The melanocyte and the epidermal melanin unit: An expanded concept

The skin has three main layers, the epidermis, dermis and subcutaneous tissue. The epidermis is composed a variety of cells but there are three main cell types, the keratinocyte, the melanocyte, and the Langerhans cell. The concept of a melanocyte interacting specifically with a specified group of keratinocytes was first proposed by Fitzpatrick and Breathnach [1] in 1963. They called the unit appropriately the epidermal melanin unit. The concept was significant. It proposed that two of the three epidermal cells, a melanocyte and a group of 36 keratinocytes, worked together to produce the color of skin, both in the basal state called constitutive pigmentation and after tanning by sunlight called facultative pigmentation [2]. The concept also emphasized that the skin had evolved to respond to environmental beneficial and noxious stimuli such as sunlight to protect the host from damaging effects of ultraviolet (UV) radiation. It is notable that the Langerhans cell was not included in this physiological functional unit.