Merkel cell
Meaning of Merkel cell: A touch sensor that also behaves like a hormone releasing cell, and the namesake of a cancer it may not actually produce.
Definition of Merkel cell
A Merkel cell is a specialized touch receptor cell sitting in the deepest layer of the epidermis, the outer sheet of the skin, where it forms a close junction with the ending of a sensory nerve fiber. Together they register sustained light pressure and fine surface detail. The cells are packed most densely in fingertips, lips and the base of hair follicles.
Where exactly does it sit?
Within the outer sheet of skin, not underneath it. Merkel cells lie in the basal layer of the epidermis, the row of dividing cells resting on the boundary with the tissue below. Older descriptions place them beneath the epidermis, which misreads their position: they are epidermal cells, held to their neighbors by the same junctions that bind ordinary skin cells together. The nerve fiber they partner with does arrive from below, crossing that boundary to meet them.
How does it sense touch?
Pressure on the skin deforms the cell, opening channels in its membrane so charged particles flow in. The cell then releases a chemical signal onto the nerve ending pressed against it, and the nerve cell carries the message onward. This partnership registers steady pressure and fine texture, which is what allows the edge of a coin or a raised dot of braille to be told apart. It adapts slowly, meaning it keeps firing while contact continues rather than reporting only the moment of contact, which is why an object held still can still be felt.
What else do these cells contain?
Dense granules packed with signaling peptides, of the kind found in hormone releasing cells elsewhere in the body. This neuroendocrine character is well established, and it is why the cells take up stains used for nerve and endocrine tissue rather than those for ordinary skin. What the peptides do in intact skin is less settled, with proposed roles in wound healing, hair follicle behavior and local blood flow.
How does the cancer relate to it?
Loosely, and the name may mislead. Merkel cell carcinoma is an uncommon and aggressive skin cancer whose cells share the granules and staining pattern of Merkel cells, which is how it came by the name. Whether it truly arises from them is unresolved, and other cells of origin have been proposed. Most cases carry a common virus built into the tumor DNA, while the rest show the heavy ultraviolet damage typical of long exposed skin, so two different routes lead to the same tumor.
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Immunostaining highlighted Merkel cells scattered along the basal layer.
Good to know
Merkel cells sit within the basal layer of the epidermis rather than beneath it, and whether Merkel cell carcinoma arises from them is still unresolved despite the shared name.