Background: One of the hallmarks of the psoriatic plaque is increased epidermal proliferation. Whether this is the result of an increased recruitment of cycling epidermal cells or a decrease in cell cycle time has been a matter of debate for years. Objective: Calculating cell-kinetic information from the number of S phase cells in psoriasis by in situ hybridisation using a histone probe and the number of cycling epidermal cells by immunohistochemistry using the MIB-1 antibody. Methods: Immunohistochemistry and non-isotopic in situ hybridisation were performed on serial sections of 33 untreated psoriatic samples and 14 tacalcitol-treated samples. Results: The labelling index (number of cells in S phase/number cycling cells per millimetre length of section) in psoriatic untreated as well as in treated plaques is 16%. The amount of S phase cells in our experiment is equal compared with the number of cells in S phase as determined by BrdU incorporation. Conclusion: Using this direct approach to study cell-kinetic behaviour of psoriatic skin, we reconfirm that the psoriatic abnormality is due to a defect in the G₀–G1 recruitment mechanism (by increased recruitment of G₀ cells), a decrease in apoptosis or an increase in the number of cell divisions in the transit-amplifying compartment, rather than a reduction in the cell cycle time.

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