CYTOKERATIN EXPRESSION IN NORMAL HUMAN THYMUS AT DIFFERENT AGES

Subsets of thymic epithelial cells were examined immuno-histochemicall y to determine whether or not their phenotypes change during thymic gr owth and at early involution in terms of cytokeratin (CK) expression. Five monoclonal antibodies specific for CK4, CK8, CK13, CK18 and CK19 were used and applied to 16 neonatal, three infantile and one adult th ymus specimen, which had been obtained at autopsy, that were normal ma croscopically and microscopically. CK4, CK8, CK13, CK18 and CK19 were expressed simultaneously in the cortex, medulla and subcapsular area w ith the exception of CK4, which showed expression on the adult thymus. Light and electron microscopy showed that CK8 and CK19 expression was overlapped. Thus, it was thought that CK8 and CK19 formed complexes i n the cytoplasm of thymic epithelial cells. The immunoreactivity to CK 4, CK13 and CK18 were attenuated or disappeared in the subcapsular are a during the early involution stage. Interestingly, two patterns of CK 18 expression were observed in the neonatal and infantile thymus tissu es, which indicated that the thymic microenvironment was changeable ev en under normal conditions.