Development of the gonadotrophic and somatotrophic axes of sheep.

The hypothalamo-pituitary-gonadotrophic axis develops in the sheep fetus from midgestation to late gestation. The GnRH neuronal centres seem to be fully developed in the fetus and their localization complies with the adult pattern. Pituitary gonadotrophs are responsive to exogenous GnRH and release LH and FSH in a pulsatile fashion; the highest concentrations in plasma are found during late gestation. In sheep, maturational changes of this axis continue through to the prepubertal period. The GnRH neuronal system is established at about 12 weeks of age. The pattern of LH and FSH release is characteristic for each gonadotrophin depending on age and sex. The responsiveness of the gonadotrophs to GnRH increases up to 3 weeks of age. It is concluded that the changes in morphology and physiology of the hypothalamo-pituitary-gonadotrophic axis reflect the progressive maturation of the central mechanisms involved in the control of gonadotrophin secretion throughout fetal and prepubertal growth in sheep. Development of the hypothalamo-pituitary-somatotrophic axis begins in the fetus around mid-gestation. The central regulation of growth hormone (GH) in the fetus probably has a dual character, although the growth hormone releasing hormone (GHRH) neuronal system has not yet been observed in sheep. The somatostatin neuronal system develops in diverse neuronal centres in the fetus. The somatostatin centre involved in hypophysiotrophic functions does not develop fully before birth and is established over the first 10 weeks after birth. Plasma GH concentrations are very high in the fetus and fall suddenly in the perinatal period, and after a temporary increase they decline with age.(ABSTRACT TRUNCATED AT 250 WORDS)