Cytokeratins in intracranial and intraspinal tissues

The intermediate filament distribution pattern in cells and tissues of vertebrates reflects their differentiation or functional specialization state, their histogenesis, and their malignant transformation. In the case of cytokeratins, the characteristic epithelial intermediate filaments representing a complex group of about 30 polypeptides, extensive attention has been given to their expression in diverse epithelial and epithelioid cells. However, little is known about their distribution during fetal development and in neuroectodermal cells. This review specifically focuses on the data concerning cytokeratin expression in intracranial and intraspinal tissues, as expressed alone or as coexpressed with other intermediate filament proteins. Furthermore, the expression pattern of individual cytokeratin polypeptides was investigated by immunocytochemistry in diverse human and animal tissues using a broad panel of monoclonal antibodies. Only the cytokeratins typical of simple epithelia with the primary keratin pair 8/18 as a significant component have been detected in neuroectodermal tissues such as the choroid plexus and ciliary body epithelia, the retinal pigment epithelium, the subcommissural organ, and the ependymal cell clusters in fetal pineal gland (only in humans) as well as in various "unspecialized" ependymal cells of brain ventricles and spinal cord ependyma. Focal cytokeratin 19 expression in rat ciliary body and ventricle ependyma represents a rare exception. In addition, a group of intracranial and intraspinal tissues with controversial histogenesis express solely the cytokeratins 8 and 18: endocrine pituitary cells, arachnoid cells, and corneal endothelium. In most cases of tissues with neuroectodermal derivation, coexpression of cytokeratins and vimentin, or triple expression of cytokeratin, vimentin, and GFAP (fetal and neonatal choroid plexus of humans, rat and guinea pig ependymal cells in the neighborhood of the subcomissural, folliculostellate cells of human and guinea pig pituitary) is detectable. The coexpressions are discussed in the light of several hypotheses based on morphological and functional data concerning intermediate filament protein expression. Both the occurrence of more than one intermediate filament protein and the individual cytokeratin composition in the corresponding tumors of neuroectodermal origin reflect, in principle, the patterns found in their normal tissues. In the fetal neuroectodermal tissues studied, the cytokeratin pair 8/18 is the first one to be expressed during embryonic development.(ABSTRACT TRUNCATED AT 400 WORDS)