The influence of maternal size on placental, fetal and postnatal growth in the horse. II. Endocrinology of pregnancy.

The research explores how the size of a horse maternal breed influences hormonal changes during pregnancy. The study showed that various hormonal concentrations during pregnancy depend on the breed of the mare and the breed of the fetus.

Experiment Setup

• The experiment was established using within-breed artificial insemination and between-breed embryo transfer.
• The horses chosen for the experiment were small pony mares (P) and large Thoroughbred mares (Tb) to form four types of horse pregnancy. The categories were designed in such a manner that the unborn horse either experienced spatial and nutritional deprivation, luxury, or normality while in the womb.

Hormonal Monitoring and Discoveries

• Throughout the gestation period, samples of peripheral serum were taken from all mares. Three types of hormones were focused on in the measurements: equine chorionic gonadotrophin (eCG), total conjugated oestrogens and progestagen.
• The researchers discovered that the amount of eCG produced in the first part of gestation relied on the mare’s breed, not the fetus breed. Conversely, the total amounts of oestrogens produced were significantly higher when a Thoroughbred fetus was gestated irrespective of the breed of the mare. This suggests that a Tb fetus might have larger gonads than a P fetus, thus leading to the secretion of more C-19 precursor steroids, which then convert to oestrogens in the placenta.
• In the last weeks of gestation, plasma progestagen concentrations rose much earlier, to significantly higher levels in Tb-in-P than in P-in-Tb pregnancies, indicating increased fetal stress in the former. This stress caused premature maturation of the fetal adrenal gland, leading to an increased secretion of pregnenolone, which the placenta converts to progestagens.

Implications

• The study offers insights for breeders, veterinary specialists, and researchers working with horse breeding.
• Understanding the hormonal changes that occur as a result of the interaction between different mare and fetus breeds can be critical in the management of horse pregnancies and the postnatal health of the foals.
• The findings suggest that the physiological demands of a fetus can potentially be met differently depending on the size of the mare carrying it, thus affecting the fetus’s growth and development.