The role of relaxin in mare reproductive physiology: A comparative review with other species.

This research examines the role of the hormone relaxin in the reproductive physiology of horses (mares), particularly during pregnancy. It makes a comparative study with other species and discusses the potential for this hormone to assist in the healthy development and successful birth of a foal.

Role of Relaxin in Pregnancy

• The study identifies relaxin as a peptide hormone primarily known for its role during the latter half of pregnancy. It’s mostly recognized for softening pelvic ligaments to prepare for birth.
• The source of relaxin during early pregnancy differs across species. In many species, the corpus luteum is the main source. In late equine pregnancy, it’s the placenta.
• The study suggests that in mares with impaired placental function, relaxin levels decrease before an abortion.
• During early pregnancy, relaxin supports endometrial angiogenesis by augmenting endometrial expression of vascular endothelial growth factor.

Unique Aspect of Relaxin in Horses

• In horses, the conceptus expresses relaxin mRNA as early as 8 days post-ovulation, with levels rising as conceptus development continues.
• While the secretion of functional relaxin has not been confirmed, it appears probable given that the embryo also expresses transcripts coding for enzymes processing the prohormone to create the mature hormone.

Relaxin and Its Role in the Ovulatory Process

• Follicular expression of relaxin is relatively similar among follicles of varying sizes in mares. It has been identified in granulosa and theca cells.
• In humans and pigs, relaxin seems to stimulate follicular development. In rats, it is implicated in the ovulatory process.
• In horses, relaxin likely plays a similar role in the ovulatory process since it promotes the secretion of gelatinases and tissue inhibitors of metalloproteinases, which are necessary for local proteolysis of the follicular wall—a key step in the ovulatory process.
• However, there are no functional studies addressing relaxin’s role in the ovulatory process in mares explicitly.

Overall, this study highlights the vital role of relaxin in equine reproductive physiology and hints at potential areas for further research to further understand this hormone’s function. People who raise and breed horses, veterinarians and animal researchers might find this information useful.