Ontogenesis of pituitary prolactin in the human fetus

To study the ontogenesis of PRL synthesis and secretion in the human fetus, PRL mRNA was measured in 12 pituitaries from fetuses of 16-27 weeks of gestation by hybridization of cytosol RNA with 125I-labeled single stranded complementary DNA (cDNA). Pituitary PRL content and serum PRL concentration were assessed by RIA. Pituitary weight increased with fetal age, ranging from 12 mg at 16 weeks of gestation to 41 mg at 27 weeks. The increased weight was due to an increase in cell number. PRL mRNA content did not change during weeks 16-20 of gestation. However, it increased rapidly after 21 weeks, reaching 11.5 times the 16-20 week value at 27 weeks. Pituitary PRL content was constant at a low level until 21 weeks, but thereafter it increased markedly. The increase was greater than that in PRL mRNA, and therefore, the PRL to PRL mRNA ratio was 10-fold greater at 27 weeks of gestation. Serum PRL concentrations also gradually increased after 21 weeks of gestation. These results indicate that marked increases in fetal pituitary PRL synthesis and release occur after 21 weeks of gestation.