Prolactin-synthesizing and prolactin-releasing activity of fetal and early postnatal rat pituitaries: in vivo and in vitro studies using RIA, reverse hemolytic plaque assay and immunocytochemistry.

In vivo and in vitro prolactin (PRL)-synthesizing and PRL-releasing activity of fetal (days 12-22) and early postnatal (days 1-10 after birth) rat pituitaries were studied by means of radioimmunoassay (RIA), reverse hemolytic plaque assay and immunocytochemistry. Using RIA, PRL could first be detected, both in the pituitary and in the serum, on day 17 of fetal development. From this day on, pituitary PRL gradually increased, the rise was particularly marked during the postnatal period and became depressed for the first 10 days of postnatal life. On fetal day 18, 12-15% of monodispersed pituitary cells displayed PRL immunopositivity, but only 3-5% of PRL-positive cells were plaque-forming, i.e. released PRL. By the end of gestation 19-25% and on postnatal day 10 42-45% of all pituitary cells were PRL cells and 31-35 and 15-17% of PRL-positive cells, respectively released PRL. Both pre- and postnatal PRL cells in monolayers were insensitive to TRH treatment. Pituitary primordia immunocytochemically and radioimmunologically negative for PRL (13- to 14-day-old fetal) when placed in serum-free organ culture were able to synthesize and release PRL. Fetal pituitary exhibited a highly regular increasing pattern of daily PRL release during a 7-day-culture period. Data obtained both in vivo and in vitro did not exhibit any sex differences. The present findings are consistent with all those observations suggesting an early emergence of fetal rat pituitary lactotrophs. The in vitro results support the concept that Rathke's pouch cells have substantial degree of independence from extrapituitary regulatory actions in the expression and further progression of specific functions.(ABSTRACT TRUNCATED AT 250 WORDS)