Cytoplasmic localization of FTS (facteur thymique sérique) in thymic epithelial cells. An immunoelectronmicroscopical...

This study was concerned with the localization of the facteur thymique sérique (FTS) by immunoelectronmicroscopy in cultured thymic epithelial cells. The FTS antibodies labelled differently sized cytoplasmic vacuoles containing a substance of variable density. This substance occurs occasionally in crystalline structures which reacted strongly with purified FTS antibodies. On the ultrastructural level the FTS-positive cells were demonstrated to fix specifically keratin antibodies and to contain tonofilaments. Some of these cells originated from the thymic medulla proved by the presence of microvilli-forming cytoplasmic vacuoles. The existence of FTS-positive crystalline inclusions confirmed the hypothesis of a storage process in the cytoplasm of thymic epithelial cells in vitro or in pathological conditions.