MELANOCYTES AND MELANIN PIGMENTATION 575 Figures 7 and 8. Twelve-day monolayer culture. Phase contrast. 256 X. Figure 7. Con- trol. Figure 8. ACTH 5 t•g/xnl. Compare the cell sizes of both pigmented and nonpig- mented cells in control and treated cultures and Breathnach (34). The epidermal melanin unit has been defined as a melanocyte with an associated pool of malpighian cells (Fig. 9, 10). In this view, factors influencing any component in the epidermal melanin unit can be expected to modify the function of the entire system. Per- haps ultraviolet stimulates melanogenesis, not as a direct action on the

576 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Figure 9. Epidermal melanin unit. Modified aftcr Fitzpatrick et al. (49) melanocytes, but indirectly, by inducing proliferation of malpighian cells and thereby providing more vehicles for the transportation and/or storage of melanin. In discussing pigmentary disturbances in vitiligo, Hu et al. (35) suggested that the defect of vitiligo may lie not only in melanoctyes but also in the basal and malpighian cells of the epidermis. A block of transfer of the pigment between the melanocytes and the epithelial cells may account partially for the lack of pigment in the diseased areas. The melanocytes in the hyperpigmented borders of the vitiliginous skin are often hypertrophic and engorged with melanin granules while the surrounding epithelial cells are completely devoid of pigment. Hypertrophic melanocytes also were seen in inflammatory conditions of the skin. Pinkus et al. (36) observed that inflamed skin exhibited large melanocytes with widespread pigmented processes in the inter- cellular spaces, while the edematous malpighian cells, on the other hand, contained little or no pigment. Rappaport (37), who studied this phenomenon in atopic dermatitis, has suggested that the malpighian