Characterization of TSH-positive cells in foetal rat pars tuberalis that fail to express Pit-1 factor and thyroid hormone beta2 receptors

The thyroid stimulating hormone (TSH)-immunoreactive cells (TSH cells) in the pars tuberalis (PT-TSH cell) of the male rat pituitary gland show an intense spot-like TSH immunoreaction in the paranuclear cytoplasm. However, the ontogenic origin and characteristics of these spot-like stained PT-TSH cells remain to be elucidated. The present study was designed to investigate the distribution and characteristics of PT-TSH cells in the foetal and adult rat pituitary by immunostaining for Pit-1 factor and thyroid hormone receptors (TRs) and reverse transcriptase-polymerase chain reaction (RT-PCR). TSH cells first appeared in the PT at 15.5 days of gestation and were either stained diffusely throughout the cytoplasm or displayed a strongly stained, spotty appearance in the paranuclear region. By 15.5 days of gestation, the rostral part of the PT consisted of columnar epithelium, in which TSH immunoreactivity was spot-like in the apical region of cytoplasm corresponding to the Golgi apparatus. At the 16.5 days of gestation, TSH cells were present in the pars distalis (PD); however, the cells were mostly larger and polygonal with strong staining throughout the cytoplasm. These differences between the PT and PD were retained throughout foetal and neonatal rat development. The TSH cells in the PD of the adult or gestational rat were observed to contain Pit-1 factor by double immunostaining. However, TSH cells in the PT lacked Pit-1 factor. RT-PCR confirmed the absence of Pit-1 and TRbeta2 mRNA in the PT of the adult and late gestation rat pituitary gland. These results suggest that apparently distinct types of TSH cells in the PT develop independently from TSH cells in the PD.