The Pars tuberalis of the monkey (Macaca fascicularis) hypophysis: cell types and hormone expression

The pars tuberalis (Pt) of most mammalian species contains specific cells which are structurally and functionally different from the pars distalis (Pd) cells. Pt-specific cells possess melatonin receptors and reveal morphological changes dependent on the duration of the photoperiod. Furthermore, in hamsters the transmission of photoperiodic stimuli to the endocrine system is influenced by melatonin, an effect which is likely to be mediated by Pt-specific cells. In monkeys, however, only little is known about this cell type. Therefore, we studied the ultrastructural differentiation of Pt-specific cells and describe the expression of different hormones and their mRNA by immunohistochemistry and in situ hybridization. Apparently the Pt consists of (1) cells similar to gonadotropic cells of the Pd, (2) folliculostellate cells and (3) a cell population which is morphologically and functionally clearly distinct from all other cell types found in the Pd. Morphologically they resemble the Pt-specific cells found in other species. Regarding the expression of secretory products there is evidence that they transcribe and translate the beta-TSH subunit. Although there is a strong signal for the mRNA of the common alpha-chain, protein staining is much weaker. POMC mRNA is expressed in the Pt while there is no evidence for PRL mRNA. The present results lead to the conclusion that the Pt of the monkey contains Pt-specific cells which express different hormonal subunits as was already shown for other species. In context with previous findings of melatonin receptors in the monkey Pt further investigations are necessary to establish the possible role of Pt-specific cells in the photoperiod-dependent generation of endocrine rhythms.