Ultrastructural demonstration of cystine in guinea pig back stratum corneum

A method for the ultrastructural demonstration of cystine in keratinized epidermal cells of guinea pig back hairy skin is described. This involves selective oxidation by peracetic acid and formation of an electron dense reaction product with uranyl acetate. Unkeratinized cells were unstained except for nuclei and keratohyalin. The thickened plasma membranes of transitional and keratinized cells were darkly stained. In transitional keratinized cells darkly stained membranes occurred initially in relation to desmosomes and then spread to intervening membrane areas. Dense material, probably matrix keratin containing cystine was found in the interiors of basal horny cells but not in the intermediate or superificial cells of the stratum corneum. It is suggested that the unstable matrix keratin is probably broken down by enzymatic hydrolysis above the basal zone of the stratum corneum.