Ontogenesis and Sexual Dimorphism of Rat Growth Hormone Messenger Ribonucleic Acid as Studied by in situ Hybridization.

Abstract We have investigated the ontogeny of growth hormone (GH) mRNA in the developing rat foetus and also from birth to adulthood. Using quantitative in situ hybridization, we studied the variations in the levels of GH mRNA during foetal and postnatal life in the pituitary of both male and female rats. A cDNA probe to GH mRNA was used to detect GH transcripts on fixed pituitary sections at different stages of development. Few labelled cells were observed in the lateral wings of the anterior pituitary from the 17th to 19th day of gestation in both sexes. The amounts of GH mRNA significantly increased in both male and female rats from neonatal to adult life, reaching the highest levels after puberty. A clear sexual dimorphism was observed at the 20th day of foetal life, GH mRNA levels being higher in male than in female rats. After birth, no significant differences of GH mRNA levels could be observed between male and female rats until 30 days of age (prepubertal period) when male rats exhibited GH mRNA levels higher than females. This sexual difference in GH mRNA levels remained constant throughout adult life. Moreover, gonadectomy performed at neonatal, prepubertal and adult periods in both male and female rats did not modify GH mRNA levels in either sex. These results indicate that an early synthesis of GH mRNA occurs in foetal pituitary and that the sexual dimorphism of GH mRNA observed from 30 days of age is not related to sex steroids.