Melanocyte changes in epidermal hyperplasia--an experimental study.

Subepidermal injection of suspensions of silica in guinea-pigs produces granulomatous inflammation in the dermis and hyperplasia of the overlying epidermis. In the experiments described here this procedure was adopted in 16 animals in order to investigate the behaviour of the melanocytes in the hyperplastic epidermis. The DOPA reaction was used to identify melanocytes and the number of DOPA positive cells per 100 basal epidermal cells (DOPA index) was correlated with the degree of epidermal hyperplasia. The epidermal reaction was charted by estimating the autoradiographic labelling index after injection of tritiated thymidine and measuring the epidermal thickness. The DOPA index fell in the hyperplastic epidermis and had not returned to control levels by three weeks. However, there were foci within the epidermis where the number of melanocytes and their reactivity appeared to be increased. The number of “clear cells” in the basal epidermis was also estimated and the numbers of these cells approximately paralleled the changes in the DOPA positive cells. Silver staining for melanin did not demonstrate pigmentary incontinence.