DEVELOPMENTAL-CHANGES IN THE DISTRIBUTION OF PROOPIOMELANOCORTIN AND PROLACTIN MESSENGER-RNA IN THE PITUITARY OF THE OVINE FETUS AND LAMB

Ontogenic changes in pituitary pro-opiomelanocortin (POMC) mRNA and pr olactin (PRL) mRNA were examined during gestation and early neonatal l ife using in situ hybridization histochemistry. Pituitaries were harve sted from fetuses at days 60-80, 100-120, 135-140 and 142-143 of gesta tion and at term, and from lambs at days 1-7 and 30-60 of age and adul ts. POMC mRNA, present by day 60, rose during mid- and late gestation. Concurrently there was a change in corticotroph distribution, resulti ng in a relatively greater quantity of POMC mRNA at the base of the pa rs distalis. At term, there was a significant (P < 0.05) further eleva tion of POMC mRNA. POMC mRNA levels remained high in the newborn lamb but decreased in the adult. Cells in the pars intermedia expressed lar ge amounts of POMC mRNA early in fetal life and this pattern persisted throughout gestation and into the neonatal period. Changes in the exp ression of the POMC gene correlated closely with the immunoreactive (i r)ACTH in the fetuses the proportion of irACTH-positive cells rose to 10% of pars distalis cells by day 100 and did not change significantly thereafter. The lactotrophs contained PRL mRNA by day 60, and the qua ntity increased towards parturition (P < 0.05). PRL mRNA subsequently decreased in the neonate, but rose as the lamb matured. These results indicate that in the fetal pituitary: (1) the POMC gene is highly expr essed during gestation in both the pars distalis and the pars intermed ia, (2) changes in the amounts of POMC mRNA and PRL mRNA in the pars d istalis correlate with the distribution of irACTH and irPRL respective ly, and (3) POMC mRNA is distributed primarily in the inferior aspect of the pars distalis, and in this region its quantity is highest immed iately prior to parturition.