Melanin is found in the small melanosomes, particles formed in melanocytes from where they are transferred to the surrounding keratinocytes. The size, number, and arrangement of the melanosomes vary between racial groups, but while the number of melanocytes can vary between different body regions, their numbers remain the same in individual body regions in all human beings. In white and Asian skin the melanosomes are packed in "aggregates", but in black skin they are larger and distributed more evenly. The number of melanosomes in the keratinocytes increases with UV radiation exposure, while their distribution remain largely unaffected. Clinical significance
Laboratory culture of keratinocytes 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 (Hecks disease) is an asymptomatic, benign neoplastic condition characterized by multiple white to pinkish papules that occur diffusely in the oral cavity. : 411 
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 ==