Localization of sex steroid receptor cells, with special reference to thymulin (FTS)-producing cells in female rat thymus

Using monoclonal antibodies against progestin receptors (PR) and estrogen receptors (ER), and polyclonal antibodies to thymulin (FTS) and keratin, localization of the sex steroid receptors was studied immunohistochemically in ovariectomized estrogen-treated rat thymus, with special reference to FTS-producing cells. Both ER- and PR-immunostained cells were mainly localized in the medullary region, especially at its periphery (i.e., the corticomedullary junction). A few cells were also situated in the subcapsular area. They were medium- to large-sized and had a dendritic cell process, some of which were immunohistochemically keratin- and FTS-positive, indicative of reticuloepithelial (RE) cells. Hassall's corpuscles were also receptor-positive and FTS-positive. T-cells were not immunostained with anti-ER, anti-PR or anti-FTS. Light microscopically, both ER and PR immunostainings were localized in the cytoplasm and/or nucleus of keratin-stained RE cells. Electron microscopically, both steroid receptors were shown more precisely to distribute as aggregates of osmiophilic black dots on polysomes and perinuclear space in the cytoplasm and on the euchromatin area in the nucleus. These results suggest that the sex steroids E and P exert their effects through receptors within RE cells which produce FTS to regulate T-cell differentiation.