Early Expression of the Glycoprotein Hormone Alpha-Subunit in the Pars Tuberalis of the Rat Pituitary Gland during Ontogenesis

The expression of the alpha-subunit of the glycoprotein hormones was studied in the developing rat pituitary by in situ hybridization and immunocytochemical techniques. The alpha-subunit mRNA labelling was detected in Rathke's pouch at 12 days' gestation (E12); at E13, it was exclusively concentrated in the pars tuberalis primordium (the lateral lobes) where its intensity rapidly increased at following stages. The alpha-subunit was first immunocytochemically detectable in the pars tuberalis at E14. Labelling of the whole pars tuberalis primordium with both techniques indicated that the specific glandular cells were involved. Immunocytochemical detection of bromodeoxyuridine, injected into the pregnant rat 4 h prior to fixation, revealed a total arrest of cell divisions in the pars tuberalis between E13 and E17, which was not the case in the pars distalis or the pars intermedia at any stage. In the pars distalis, the onset of alpha-subunit expression was detected much later than in the pars tuberalis, at E16-17 by in situ hybridization and at E17-18 by immunocytology. The functional significance of the early secretory differentiation of the pars-tuberalis-specific cells remains to be established.