Immunocytochemical evidence for a possible role of cross-linked keratinocyte envelopes in stratum corneum cohesion.

Cross-linked cornified envelopes are cell structures specifically synthesized by terminally differentiating keratinocytes. They are composed of proteins deposited at the cell periphery under the plasma membrane, and can be purified from epidermis by physicochemical extractions. The resulting keratinocyte "shells" are highly insoluble structures devoid of cytoplasmic components. The rigidity of the stratum corneum cell envelope seems to be one of the essential factors contributing to the physical resistance of this most superficial epidermal layer. We studied the purified cell envelopes from human plantar horny layer to determine their antigenic composition and protein distribution. The extraction protocol consisted of four 10-min cycles of boiling in 10 mM Tris-HCl buffer containing 2% SDS and 1% beta-mercaptoethanol. The absence of any extractable proteins persisting in the purified pellets was checked with SDS-PAGE of the sample electroeluates. Indirect immunofluorescence as well as pre- and post-embedding immunogold labeling for electron microscopy revealed the persistence of several keratinocyte antigenic determinants on the purified substrates. The antibodies directed against involucrin, keratin 10, desmoplakin I + II, desmoglein (intracellular epitope), intercellular corneodesmosome proteins, and filaggrin (a considerably weaker reactivity) labeled the cell envelopes according to the ultrastructural localization pattern characteristic for a given antigen. We conclude that the cytoskeletal and desmosomal components become "embedded" in the highly cross-linked cornified envelope structures during the process of keratinocyte terminal differentiation. This underlines the central role of cornified envelopes in the physical resistance of superficial epidermal layers and indicates a possible importance of junctional proteins in this function.