Embryologic, Histologic, and Anatomic Aspects
In 1875, Friedrich Merkel, a 30-year-old physician at the University of Rostock in Germany, noted unique cells at the base of epidermal rete ridges, those cells being in contact with nerve fibers. He considered the cell-neurite complexes to be touch receptors and named the cells “Tastzellen” or “touch cells.” The close proximity of the cells, named subsequently for Merkel, to neurites has been recorded in surface epidermis on palms and soles, infundibular epidermis, and epithelium of nail beds, oral mucosa, and genital regions. On rare occasions, Merkel cells have been observed in the epidermis and the dermis in the absence of any seeming association with neural elements. Merkel cells appear in fetal skin by the 16th week of gestation, and, unlike melanocytes, they do not come from the neural crest, and, unlike Langerhans” cells, they do not come from the bone marrow; they originate in the epidermis itself, presumably from germinative keratocytes. Merkel cells in a fetus cannot be differentiated, by conventional microscopy alone, from melanocytes or Langerhans” cells. By electron microscopy, however, Merkel cells are seen to possess within their cytoplasm distinctive electron-dense granules (Fig. 1.69). The cells are thought to participate in providing feelings of sensation in the skin, either as energy transducers or structural components of slowly adapting touch receptors.
Schematic representation of a Merkel cell-neurite complex as observed ultrastructurally. This diagram depicts (1) a dendritic Merkel cell (Mc) with its desmosomal attachments to adjacent keratocytes (K), intranuclear “rodlet,” and membrane-bound dense core granules (G) and (2) a mitochondria-rich myelinated axon (A) with postsynaptic thickening of its terminal membrane. (Art by Mario DiLeonardo, M.D., from AB Ackerman, H Jacubovic. In: Moschella SL, Hurley HJ, eds. Dermatology, 3rd ed. Philadelphia: W.B. Saunders, 1992.)
In 1902, Felix Pinkus, by observation grossly and examination histologically, was able to recognize structures in hairy skin that later would be found to consist of cell-neurite complexes similar to those reported on by Merkel. Because of the disc-like appearance of these complexes that often were situated next to infundibular ostia, which in those days were regarded universally as follicular ostia, Pinkus termed them Haarscheiben or “hair discs.” After nearly 100 years, Haarscheiben have yet to be characterized compellingly morphologically or elucidated convincingly biologically.