Species-Specific Mechanisms of Sperm Transport: Predominant Roles of Motility in Ewes and Uterine Contractility in Mares.

Overview

• This study investigates how sperm are transported through the female reproductive tracts of two different species, mares (female horses) and ewes (female sheep).
• The research aims to determine whether sperm motility (the ability of sperm to move) or uterine contractions (muscle movements of the uterus) are more important for sperm transport in these species.

Background

• Sperm transport through the female reproductive tract is a critical step for successful fertilization.
• Different species show varied mechanisms controlling sperm movement inside the uterus and related structures.
• Understanding these species-specific mechanisms can enhance reproductive management and breeding strategies.

Objective

• To quantify the roles of sperm motility and uterine contractions in sperm passage within the reproductive tracts of mares and ewes.
• To compare sperm distribution during stages of the reproductive cycle characterized by high and low uterine contractility.

Methods

• In vivo sperm imaging was performed using probe-based confocal laser endomicroscopy (pCLE), allowing visualization inside the female genital tract.
• Sperm was inseminated intrauterinely during two cycle phases:
  • Oestrus (high uterine contractility)
  • Luteal phase (low uterine contractility)
• Fluorescent microbeads (2 µm diameter), which are non-motile, were inseminated alongside sperm to serve as controls for movement caused by uterine contractions rather than sperm motility.
• Sperm and bead distributions were analyzed along different parts of the reproductive tract: vagina, uterine body, mid-horn, and upper uterine horn.

Results

• In Oestrous Mares (High Uterine Contractility):
  • Sperm exhibited a descending gradient from the uterine body to the upper uterine horn, with a strong selection for sperm in the upper horn.
  • Fluorescent beads showed a uniform distribution throughout the tract, from vagina to the tip of the horn, indicating beads moved passively with uterine contractions.
• In Oestrous Ewes (High Uterine Contractility):
  • Sperm exhibited an ascending gradient from the uterine body toward the upper uterine horn, suggesting active sperm movement against uterine flow.
  • Beads were mostly expelled toward the vagina and did not reach the horn, indicating passive movement could not overcome uterine expulsion forces.
• In Both Species During the Luteal Phase (Low Uterine Contractility):
  • Uterine contractions were reduced.
  • Both sperm and beads showed similar, homogeneous distributions along the uterine horn.
  • This implies reduced uterine contractions result in less differentiated movement patterns.

Interpretation and Conclusion

• The study demonstrates species-specific mechanisms of sperm transport influenced by both sperm motility and uterine contractility.
• In ewes, strong uterine contractions prevail, but sperm motility is essential as sperm actively swim against uterine flow to reach the upper horn.
• In mares, moderate uterine contractions effectively transport sperm; sperm motility plays a lesser role since passive transport by muscle contractions predominates.
• The role of uterine contractions is dynamic, varying with the reproductive cycle stage, influencing sperm and particle movement inside the reproductive tract.
• These findings highlight the importance of considering species differences when developing reproductive technologies and treatments for fertility.