The ontogeny of pit-1 expression in the human fetal pituitary gland

Pit-1 expression has been reported to be cell-type-specific in the adult human pituitary and in human pituitary adenomas. In contrast, studies of rodent fetal adenohypophysial development as well as mature rodent glands have indicated that the pit-1 mRNA is ubiquitously expressed and the protein is under translational control. To determine the ontogeny and localization of Pit-1 expression in the human fetus, we examined fetal pituitaries (n = 23) at various stages of gestation from 6 weeks to term using in situ hybridization (ISH) for pit-1 mRNA, immunohistochemical localization of Pit-1 protein, and combined ISH for pit-1 mRNA with immunohistochemistry for pituitary hormones. At 6 and 7 weeks of gestation, the cells surrounding both limbs of Rathke's cleft showed a moderate specific signal for pit-1 mRNA. At 7 weeks, only a few cells were immunoreactive for ACTH and there was no definite colocalization of that hormone with pit-1 mRNA. At 8 and 9 weeks of gestation, there was definite preferential expression of pit-1 mRNA in cells containing growth hormone (GH) but not ACTH, as well as in cells with no detectable hormone immunopositivity. At midgestation and after, there was clear correlation between pit-1 mRNA expression and hormone content; cells with GH, prolactin and/or thyrotropin immunoreactivity had abundant pit-1 mRNA, whereas those containing ACTH, FSH or LH were negative for pit-1 mRNA by ISH and its protein by immunohistochemistry. The signal for pit-1 mRNA was stronger in intensity and present in more cells in fetal than in adult adenohypophyses. The ontogeny of pit-1 mRNA expression indicates that it precedes the onset of pituitary hormone detection. Its abundance in the human pituitary early in gestation may reflect its role in cytodifferentiation and cell proliferation. The correlation between pit-1 mRNA detection and Pit-1 protein localization is consistent with a cell-type-specific pretranslational regulatory mechanism for Pit-1 expression in the developing human adenohypophysis.