Fetal and maternal gonads and gonadotropins in the pony.

This research examined the concentrations of hormones FSH and LH during the gestation period of seven pony mares from approximately day 100 until birth. Additionally, over a three-year period, 125 pregnant pony reproductive tracts and fetuses were investigated. The study found varying hormone concentrations across different stages of pregnancy and suggested a functional relationship between gonadal hormones and pituitary LH in the pony fetus that requires further examination.

Hormone Analysis During Pregnancy

• The research closely monitored seven mares during their gestation period, which was normalized to 332 days for the purpose of the study.
• The researchers could not determine LH concentrations prior to the 200th day due to interference from the Pregnant Mare Serum Gonadotropin (PMSG). Post day 200, the concentrations were low and consistent.
• FSH concentrations, on the other hand, varied significantly among the mares. Some experienced large elevations irregularly during their pregnancy.
• On the day of parturition (birth), the concentration of FSH was higher in the mare compared to the foal, whereas LH concentration was typically higher in the foal.

Analysis of Reproductive Tracts and Fetuses

• Over three years, 125 pregnant pony reproductive tracts and fetuses were collected and examined.
• It was found that mare ovarian, follicular, and luteal activities were highest between days 50 and 150 of pregnancy, aligning with the period when PMSG concentrations were the highest.
• The number of corpora lutea (release sites of mature eggs in ovaries) during pregnancy was found to be greater in mares bred in spring than those bred later.
• Regression analysis revealed that fetal gonad weight was greatest between days 220 and 250 of pregnancy, and fetal pituitary weight increased continuously throughout the pregnancy.
• There was an increase in fetal pituitary LH concentrations between days 90 and 150, which then remained relatively constant. This is aligned with the period of rapid growth in fetal gonads.
• As fetal gonad weights reached a maximum, the fetal blood LH concentrations declined.

Hypothesized Relationship and Further Research

• The study suggested a functional relationship between gonadal hormones and pituitary LH in the pony fetus, based on the temporal associations observed, but emphasized that more detailed study is needed.
• FSH hormone levels were below the detectable limit in either fetal pituitary or fetal blood throughout pregnancy, though it was measurable on the day of birth in the newborn foal.