CYTOKERATIN EXPRESSION IN HUMAN THYMUS - IMMUNOHISTOCHEMICAL MAPPING

Cytokeratin expression in normal postnatal human thymus was studied im munohistochemically by using monoclonal antibodies against various cyt okeratin polypeptides. An attempt was made to characterize cell popula tions giving rise to the cornified structures of Hassal's corpuscles. Monoclonal antibody KB-37, a marker of squamous epithelium basal cells , was applied to distinguish the earliest cells capable of undergoing squamous differentiation. Parts of the subcapsular epithelium were ext ensively stained with this reagent. This epithelium, like the basal la yer of certain squamous epithelia, exibited a high incidence of cytoke ratins 13 and 14, and pronounced expression of cytokeratin 19. Simple epithelium cytokeratins 8, 18, and 19 were present in the cortex. Scat tered cells reacted with KB-37 antibody. All stellate epithelial cells in the medulla were positive for cytokeratin 19. Most of the medullar epithelial cells were positive for cytokeratins 13, 14 and 17 of comp lex epithelium, in contrast to the cortex, where only a few cells were positive for these cytokeratins. A significant proportion of the medu llar cells was positive for KB-37 antigen. Cytokeratins 8 and 18 were expressed in single cells and in groups of cells surrounding Hassal's corpuscles. The outermost cells of these corpuscles were positive for cytokeratin 19 and KB-37. In the peripheral parts of Hassal's corpuscl es, simple epithelium cytokeratins 7, 8, 18, and cytokeratins 4, 13, 1 4, and 17, characteristic of stratified nonkeratinizing epithelia, wer e coexpressed with keratinization-specific cytokeratins 10/11. The inn er parts of the swirls were uniformly positive for cytokeratins 10/11. However, the expression of other cytokeratins was reduced.