Merkel cells are specialized cells found within the skin, recognized for their role in touch sensation. Friedrich Merkel first described these cells in 1875 while studying the skin of catfish, later identifying them in human skin. Although their full function was not immediately clear, subsequent research established their significance in tactile perception. These unique cells contribute to the intricate design of the human sensory system.
Location and Microscopic Appearance
Merkel cells are located in the basal layer of the epidermis, the outermost layer of the skin, approximately 0.1 mm from the skin’s surface. They are abundant in highly sensitive areas, such as the fingertips, lips, and around hair follicles, where touch sensation is most acute.
Under light microscopy, Merkel cells appear as clear, oval-shaped cells, roughly 10–15 micrometers in diameter. They possess a large, pale, and lobulated nucleus with few nucleoli. Their cytoplasm is clear and rich in mitochondria, indicating their energy requirements for sensory tasks.
Electron microscopy reveals ultrastructural features. They contain numerous dense-core granules, measuring about 50–110 nanometers, concentrated near their junction with nerve endings. These granules are thought to store neuropeptides and neurotransmitters. Merkel cells also form specialized connections called desmosomes, which link them to the surrounding keratinocytes, the primary cells of the epidermis. They contain intermediate filaments, notably cytokeratin 20 (CK20), which extends throughout their cytoplasm.
Immunohistochemical staining is a powerful tool for identifying Merkel cells in tissue samples. The presence of cytokeratin 20 (CK20) is a specific and widely used marker, showing cytoplasmic expression. Other markers like cytokeratin 8 and Piezo2 are also observed. This staining technique is useful for distinguishing Merkel cells from other cell types and for diagnostic purposes.
Function and Neural Connection
The primary function of Merkel cells is to act as mechanoreceptors, specialized sensory cells that convert mechanical stimuli into electrical signals. They are involved in light touch sensation and the ability to discriminate shapes and textures. Merkel cells are classified as slowly adapting type I mechanoreceptors (SA1), meaning they produce sustained responses to static stimulation and have small receptive fields, allowing for detailed information perception.
Merkel cells form associations with afferent nerve endings, creating a structure known as the “Merkel cell-neurite complex” or Merkel nerve ending. This complex is a unique touch receptor comprising both the Merkel cell and its associated sensory neuron. The sensory neurons that innervate Merkel cells are low-threshold mechanoreceptors, specifically Aβ fibers.
The interaction within the Merkel cell-neurite complex allows for the transmission of touch-related information to the brain. Recent research indicates that Merkel cells themselves are mechanosensitive, detecting mechanical forces through ion channels like Piezo2. Upon mechanical stimulation, Merkel cells depolarize and release neurotransmitters, such as norepinephrine, from their dense-core granules, which then activate the associated nerve endings. This chemical signaling enhances the excitability of the nerve fibers and initiates action potentials, contributing to the detailed perception of touch.
Developmental Origin
The developmental origin of Merkel cells has been a topic of scientific debate. Historically, two main hypotheses existed: that Merkel cells originated from epidermal stem cells or from neural crest cells. Neural crest cells are migratory cells that give rise to various tissues, including parts of the nervous system.
Recent lineage-tracing experiments have provided evidence supporting an epidermal origin for Merkel cells. Studies have shown that Merkel cells arise through the differentiation of epidermal progenitors during embryonic development. For instance, the transcription factor Atoh1 is required for epidermal progenitors to differentiate into Merkel cells; mice lacking Atoh1 in their skin fail to develop these mechanotransducing cells.
In adult skin, Merkel cells undergo a slow turnover and are replenished by cells originating from epidermal stem cells. This indicates that the epidermal lineage maintains the Merkel cell population throughout life. While neural input is recognized as important for their differentiation and proper function, the prevailing scientific consensus is that Merkel cells themselves are derived from the epidermis.