Follicular growth and atresia at the time of oocyte aspiration differentially affect in vitro embryo production in mares.

Follicular growth phase versus atresia phase in mares at the time of oocyte aspiration differentially affects the developmental competence of oocytes, influencing in vitro embryo production success.

Background and Purpose

• Ovum pick-up (OPU) combined with intracytoplasmic sperm injection (ICSI) is a technique used to produce embryos in horses.
• Despite its increasing use, embryo production rates vary significantly, possibly due to the developmental competence of oocytes at the time of collection.
• This study aimed to investigate how the stage of follicular development—specifically growth phase (GP) versus atresia phase (AP)—and related hormonal profiles affect oocyte quality and embryo production in mares.

Methodology

• Follicular waves were synchronized in mares using follicular ablation.
• Oocytes were aspirated either during the growth phase (GP) or the atresia phase (AP) of the follicle.
• Collected oocytes underwent in vitro maturation and embryo production through ICSI.
• Blood samples were collected daily to measure hormone concentrations: progesterone (P4), luteinizing hormone (LH), follicle-stimulating hormone (FSH), inhibin A, inhibin B, and anti-Müllerian hormone (AMH).
• Chromatin analysis assessed the maturation stage of oocytes.

Main Findings

• Oocyte maturation rate was significantly higher in oocytes collected during the atresia phase (62.8%) compared to the growth phase (17.1%), indicating better developmental competence in AP oocytes.
• Blastocyst formation rate per recovered oocytes was numerically greater in the AP group (20.0%) compared to the GP group (7.3%), though this difference was not statistically significant.
• At the individual mare level, 66.7% of mares in the AP group produced at least one blastocyst, compared to only 14.3% in the GP group, suggesting enhanced embryo production potential during AP.
• Chromatin analysis demonstrated more oocytes in post-germinal-vesicle non-matured stages in AP mares (30.8%) compared to GP mares (2.9%), reflecting differences in nuclear maturation dynamics.

Hormonal and Endocrine Insights

• Progesterone (P4) levels were lower and inhibin A levels higher in mares at the atresia phase compared to those in growth phase, indicating a distinctive hormonal environment associated with follicular atresia.
• There were significant interactions over time for LH, FSH, and inhibin B levels:
  • Higher LH and inhibin B concentrations were observed in AP mares.
  • FSH levels were lower in AP mares relative to GP mares.
• These hormonal changes may reflect shifts in follicular physiology linked to atresia and influence oocyte competence.

Implications and Conclusions

• The physiological status of follicles at the time of oocyte retrieval directly impacts oocyte developmental competence and subsequent embryo production in mares.
• Contrary to expectations, follicles entering early atresia may provide more developmentally competent oocytes than those in active growth phase.
• Targeting follicles in early atresia when performing OPU-ICSI could improve embryo production outcomes in equine assisted reproductive technologies.
• Understanding associated hormonal profiles provides insight into follicular dynamics and suggests potential biomarkers to optimize timing for oocyte collection.