The Endocrinology of equine parturition.

This research explores the endocrine processes involved in equine birth, focusing on the hormonal changes and interactions between the mother and fetus in the period leading up to and during delivery. It notes unique characteristics in these processes in horses compared to other mammals.

Understanding Endocrine Processes in Equine Parturition

• The researchers discuss how successful delivery of young depends on coordination between fetal maturation and the onset of labour. They indicate that a cascade of endocrine events between the fetus and mother underscore this process.
• While there are similarities between different mammals in this process, unique aspects in placental hormone synthesis, and variations in timing and magnitude of key prepartum endocrine changes are noted between different species, such as horses.
• The paper points out that unlike many other farm animals where maternal progesterone levels decline and oestrogen levels increase days before delivery in response to fetal hypothalamic-pituitary-adrenal (HPA) axis activation, these hormonal changes are complex and different in horses.

Features Unique to Equine Parturition

• During late gestation in mares, the progesterone levels are almost negligible − unlike other animals. Both progestagens and oestrogens are produced by a feto-placental unit, which uses precursors supplied by the fetus to synthesise various C21 and C18 steroids, many exclusive to horses. Consequently, the regulation of uterine quiescence and activation grows more complex in horses nearing term.
• The study emphasizes that the concentrations of progestagen rise and oestrogen levels decrease in the mare during the last gestation phase (20 – 30 days) and only start showing the changes typical of impending delivery present in other species in the last one or two days before delivery.
• Also, a late surge in cortisol occurs in fetal horses compared to other species, owing to activation of the fetal HPA axis. Initially, pregnenolone (P5), not cortisol, appears to be the primary product of the fetal equine adrenal. This leads to increased progestagen production by the uteroplacental tissues, maintaining uterine quiescence against increasing uterine stretch caused by the quickly growing fetus. As delivery nears, an increase in fetal ACTH levels initiates a switch from P5 to cortisol production in the fetal equine adrenals.

Implications and Future Research

• This late cortisol surge contributes to rapid fetal maturation and possibly to increased uteroplacental production of oestradiol-17 beta and prostaglandin (PG). With the reduction in P5, there may be a decrease in uteroplacental progestagen production, removing the block on myometrial contractility. Increased PG secretion then stimulates myometrial contractions, which lead to oxytocin release through a neuroendocrine reflex. Overall, the endocrine regulation of equine parturition includes progestagens, oestrogens, PGs and oxytocin, similar to other species.
• The study concludes by emphasizing a need for further research to understand the causes and consequences of the fluctuating maternal progestagen levels, and to determine how much fetal HPA axis activation contributes to the initiation of equine labour.