Effects of a dimethylformamide-glycerol combination on freezing behavior, post-thaw sperm quality, and sperm-oocyte interaction in Arabian stallions.

Overview

• This study investigated how combining two cryoprotectant agents (dimethylformamide and glycerol) affects the freezing process and post-thaw quality of sperm from Arabian stallions.
• Researchers assessed the sperm’s movement, membrane integrity, and ability to bind to oocytes after freezing with each cryoprotectant alone and in combination.

Background and Purpose

• Cryoprotectant agents (CPAs) are substances used to preserve sperm by protecting them from damage during freezing and thawing.
• Glycerol and dimethylformamide are commonly used CPAs for freezing horse semen, but the effects of using them together are less well understood.
• The objective was to evaluate how glycerol alone, dimethylformamide alone, and their combination (DMF+GLY) influence:
• Freezing behavior and thermal properties during cryopreservation
• Sperm motility and movement characteristics after thawing
• Sperm membrane and acrosomal integrity (important for fertilization)
• Sperm’s functional ability to bind to and fertilize oocytes via in vitro fertilization (IVF)

Methods

• Sperm Collection:
  • Twenty-four ejaculates were collected from four Arabian stallions (six ejaculates per stallion).
• Freezing Protocol:
  • Semen was diluted in BotuSemen-Gold® extender supplemented with one of three treatments:
    • 5% dimethylformamide (DMF)
    • 5% glycerol (GLY)
    • Combination of 3% DMF + 3% GLY (lower combined concentration but total 6%)
  • Samples were loaded into 0.5-ml straws and frozen using standard cryopreservation techniques.
• Functional Assessment:
  • Sperm-oocyte interaction was tested using a heterologous IVF system with in vitro-matured bovine oocytes.
  • Parameters measured included:
    • Sperm motility and velocity metrics post-thaw
    • Plasma membrane and acrosomal integrity (acrosome is critical for fertilization)
    • Sperm binding to oocytes after 2.5 hours
    • Pronuclear formation as an indicator of fertilization success

Key Findings

• Freezing Behavior:
  • The combination (DMF+GLY) showed an intermediate freezing temperature for ice crystal formation (“seeding” at -9.0°C) and a quicker release of latent heat, indicating a more controlled and balanced freezing process.
• Sperm Quality Post-Thaw:
  • DMF+GLY treatment led to better overall motility, more curved swimming patterns (curvilinear velocity), more frequent flagellar beats (beat-cross frequency), and better acrosomal integrity compared with glycerol alone.
  • The combination treatment improved sperm viability, straight-line velocity (movement in a straight direction), and average-path velocity compared to both glycerol or dimethylformamide alone.
  • In many motility parameters, DMF alone performed similarly to the combination, but the combination outperformed glycerol alone.
• Sperm-Oocyte Interaction:
  • At 2.5 hours, sperm frozen with the DMF+GLY combination showed significantly higher binding to oocytes than sperm frozen with either cryoprotectant alone.
  • Despite increased binding, pronuclear formation (a sign of successful fertilization) was not significantly different among treatments, indicating fertilization potential was not adversely or advantageously affected in the short term by CPA choice.

Conclusions and Implications

• The combined use of 3% DMF and 3% GLY as cryoprotectants improves certain freezing parameters and important sperm quality features after thawing in Arabian stallion semen.
• This combination led to better sperm motility metrics, membrane integrity, and sperm-oocyte binding capacity compared with using glycerol or dimethylformamide alone.
• The improved freezing behavior suggests that combining these agents might protect sperm better by balancing the freezing process.
• However, the total concentration of cryoprotectants in the combination treatment (6%) is higher than for either alone (5%), which may impact interpretation and requires consideration in practical applications.
• Since fertilization success (pronuclear formation) was not significantly different, further studies may be needed to confirm whether the improved sperm parameters translate to higher pregnancy rates in vivo.