Characterization of isolated late preantral and early antral equine ovarian follicles.

Overview

• This study investigated how the thickness of ovarian tissue slices and the location within the ovary affect the isolation, quality, and characteristics of late preantral and early antral follicles in horses.
• The research evaluated the viability, functional status, and epigenetic profiles of follicles isolated through mechanical microdissection to improve assisted reproductive technologies such as in vitro culture and cryopreservation.

Introduction and Purpose

• Follicles are the key functional units in ovaries where eggs develop, classified as preantral (without a fluid-filled cavity) or antral (with such a cavity).
• Isolating a large number of healthy follicles is critical for enhancing success rates in assisted reproductive technologies (ARTs) like in vitro culture and cryopreservation.
• The study aimed to characterize late preantral and early antral equine follicles isolated from ovarian tissue sliced at two thicknesses (0.25 mm and 0.50 mm) and from different ovarian regions (dorsal, medial, ventral).
• Characterization focused on follicle recovery, morphofunctional features (such as membrane integrity, viability, mitochondrial function, and oxidative status), and epigenetic markers.

Methodology

• Equine ovaries were sectioned into slices of either 0.25 mm or 0.50 mm thickness.
• Late preantral and early antral follicles were isolated using mechanical microdissection with needles from the intermediate region of these slices.
• Follicles were then grouped based on their location within the ovary (dorsal, medial, ventral regions).
• Several assays were performed on isolated follicles:
  • Membrane integrity assessment using trypan blue exclusion test.
  • Viability testing with propidium iodide and Hoechst 33342 staining.
  • Mitochondrial membrane potential measurement using JC-1 dye, indicative of mitochondrial health.
  • Production of reactive oxygen species (ROS) measured by H2DCFDA fluorescence, indicating oxidative stress levels.
  • Analysis of histone trimethylation marks H3K4me3 (activation mark) and H3K9me3 (repression mark) as indicators of epigenetic regulation during follicle development.

Key Findings

• Mechanical microdissection effectively isolated follicles with intact membranes regardless of slice thickness or ovarian region, confirming method robustness.
• Recovery rates of follicles varied:
  • Follicle isolation was influenced by the thickness of ovarian tissue slices, with different efficiencies at 0.25 mm vs. 0.50 mm.
  • The abundance of recoverable follicles differed depending on whether follicles were from dorsal, medial, or ventral ovarian regions.
• Viability and ROS levels of preantral and antral follicles were consistent across developmental stages and independent of slice thickness.
• Mitochondrial membrane potential was affected by tissue slice thickness, suggesting that slice preparation can impact mitochondrial health within follicles.
• In antral follicles, viability and ROS levels varied by ovarian location, indicating that microenvironment within the ovary influences follicle health.
• Epigenetic analysis showed that histone methylation patterns differ based on follicular developmental stage:
  • H3K4me3 (a mark linked to gene activation) and H3K9me3 (a repression-associated mark) patterns varied between preantral and antral follicles.
  • This suggests dynamic epigenetic regulation during follicle maturation.

Conclusions and Implications

• Optimal isolation of high-quality late preantral and early antral follicles depends on choosing the appropriate tissue slice thickness and considering the specific ovarian region.
• The study supports mechanical microdissection as an effective method to isolate viable follicles for research and ART applications in horses.
• Variations in mitochondrial function and epigenetic status highlight the importance of these factors in follicle quality and developmental competence.
• Understanding these variables is critical for improving ART outcomes, such as better in vitro follicle culture and cryopreservation protocols.
• This research is pioneering in documenting how tissue slicing parameters and ovarian follicle location affect follicle isolation success and follicle epigenetic regulation in equine species.