Development of the embryonic chicken thymus: II. Differentiation of the epithelial cells studied by electron microscopy

Abstract

The differentiation of thymic epithelial cells in embryonic chickens was studied ultrastructurally from the tenth day of incubation to hatching. Reticuloepithelial cells first displayed functionally differentiated features between the twelfth and thirteenth days of incubation, whereas medullary epithelial cells with cysts and/or numerous granules did not display such features until the fifteenth day of incubation or later. Between the twelfth and thirteenth days, reticuloepithelial cells came to possess characteristic electron-lucent vacuoles and dense bodies. Furthermore, some of the vacuoles, probably as a result of fusion, seemed to contain the electron-dense bodies. The density of the material of such inclusion vacuoles tended to decrease after its entrance into the vacuoles. These findings suggested that the material in the inclusion vacuoles may be released into cytoplasm and, thereafter, may be secreted to exterior of the cells. Since a series of distinct morphological changes of reticuloepithelial cells took place simultaneously with an abrupt increase of lymphoctes bearing the embryonic thymus-specific antigen (T antigen), it is likely that the material supposedly released from the inclusion vacuoles of reticuloepithelial cells could be responsible for differentiation of lymphoid cells into embryonic T-antigen-bearing lymphocytes.