Pituitary development, in particular the differentiation of anterior pituitary endocrine cells, remains to be fully understood, and may have implications for adult pituitary plasticity and hyperplasia. We have used prolactin-reporter transgenic rats to characterise prolactin transcription during the appearance of newly formed lactotroph cells during fetal development.
Transgenic rats with the firefly luciferase reporter gene inserted into exon 1b of a 160 kbp human prolactin BAC enabled real-time bioluminescence imaging of fetal rat pituitary glands ex vivo. Luminescence signal was detected as early as ED16.5, with cells appearing to exist initially either as isolated single cells or as small isolated cell clusters. RT-PCR analyses confirmed the presence of Pit-1, endogenous rat prolactin and hPRL-luciferase transcripts at this gestational age. RT-PCR indicated that low-level prolactin transcription may occur as early as ED15.5. These data indicate that lactotrophs appear earlier than previously suggested. This has implications for the differentiation of lactotroph cells, which has been hypothesised to occur via a somatotroph differentiation step from the Pit-1 cell lineage. Our discovery of a limited time frame between the initiation of Pit-1 expression at ED15.5 and prolactin expression suggests that transdifferentiation from the somatotroph lineage is unlikely to be a major pathway for lactotroph differentiation. This is consistent with previous data showing that only a small proportion of lactotrophs ever express GH.
Preliminary data from bioluminescent imaging of ED16.5 fetal pituitaries suggest that transcriptional activity fluctuates with time in newly emerging lactotroph cells. We aim to extend these studies to assess whether transcription dynamics are altered during development through changes to chromatin structure and the emergence of key transcriptional regulators such as ERα. Analysis of lactotroph transcriptional activity during development may also provide insight into the emergence and role of a lactotroph cell network.