Temporal characterization of conditions that promote functional capacitation of stallion sperm.

Overview

• This study investigates the optimal conditions and timing for functional capacitation of stallion sperm, a crucial step that has historically limited success in equine in vitro fertilization (IVF).
• The research identifies temperature, incubation time, and sperm isolation techniques that improve motility, capacitation markers, and fertilization potential of stallion sperm, including fresh, cooled, and frozen-thawed samples.

Background and Problem Statement

• Equine IVF has faced challenges primarily because stallion sperm require specific and not yet fully understood conditions to achieve functional capacitation.
• Capacitation is the physiological process sperm undergo to gain the ability to fertilize an egg, often thought to necessitate prolonged incubation, but the exact timing and conditions remain unclear for stallion sperm.
• Complete characterization of conditions that promote capacitation is essential to improve equine IVF success rates.

Objectives

• To characterize conditions (temperature, incubation duration, sperm processing methods) that promote functional capacitation of stallion sperm.
• To compare capacitation success among fresh, cooled, and frozen-thawed sperm samples using different isolation techniques: Percoll gradient, Swim-Up, and Microfluidics (VetMotl).
• To validate functional capacitation through fertilization outcomes in bovine (heterologous) and equine (homologous) IVF systems.

Methods

• Sperm Preparation: Stallion sperm samples were processed fresh (diluted), cooled (held at 5°C for 24-48 hours), or frozen-thawed.
• Isolation Techniques:
  • Percoll gradient separation.
  • Swim-Up technique.
  • Microfluidics method (VetMotl), a newer technology designed for sperm selection.
• Incubation Conditions:
  • Capacitating conditions maintained at either 38°C or ambient temperature.
  • Sperm were incubated up to 22 hours.
• Assessments Conducted at 4-hour intervals for:
  • Motility (movement quality and quantity).
  • Tyrosine phosphorylation as a molecular marker of capacitation.
  • Acrosome integrity, critical for fertilization capability.
• Functional Validation:
  • Fresh sperm capacitation was tested in a bovine IVF system by checking pronuclear formation and extrusion of polar bodies (signs of fertilization and oocyte activation).
  • Frozen-thawed sperm was introduced into equine IVF systems using different sperm isolation methods, with fertilization potential confirmed by pronuclear formation and embryo development.

Key Findings

• Temperature Effects:
  • Holding sperm at 38°C reduced motility, making it detrimental to sperm viability.
  • Ambient temperature incubation supported better capacitation and motility.
• Timing of Capacitation:
  • Maximum capacitation, evidenced by molecular markers and acrosome integrity, occurred at 4 hours of incubation.
  • Prolonged incubation beyond 4 hours decreased capacitation quality regardless of sperm isolation method.
• Sperm Isolation Techniques:
  • Microfluidic sperm selection (VetMotl) yielded superior sperm kinematics, capacitation, and acrosome integrity compared to Swim-Up, especially with frozen-thawed samples.
• Functional Confirmation:
  • Fresh sperm incubated at ambient temperature for 4 hours fertilized bovine oocytes successfully (heterologous system), confirmed by key fertilization indicators.
  • Frozen-thawed sperm processed with Microfluidics successfully fertilized equine oocytes, resulting in embryo development.

Implications and Contributions

• This research provides comprehensive temporal and procedural characterization for effective capacitation of stallion sperm.
• Findings suggest that shorter incubation (around 4 hours) at ambient temperature is optimal, challenging previous assumptions that prolonged incubation is necessary.
• Microfluidic sperm selection holds promise as an advanced method to improve sperm quality in equine IVF.
• The work advances equine reproductive technologies by offering validated conditions that can enhance in vitro embryo production.
• Data supports further refinements for stallion sperm handling to overcome current equine IVF limitations.