Epidermis – Wikipedia

Acanthosis nigricans is a black, poorly defined, velvety hyperpigmented acanthosis, usually observed in the back of neck, axilla, and other folded regions of the skin. Focal epithelial hyperplasia (Heck’s disease) is an asymptomatic, benign neoplastic condition characterized by multiple white to pinkish papules that occur diffusely in the oral …

https://en.wikipedia.org/wiki/Acanthosis