Differential distribution of S-100 protein and vimentin in the hypophyseal pars tuberalis of the guinea pig.

In the hypophyseal pars tuberalis of guinea pigs, I examined immunohistochemical localization and development of vimentin and S-100 protein in comparison with those of the pars distalis. In the pars distalis, almost all folliculostellate cells expressed intense immunoreactivity for vimentin. A subpopulation of vimentin-immunoreactive folliculostellate cells was also immunoreactive for S-100 protein. During fetal development, vimentin-immunoreactive cells appeared at mid-gestation in the pars distalis and became numerous at late stages, whereas only a few S-100 immunoreactive cells were observed even at late stages. In the pars tuberalis, a large number of vimentin-immunoreactive cells were distributed in the cranial region surrounding the median eminence and the dorsocaudal region surrounding the infundibular stalk. These cells, however, were sparse in the ventrocaudal region in continuity with the pars distalis. Conversely, dense distribution of S-100 immunoreactive cells was restricted in the ventrocaudal region. Vimentin-immunoreactive cells were elongated and were mostly distributed as solitary cells, whereas S-100 immunoreactive cells were gathered in large or small cell groups and frequently formed colloid-containing follicles. During fetal development, large cell groups immunoreactive for S-100 protein were detected at late stages in the ventrocaudal region. Therefore, in the pars tuberalis S-100 immunoreactive cells are distinct from the vimentin cells and do not correspond to the folliculostellate cells of the pars distalis.