Analysis of proopiomelanocortin gene expression during prenatal development of the rat pituitary gland.

The POMC gene is transcribed in 5-10% of adult rat anterior pituitary cells and nearly all intermediate lobe cells, but the end product peptide hormones produced by these two cell populations are distinct. We have here used in situ hybridization with 3H-POMC cRNA probes to investigate the temporal and spatial distribution of POMC mRNA containing cells during development of both lobes of the adenohypophysis. These hybridization patterns were, in addition, compared to the immunocytochemical distribution of POMC-related peptides on the same or adjacent sections. In both POMC populations, POMC-mRNA detection preceded POMC-derived peptide accumulation by at least 0.5 days. POMC-mRNA was first detected in the ventral aspect of Rathke's pouch (which will give rise to the anterior lobe) at e 13, which was two days earlier than POMC transcripts were first detected in the intermediate lobe; this result provides strong evidence that factors eliciting POMC transcription in these two populations act at distinct times during development. Further, since POMC mRNA containing cells were restricted to the anterior lobe during stages when intermediate lobe-like POMC peptides have been characterized biochemically, these results confirm immunocytochemical results and demonstrate that developmentally regulated alterations in the extent of POMC posttranslational processing occur during prenatal differentiation of anterior lobe POMC cells. POMC expression in the intermediate lobe was first observed in the midcaudal region in the midsagittal plane and subsequently extended in a gradient-like pattern to the rest of the intermediate lobe. Finally, semiquantitation of in situ hybridization signals documented that increases in the relative levels of POMC mRNA/cell occur at specific ages in both anterior and intermediate lobe. POMC cell populations and demonstrated that adult POMC mRNA/cell levels are not achieved before birth in either the anterior or intermediate lobe POMC cell populations.