MECHANISMS OF PARAKERATOSIS

SUMMARY. The removal of the horny layer of the guinea‐pig ear by tape stripping causes rapid development of a parakeratotic scale during the first 12 hr. Some epidermal cells, which were labelled with 3 H‐Thymidine when stripping was performed, could be detected autoradiographically in the parakeratotic stratum corneum a few hours later. Since the proliferative response of the epidermis is retarded after vigorous stripping, these cells have become parakeratotic due to exposure and subsequent cell death. Topical application of Vitamin A‐acid produces morphological alterations of the keratogenous zone together with increased DNA‐synthesis in the lower portion of the epidermis, leading to the persistence of nuclei in the stratum corneum. Prolonged treatment with Vitamin A‐acid and Hexadecane increases the epidermal thickness to values such as are found in psoriasis. The turnover time of this hyperplastic epidermis is reduced to about 3 days, yet no parakeratosis can be found. These observations lead to the conclusion that maximally stimulated epidermopoiesis does not result in parakeratosis unless there is a defect in cell differentiation.