Characterization of TSH-positive cells in foetal rat pars tuberalis that fail to express Pit-1 factor and thyroid hormone beta 2 receptors

The thyroid stimulating hormone (TSH)-immunoreactive cells (TSH cells) in t he pars tuberalis (PT-TSH cell) of the male rat pituitary gland show an int ense spot-like TSH immunoreaction in the paranuclear cytoplasm. However, th e ontogenic origin and characteristics of these spot-like stained PT-TSH ce lls remain to be elucidated. The present study was designed to investigate the distribution and characteristics of PT-TSH cells in the foetal and adul t rat pituitary by immunostaining for Pit-1 factor and thyroid hormone rece ptors (TRs) and reverse transcriptase-polymerase chain reaction (RT-PCR). T SH 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, t he rostral part of the PT consisted of columnar epithelium, in which TSH im munoreactivity was spot-like in the apical region of cytoplasm correspondin g to the Golgi apparatus. At the 16.5 days of gestation, TSH cells were pre sent in the pars distalis (PD); however, the cells were mostly larger and p olygonal with strong staining throughout the cytoplasm. These differences b etween the PT and PD were retained throughout foetal and neonatal rat devel opment, The TSH cells in the PD of the adult or gestational rat were observ ed 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 TR be ta 2 mRNA in the PT of the adult and late gestation rat pituitary gland. Th ese results suggest that apparently distinct types of TSH cells in the PT d evelop independently from TSH cells in the PD.