Fetal and maternal gonads and gonadotropins in the pony

Abstract

Concentrations of FSH and LH were determined from approximately Day 100 of pregnancy to parturition for a group of 7 pony mares. Gestation length was normalized to 332 days for all mares. The concentrations of LH could not be determined prior to Day 200 due to PMSG interference. After Day 200, LH concentrations were low and constant. FSH concentrations were inconsistent among mares, with large elevations occurring in some mares at irregular intervals during pregnancy. On the day of parturition, the concentration of FSH was higher in the dam than in the foal and LH tended to be higher in the foal than in the dam. During a 3 year period, 125 pregnant pony reproductive tracts and fetuses were collected at a slaughterhouse. Mare ovarian, follicular and luteal activity was greatest during the time period (Days 50-150) when PMSG concentrations were greatest and thereafter decreased to very low activity during the last third of pregnancy. Numbers of corpora lutea during pregnancy appeared to be greater for spring-bred mares than for late-bred mares. Regression analysis indicated that fetal gonad weight was greatest during Days 220-250 of pregnancy and that fetal pituitary weight increased throughout pregnancy. Fetal pituitary LH concentrations increased between Days 90-150 and then remained relatively constant. Fetal blood LH concentrations were highest between Days 100-150 of pregnancy during a period of rapid growth of fetal gonads, and fetal blood LH declined as fetal gonad weights reached a maximum. These temporal associations are compatible with the hypothesis of a functional relationship between gonadal hormones and pituitary LH in the pony fetus, but more critical study is needed. FSH was not detectable (below assay sensitivity) in either fetal pituitary or fetal blood throughout pregnancy, although it was measurable on the day of birth in the newborn foal.