Melanosomal pattern in phototoxic pigmentary responses induced by topical psoralens and other photosensitizers

An increase in melanosomal size, without a consistent increase in the percentage of single melanosomes, was observed in phototoxic hyperpigmentation produced by topical psoralens and long wavelength ultraviolet (UVA) irradiation. This increase in size was proportional to the degree of hyperpigmentation rather than the concentration of psoralen applied. A similar response was seen in phototoxic hyperpigmentation induced by a topical tar preparation.