Ontogeny of prolactin-secreting cells during chick embryonic development: effect of vasoactive intestinal peptide

It was previously reported that prolactin (PRL)-containing cells differentiate by days 15 to 19 of embryonic development and that vasoactive intestinal peptide (VIP) increased plasma PRL concentrations and pituitary PRL protein and mRNA levels in vivo. In the present study, anterior pituitaries derived from day 15, 16, 17, 18, and 19 embryos were subjected to reverse hemolytic plaque assays (RHPA) for chicken PRL to determine the ontogeny of lactotrophs. We found that PRL secreting cells were first consistently detected on day 17 of embryonic development, indicating that lactotroph differentiation during normal development occurs by this age. However, extended treatment in the RHPA with VIP exposed lactotrophs as early as day 15, suggesting that a lactotroph precursor population was present earlier. Next, primary cultures of embryonic anterior pituitary cells from day 12, 13, 14, 15, 16, and 17 embryos were incubated for 0, 2, or 4 days in serum-free medium or medium supplemented with 10 nM VIP. After the culture periods, cells were subjected to RHPAs for chicken PRL. PRL-secreting cells differentiated spontaneously by 2 days in culture for day 15 and 16 cells and by 4 days in culture for all embryonic ages tested, except day 17. Culturing with VIP for 2 days did not increase PRL-secreting cells at any embryonic age tested, whereas VIP treatment for 4 days increased lactotroph numbers at all ages except day 12. Cells from days 15, 16, and 17 were responsive to VIP in the RHPA after 2 days in untreated cultures, but after 4 days only day 17 cells continued to respond to VIP in the RHPA. Treatment with VIP in culture for 4 days maintained VIP responsiveness in the RHPA for cells derived from embryos as early as day 14. We conclude that lactotroph differentiation during normal chicken development occurs by embryonic day 17. Moreover, our results indicate that responsiveness to VIP is an early event in PRL cell development and that lactotroph differentiation may be stimulated in vitro by VIP as early as embryonic day 13.