Thymic epithelial cysts in the pigeon.

Medullary epithelial cells forming cysts in the thymus of pigeon chicks were examined in order to clarify these morphological characteristics by immunohistochemical and electron microscopic techniques. Light microscopically, cysts were divided into intracellular and intercellular types. Intracellular cysts were positive for the periodic-acid Schiff reaction, while the intercellular type was not. With immunohistochemical staining for keratin, the edges of the intercellular cysts were moderately stained, whereas the cytoplasm of epithelial cells was weakly positive. Heterophils, eosinophils and lymphocytes were included in most of the intercellular cysts. Ultrastructurally, there were two types of epithelial cells which contained intracellular cysts densely packed with microvilli. They differed with regard to the shape of the rough endoplasmic reticulum and bundles of keratin filaments, and in the presence of large vacuoles. The walls of the intercellular cysts were composed of a single type of epithelial cell with a pale nucleus and bundles of keratin filaments. Small vesicles measuring 150 to 200 nm in diameter were numerous in the apical cytoplasm beneath the microvilli. In intercellular cysts, there were abundant heterophils, eosinophils, lymphocytes and macrophages which mostly appeared to be living cells. This evidence suggests that the intercellular cysts might provide a specific microenvironment for leukocytes in the pigeon thymus.