Ontogeny of pro-opiomelanocortin (POMC) gene expression and translated products adrenocorticotrophin (ACTH) and alpha-melanocyte stimulating hormone (alpha-MSH) in the ovine fetal pituitary gland.

This study sought to determine the ontogeny of pro-opiomelanocortin (POMC) mRNA by in situ hybridization, and the expression of alpha-melanocyte stimulating hormone (alpha-MSH) and adrenocorticotrophin (ACTH) by immunohistochemistry, in the ovine fetal pituitary gland. Detection of POMC mRNA, and of ACTH and alpha-MSH immunoreactivity was first noted at Day 40 of gestation in the pars intermedia (PI) and the pars distalis (PD). After Day 70, alpha-MSH immunoreactivity was confined to the PI whereas POMC mRNA and ACTH immunostaining remained in both the PI and the PD. Increases (P < 0.05) of POMC mRNA expression were observed in the PI between Days 50-70 and levels then remained constant to Day 141 (term, 145 days). In the basal region of the PD, POMC mRNA expression was high at Day 40, declined (P < 0.01) by Day 50 and then increased progressively to Day 141 of gestation. The proportion of ACTH-immunopositive cells in the PD also fell from 14% at Day 40 to 11.4% at Day 70 and then increased to 15.3% by Day 141. In contrast, neither the level of POMC mRNA expression nor the percentage of corticotrophs changed in the region of the PD immediately adjacent to the PI. These data provide evidence for differential processing of POMC in a tissue-specific manner during early fetal pituitary development.