The yeast, Pityrosporum orbiculare, isolated from lesions from lesions of tinea versicolor, grows in vitro only if fatty acids from the C12 to C24 series are added to the culture medium. Except for elaidinic and nervonic acids, all saturated and unsaturated fatty acids tested support growth. P. orbiculare can synthesize various lipid fractions containing both saturated and unsaturated fatty acids from a single fatty acid. Glucose and asparagine stimulate growth but exogenous vitamins do not.

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The effect of cholesterol and cholesterol esters on Pityrosporum cultures has been studied. A mixture of 0.25% to 2.0% of cholesterol:cholesteryl stearate:glyceryl monostearate (2.0:1.5:2.0) added to Bacto Yeast Morphology Agar plus oleic acid was able to induce hyphae in cultures of both Pityrosporum orbiculare and P. ovale. This result is discussed with respect to the cholesterol effect on cell membranes and to the occurrence of cholesterol and cholesterol esters in the scaling patches of Pityriasis versicolor.

Riley's classic 1970 experiment showing a specific cytotoxic effect of 4-hydroxyanisole (4-OHA) on tissue-cultured melanocytes of black guinea pig ear skin was repeated on normal human melanocytes, and the results were examined by electron microscopy. In dispersed tissue culture, no specific toxic effect on human melanocytes was observed following equally timed exposures to similar (10(-3) M) or even higher (10(-2) M) concentrations of the drug; plasma membrane, nucleus, and cytoplasmic organelles, including melanosomes were unaffected. The same applied to melanocytes of whole epidermis exposed for 5 hr to the same concentrations of 4-OHA in culture medium. Melanocytes of PUVA treated skin similarly exposed for up to 24 hr to 10(-2) and 10(-3) M 4-OHA, likewise exhibited no evident morphological damage at the ultrastructural level. The discrepancy of results between guinea pig and man could have a variety of explanations, one of which could be due to a possible relatively low level of active tyrosinase in the human melanocytes (Riley believes the cytotoxic effect of 4-OHA to be due to the fact that it acts as a substrate for tyrosinase, toxic intermediates being liberated as a result). However, the lack of effect on the PUVA-activated melanocytes indicates that this cannot be the entire explanation.

Since dicarboxylic acids are competitive inhibitors of tyrosinase, and effective in treatment of hyperpigmentary disorders, such as chloasma and lentigo maligna, probably due to a cytotoxic effect on abnormal melanocytes, it is of interest to examine their effect on normal melanocytes in tissue culture. Azelaic or dodecandioic acids were added (150-200 micrograms/ml) to dispersed cultures of epidermal cells, and melanocytes were examined by electron microscopy after 7, 10, 15, 20 and 30 days. Apart from a stimulation of melanogenesis, the presence of dicarboxylic acids in the culture medium caused no detectable damage to melanocytes, nor did they prevent growth of a second generation of cells.