By contrast, the early basal cell carcinoma—or rodent ulcer— presents as a firm nodule, clearly growing within the skin and below it, rather than on the surface. The colour varies from that of normal skin to dark brown or black, but there is commonly a "pearly" translucent quality. As its name implies, the tumour is composed of masses of dividing basal cells that have lost the capacity to differentiate any further. As a result no epidermis is formed over the tumour and the surface breaks down to form an ulcer, the residual edges of the nodule forming the characteristic "rolled edge". Once the basal cells have invaded the deeper tissues the rolled edge disappears.
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