The corpus luteum: source of oestrogen during early pregnancy in the mare.

The aim of the study involved 30 pregnant mares, divided into three groups, was to discover the main source of oestrogen during the early stages of pregnancy by measuring levels of progesterone, chorionic gonadotrophin, and oestrogen conjugate. The research revealed that the corpus luteum was necessary for increased oestrogen secretion during the 35 to 50-day timeframe.

Research Procedure and Grouping

• The study involved 30 pregnant mares which were divided into three groups–Group 1, Group 2, and Group 3–consisting of 10 mares each.
• The mares in Group 1 served as control subjects, while the mares in Group 2 and Group 3 received different treatments.
• Mares in Group 2 were administered with 44 mg/day of altrenogest from the 16th day to the 80th day, and Group 3 mares were given a luteolytic dose of PGF2 alpha on Day 16 followed by the same dosage of altrenogest until Day 80.

Research Findings

• Between Days 16 and 80 of gestation, concentrations of progesterone and chorionic gonadotrophin (CG) in plasma and oestrogen conjugate (OC) in urine of the mares were measured.
• In Group 3, all 10 mares experienced complete luteolysis after PGF2 alpha administration. Six out of these did not have an active CL (corpus luteum) until after Day 60 of pregnancy (Group 3a), while the remaining 4 mares developed a new CL (Group 3b).
• In Groups 1 and 2, an escalation in oestrogen secretion was observed between Days 35 and 40. There was, however, a significant decrease in oestrone concentrations in Group 3a mares without an active CL compared to Groups 1 and 2. Also, oestrogen secretion did not show any significant increase between Days 20 and 50 in Group 3a.
• For Group 3b, OC levels stayed low in the absence of an active CL. However, after the commencement of CG secretion, OC levels increased in parallel with the development of a new CL. They reached similar values to those observed in Groups 1 and 2.

Conclusions

• The results of this experiment demonstrated that to see an increase in oestrogen secretion between Days 35 and 50 of pregnancy, the presence of an active corpus luteum is necessary.