Involucrin: A Constituent of Cross-Linked Envelopes and Marker of Squamous Maturation
The coordinated program of terminal differentiation in epidermis yields a superficial layer of tough dead squames well suited to the protective function of the integument. These mature cells consist primarily of insoluble disulfide-bonded keratin tonofilaments, but also exhibit a “cornified envelope” immediately beneath the plasma membrane. This structure, consisting of protein, is resistant to keratinolytic agents (alkali, detergent and reducing agent) and organic solvents, but is sensitive to proteolytic digestion (Matoltsy and Balsamo 1955; Sun and Green 1976). The chemical stability of the envelope is attributable to a high degree of ε-(γ-glutamyl)lysine cross-linking arising from cellular transglutaminase activity (Sugawara 1977; Rice and Green 1977). Envelopes are conspicuous in hair and nail samples boiled in the presence of sodium dodecyl sulfate (SDS) and reducing agent (Rice and Green, unpublished), where their rigid interlocking convolutions contribute to the exceptional cohesiveness of the cells in these appendages (Green et al. 1982). Envelopes are present on the surface of all the stratified squamous epithelia of the human and have been observed in the cornified layer of epidermis of mammals, birds, reptiles and amphibians but not fish (Matoltsy 1977).
KeywordsStratify Squamous Epithelium Human Epidermis Squamous Differentiation Cornified Envelope Retinyl Acetate
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