The human sense of touch is complex and relies on several types of cells in the skin to detect different types of stimuli. Touch receptors can be divided into two categories: mechanoreceptors, which detect pressure or movement, and thermoreceptors, which detect changes in temperature. Most touch receptors are specialized nerve cells. The cells responsible for the sensation of light touch or pressure are known as Merkel cells, named after the German anatomist Friedrich Sigmund Merkel (1845—1919) who discovered them in the late 19th century. Friedrich Merkel originally named them “tastzellen” or “touch cells” before other types of touch receptors had been discovered. It has been more than 100 years since their discovery and the origin and role of Merkel cells are not yet fully understood; in fact, it was only recently that scientists discovered that Merkel cells are highly specialized epithelial (skin) cells rather than neural receptor cells. It is also thought that Merkel cells can detect texture and shape of stimuli in addition to faint touch sensations, although there is ongoing research to confirm these functions.
Skin is a complex organ made of three distinct tissue layers. The outermost layer (the one we are able to see and feel) is called the epidermis; the middle and thickest layer is called the dermis; and at the bottom is a layer of fat known as the subcutaneous tissue. The epidermis contains five of its own distinct layers, and Merkel cells are located near the bottom of the epidermis in the stratum basale (basal layer). Interestingly, this is the same layer where melanocytes, or skin pigment cells, are located. This location is vital to the functional success of Merkel cells; the stratum basale is deep enough to provide adequate protection of sensory cells but close enough to the surface that light touch can be detected. Other types of touch receptor cells are located deeper in the skin and are responsible for sensation of stronger stimuli, such as pain and temperature. Merkel cells have been found to cluster around sweat glands and hair follicles and are present in certain mucosal tissues.
The most common disease that affects Merkel cells is Merkel cell carcinoma, an aggressive but rare skin cancer. The first signs are small, painless, firm nodules in the skin that grow rapidly after detection. Merkel cell carcinoma usually appears on areas of the skin that face the most sun exposure, such as the head, face, and neck, and can spread aggressively to many other tissues and organs. It most commonly affects adult Caucasian males and individuals with weakened immune systems. It is often confused with other carcinomas and may be misdiagnosed, but the treatment procedure for Merkel cell carcinoma is similar to that for other skin cancers. Once diagnosed, it is crucial to receive treatment as soon as possible.
Up to 80 percent of Merkel cell carcinomas are caused by Merkel cell polyomavirus (MCV), which is one of the few known human oncoviruses (cancer-causing viruses). The mode of transmission of MCV is unknown; fortunately, a person infected with MCV is not contagious and will not transmit the virus further. The remaining 20 percent of Merkel cell carcinomas have unknown origins. Health care providers suggest that Merkel cell carcinoma may be avoided by taking the same precautions one would take for other skin cancers: limit sun exposure and use sunscreen if exposure cannot be avoided.
See also: Discriminative Touch; Meissner’s Corpuscles; Mirror-Touch Synesthesia; Pacinian Corpuscles; Touch
Lumpkin, Ellen A., Kara L. Marshall, & Aislyn M. Nelson. (2010). The cell biology of touch. Journal of Cell Biology, 191(2), 237—248. Retrieved from http://jcb.rupress.org/content/191/2/237.full
Moll, Ingrid, Marion Roessler, Johanna M. Brandner, Ann-Christin Eispert, Pia Houdek, & Roland Moll. (2005). Human Merkel cells—aspects of cell biology, distribution and functions. European Journal of Cell Biology, 84(2—3), 259—271.
National Cancer Institute. (2015). Merkel cell carcinoma treatment. Retrieved from http://www.cancer.gov/types/skin/patient/merkel-cell-treatment-pdq