Ultrastructural immunogold localization of vimentin and S-100 protein in guinea pig pars tuberalis

Ultrastructural features of vimentin- and S-100 protein-positive cells in guinea pig pars tuberalis were examined by immunoelectron microscopy with the postembedding immunogold method. Interstitial cells that exhibited elongated cytoplasm and partly surrounded the specific secretory cells were filled with intermediate filament bundles on which immunogold particles for vimentin were densely located. Small colloid-containing follicles were occasionally distributed in the cranial region surrounding the median eminence. The follicular cells lining the luminal cavities contained considerable amounts of intermediate filaments immunoreactive for vimentin. Some specific secretory cells displayed various amounts of intermediate filaments immunoreactive for vimentin, which were scattered throughout the cytoplasm or concentrated around the nucleus. The specific secretory cells in which many parallel profiles of rough endoplasmic reticulum (RER) or a large number of mitochondria were dispersed throughout cytoplasm, i.e., being highly metabolically active, were devoid of intermediate filaments. The ventrocaudal region in continuity with the pars distalis was occupied by cells that were packed with vesicular inclusions, considered to be dilated cisternae of RER, and frequently formed colloid-containing follicles. Immunoreactivity for S-100 protein was located exclusively in this novel cell type. Immunogold particles for S-100 protein were distributed in the cytoplasmic matrix and were also associated with the membrane of vesicular inclusions. Gold particles were also detected on the tight junctions, desmosomes, and fibril materials at the apical cell regions of the colloid-containing follicles. The finding of two separate populations of cells containing vimentin and S-100 protein, respectively, supports the idea of functional separation in the pars tuberalis.