S100 protein in folliculostellate cells of the rat pituitary anterior lobe.

The anterior lobe of the rat pituitary was investigated immunohistochemically as to nervous tissue-specifis S100 protein. Granulated cells, both acidophils and basophils, were not stained with peroxidase-labelled anti-S100 rabbit serum (Fab) by the direct staining method. Only stellate-shaped cells with long slender cytoplasmic processes contained nervous tissue-specific S100 protein in the nucleus and cytoplasm, the presence of which was verified by both light and electron microscopic immunohistochemistry. Folliculostellate cells were devoid of specific secretory granules and in areas formed follicles with numerous microvilli and junctional complexes. They often extended long cytoplasmic processes between granulated cells. Ouchterlony double diffusion tests confirmed the presence of S100 protein in the pars distalis of rat pituitary glands. These findings lead to the hypothesis that folliculostellate cells may belong to the neuroectodermal cells and may be specialized cells playing some unknown role in rat adenohypophysis.