A quantitative histochemical study of sulphydryl and disulphide content during normal epidermal keratinization

A quantitative histochemical study was carried out on the distribution of protein thiol and disulphide groups in normal human plantar epidermal tissue. Histochemical demonstration of reactive groups was achieved by addition of N-(4-aminophenyl) maleimide, subsequent diazotization and final coupling with a Nitro Red or chromotropic acid label as first described by Sippel. The quantitative reliability of the method was tested by absorption cytophotometry, and evaluated on the basis of the internal consistency of the results reported. Our histological observations and histophotometric data support accepted views on epidermal keratinization. A limited, though reproducible, amount of disulphide bonds was observed near the basement membrane. The free thiol concentration in basal and prickle cells was low and almost constant, but was higher in the granular cells, where deposition of sulphur-containing proteins on cell membranes is initiated. In Malpighian layers, disulphide cross-links only occurred just beneath the transition zone in thickened cell membranes. The staining pattern of the inner stratum corneum resembled a mosaic and was characterized by a Sharp rise of the disulphide content, which exceeded the decrease in free thiol groups. The free thiol concentration decreased further throughout the cornified layers whilst the disulphide content remained fairly constant. Staining of thiol and disulphide groups together corresponded, within the limits of the standard error, to the sum of the thiol and disulphide concentrations when they were assayed separately in living ahd horny cells. These results confirm that living cells are the main site of free thiol groups, while horny cells are the most prominent of site of disulphide cross-links.