CYTOKERATIN EXPRESSION IN NORMAL HUMAN THYMUS AT DIFFERENT AGES

Subsets of thymic epithellal cells were examined Immuno-histochemically to determine whether or not their pheno-types change during thymic growth 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 for 16 neonatal, three Infantile and one adult thymus speeimen, which had been obtained at autopsy, that were normal macroscopically and microscopicaily. CK4, CK8, CK13, CK18 and CK19 were expressed simultaneously in the cortex, medulla and subcapsular area with 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 in the cytoplasm of thymic epithelial cells. The Immunoreactivity to CK4, CK13 and CK18 were attenuated or disappeared In the subcapsular area during the early involution stage. Interestingly, two patterns of CK18 expression were observed in the neonatal and Infantile thymus tissues, which Indicated that the thymic microenvironment was changeable even under normal conditions.