to form a 3D structure (artificial skin) recapitulating most of the properties of the epidermis is routinely used as a tool for drug development and testing. Hyperplasia Epidermal hyperplasia (thickening resulting from cell proliferation) has various forms: Acanthosis is diffuse epidermal hyperplasia (thickening of the skin, and not to be confused with acanthocytes). It implies increased thickness of the Malpighian layer (stratum basale and stratum spinosum). 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 cavity. Pseudoepitheliomatous hyperplasia (PEH) is a benign condition characterized by hyperplasia of the epidermis and epithelium of skin appendages, with irregular squamous strands extending down into the dermis, and closely simulating squamous cell carcinoma (SCC). In contract, hyperkeratosis is a thickening of the stratum corneum, and is not necessarily due to hyperplasia. Additional images See also Skin repair References Skin anatomy