Source of oestrogen in early pregnancy in the mare.

This study explores oestrogen secretion in early pregnancy in mares, using three different groups of pregnant mares under different conditions to understand the source of this hormone.

Research Study Groups

In the study, researchers observed three distinct groups of pregnant mares:

• Group I: Six mares that had been ovariectomized (ovaries removed) on Day 18-19 of gestation and whose pregnancies were sustained by a daily oral progestagen, altrenogest.
• Group II: Nine untreated, pregnant mares.
• Group III: Five intact, pregnant mares treated daily with altrenogest.

Methodology and Observations

Oestrogen secretion was determined by analysing oestrogen conjugate (EC) in the urine of the mares. The EC concentrations were examined and compared across the three groups at different gestation stages.

• In Group I, researchers noticed a steady linear increase in mean EC concentrations from 17 ng/mg Cr on Day 20 to 291 ng/mg Cr on Day 70, without an observable surge in oestrogen secretion on about Day 39.
• However, the intact mares in Groups II and III showed different results. Between Days 20 and 33 of gestation, mean urinary EC concentrations rose quickly from 172 (77) ng/mg Cr on Day 33 to a peak of 1066 (895) ng/mg Cr on Day 39, followed by a decrease to 637 (719) ng/mg Cr on Day 44. After Day 44, EC concentrations continued to increase in a linear way, reaching 1191 (842) ng/mg Cr on Day 70.

Conclusion

Overall, the study found a significant difference in oestrogen secretion between ovariectomized mares and intact mares either untreated or treated with altrenogest. The intact mares showed a rapid increase and then decline of EC concentrations around Day 39, followed by a continued linear increase; however, ovariectomized mares displayed a consistent linear growth without any surge.

This study suggests that the ovaries play a role in the surge of oestrogen secretion around Day 39 of gestation in pregnant mares. However, even when the ovaries are removed, oestrogen continues to steadily increase, suggesting that other sources also contribute to oestrogen secretion in pregnant mares.