanding temperature dynamics could improve knowledge about oocyte competence both within the body (in vivo) and in laboratory settings (in vitro).

Study Design

• Subjects: 11 mares monitored over 14 estrous cycles.
• Inclusion criteria: Presence of an ovarian follicle equal to or larger than 38 mm to identify the pre-ovulatory follicle (POF).
• Intervention: Administration of deslorelin (a hormone to induce ovulation) when the POF was identified.
• Timing: Temperature measurements taken approximately 30 hours after deslorelin administration.
• Measurement techniques:
  • Rectal and cervical temperatures measured to provide baseline reference points.
  • Ovarian temperatures measured using a thermocouple probe inserted via a 12G needle attached to a transvaginal probe.
  • Temperatures recorded in POFs and non-dominant follicles (NDFs) on the contralateral ovary, with additional stromal temperature measurements in eight mares.

Key Findings

• The pre-ovulatory follicle was significantly cooler than the non-dominant follicle on the opposite ovary:
  • Mean temperature difference: 0.30 ± 0.17°C.
  • Statistical significance: p < 0.001.
• The ovarian stroma on the same side as the POF was cooler than the POF itself:
  • Indicates that the cooling effect extends beyond the follicle to surrounding tissues.
  • Statistical significance: p = 0.003.
• In contrast, within the ovary containing a non-dominant follicle, stromal temperature was not significantly different from the NDF temperature:
  • p = 0.18, showing no cooling effect in these regions.

Interpretation and Implications

• The study confirms that cooling of POFs occurs in mares, consistent with findings in other species.
• However, unlike other species where only the follicle is cooler, in mares the adjacent ovarian stroma ipsilateral to the POF also shows cooling, suggesting the entire ovary on that side plays a role.
• This difference may relate to the unique anatomical and physiological features of the equine ovary, such as:
  • Unusual positioning of follicles.
  • Distinct stromal and vascular structures.
• Understanding this cooling and temperature distribution in the ovary can improve insights into:
  • Mechanisms underlying oocyte maturation and ovulation timing.
  • Potentially enhancing fertility treatments and in vitro fertilization techniques in horses.

Conclusion

• Pre-ovulatory follicles and associated ovarian stroma in mares are cooler than non-dominant follicles and stroma on the opposite ovary.
• These temperature variations could be crucial for reproductive success in mares and highlight species-specific ovarian physiology.