Hormone Ontogeny in the Ovine Fetus. I. Circulating Growth Hormone in Mid and Late Gestation**

Abstract

Ovine GH (oGH) was determined by homologous RIA on 118 samples from 45 chronically catheterized ovine fetuses from 66 days of gestation to term (147 days). A triphasic pattern of oGH concentration was demonstrated with high circulating GH concentrations (148.0 ± 9.6 ng/ml) at 60–70 days gestation, falling by 100–110 days (49.0 ± 7.3 ng/ml), increasing again to peak at 130-140 days (150.4 ± 10.5 ng/ml), and again decreasing to lower concentrations characteristic of the neonate before delivery. In samples obtained during fetal surgery (n = 63), a similar pattern was observed, but after 120 days of gestation, 15 of 36 plasma oGH values were above the upper limit of the normal range calculated from the chronic group. Similar findings were observed in 42 samples from fetuses that had developed hypoxia or acidosis, suggesting that stress-related GH release develops in late gestation. Maternal GH concentrations did not correlate with gestational age and were low throughout pregnancy (4.4 ± 0.7 ng/ml). The pattern of GH in the ovine fetus is compared to that of the human fetus. In both species, high fetal GH concentrations were noted in midgestation, followed by a subsequent fall in plasma GH; however, in the sheep, in contrast to the human, a second peak of plasma GH was noted in late gestation. It is postulated that in both species, the changing pattern of fetal plasma GH represents different stages in the development of hypothalamic regulation of the fetal pituitary.