Why doesn’t conventional IVF work in the horse? The equine oviduct as a microenvironment for capacitation/fertilization.

The research in question aims to understand why conventional in vitro fertilization (IVF) techniques do not consistently work in horses. The researchers believe that the issue lies in the sperm’s lack of fully activating, or “capacitation,” and propose further investigation into the environment in which fertilization takes place: the equine oviduct.

Problem with In Vitro Fertilization (IVF) in Horses

• Unlike in humans and other mammals, conventional in vitro fertilization (IVF) is not consistently successful in horses.
• The difficulty seems to stem from the horse sperm failing to fully activate – a process known as capacitation – due to improperly balanced or inadequately provided fertilizing mediums.
• Capacitation, or the activation of spermatozoa, is necessary for the sperm to penetrate the protective layer around the egg, known as the zona pellucida.

The Importance of the Oviduct

• The stallion’s spermatozoa does successfully complete capacitation in vivo, or within the female horse’s body.
• The oviduct, or fallopian tube, and its secretions provide the environment that reliably supports and regulates the interaction between the sperm and egg needed for fertilization.
• However, the details of this interaction and what makes the oviduct environment conducive for fertilization are not fully understood.

Role of the Oviduct’s Secretions

• Fertilization is thought to be regulated by certain events triggered by the oviduct’s secretions. These events include the release of sperm from the oviduct’s epithelial cells and the onset of sperm capacitation.
• However, our understanding of these secretion-triggered events is vague, since few of the secreted factors have been identified and studied.

Acid-Base Balance and its Impact

• The oviduct’s acid-base balance plays an important part in sperm capacitation.
• Stallion sperm feature a protein tyrosine phosphorylation that is activated on exposure to alkaline secretions within the oviduct.
• This acid-base balance and resulting phosphorylation is important for both hyperactivated motility, the vigorous movement necessary for the sperm to penetrate the egg, and release from the oviduct onto the egg. However, this process is not enough to induce the acrosome reaction, the event in which the sperm penetrates the egg, and ultimately fertilization.

Inference and Conclusion

• The researchers concluded that to develop a successful IVF method for horses, we need to further study and replicate the equine oviduct environment.
• Specially, we need to further understand the triggers within the oviduct that lead to successful capacitation of the stallion spermatozoa.