Transient expression of S-100 by melanotropes of the rat pituitary intermediate lobe during development.

S-100, an acidic calcium-binding protein, is co-localized with vimentin in glial-like cells in the adult rat pituitary intermediate lobe. S-100 and melanotrope markers were not co-localized in the adult. During development, S-100 and vimentin were not co-localized but appeared in cells with different morphological characteristics. S-100 was co-localized with POMC mRNA and beta-endorphin during prenatal time and the first three postnatal weeks. This was demonstrated by double-label immunohistochemistry, using combinations of antisera against S-100, vimentin and beta-endorphin, and in situ hybridization histochemistry for POMC mRNA combined with immunohistochemistry for S-100. In the second and third weeks of postnatal development, S-100 was observed in fewer melanotropes and more frequently in stellate cells, which also expressed vimentin. Thus, S-100 appeared to be transiently expressed in melanotropes during prenatal and early postnatal development. S-100 serves as a neurotrophic and glial maturation factor in the CNS. Since S-100 expression in melanotropes coincides with the onset of dopaminergic innervation and morphological changes in glial-like cells of the lobe, it could have similar functions in the rat pituitary intermediate lobe.