A Merkel cell is a touch receptor found in the skin. Merkel cells, or Merkel-Ranvier cells, are involved in the sensation of light touch, for example when feeling the texture of an object or determining its shape using the fingertips. High concentrations of Merkel cells are found in the fingertips and also in the lips, but they are also present in areas of hairy skin. Sometimes a type of cancer known as a Merkel cell carcinoma arises from Merkel cells. Merkel cells are named after Friedrich Sigmund Merkel, the German scientist who discovered them.
In the skin, Merkel receptor cells are typically situated near sensory nerve endings, with each Merkel cell and each nerve ending forming what is known as a Merkel cell-neurite complex. When the sensation of light touch is detected, the Merkel cell-neurite complex acts as what is called a mechanoreceptor. Mechanoreceptors respond to a particular stimulus, in this case touch, and react by producing electrical nerve impulses which travel along sensory nerves, eventually reaching the brain.
The type of mechanoreceptor formed by a Merkel cell is described as being slowly adapting, which means it can take a number of seconds to return to normal after electrical impulses have been produced by a stimulus. This can be useful in practice, for situations when an object, such as a cup, has to be gripped, because the brain will remain aware of the sensation for longer, decreasing the risk of dropping the cup. Some other types of mechanoreceptors adapt much more quickly — in a fraction of a second — with the result that the fingers would have to be moved over the surface of an object to keep causing new stimulus to maintain the sensation.
A rare type of skin cancer can form from Merkel cells, known as a Merkel cell carcinoma. The cancer is associated with exposure to sunlight, and generally appears on the limbs, head, or neck as a round, red lump, which is firm to the touch and can be mistaken for a harmless skin blemish such as a cyst. It is thought that cancerous changes inside the Merkel cell could be associated with a viral infection. If the tumor is diagnosed and surgically removed early in the disease, before the cancerous cells have had time to spread, the outlook is positive. In cases where the cancer has already spread, chemotherapy and radiotherapy are normally used in addition to surgery to improve symptoms and increase life expectancy.