Basic Science of the Merkel Cell
In 1875, Friedrich Sigmund Merkel identified a unique epidermal cell type closely associated with sensory nerve endings. These “Tastzallen” or “touch cells” have subsequently been renamed Merkel cells in his honor. Recent genetic and electrophysiological
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Basic Science of the Merkel Cell Stephen M. Ostrowski and Stephen Michael Maricich
Merkel Cell Structure Ultrastructure Merkel cells are a distinct epidermal cell type found at the dermal/epidermal junction just below the basal layer of the epidermis. They are roughly the same size as keratinocytes, with oval or round cell somata ~10 mm in diameter and large, lobulated nuclei (Fig. 1.1a) [1–4]. Merkel cell cytoplasm is relatively clear and contains intermediate filaments that are thinner, less dense, and ultrastructurally distinct from those of other skin cells. These filaments are most abundant near desmosomal contacts that link Merkel cells with adjacent keratinocytes (Fig. 1.1b, c) [5]. Each Merkel cell extends many fine, spine-like microvilli that contact nearby keratinocytes and may allow increased sensitivity for detecting mechanical stimuli (Fig. 1.1d, e) [6]. Typical cytokeratins (CK) found in immature
S.M. Ostrowski Department of Dermatology, University Hospitals Case Medical Center, 11100 Euclid Avenue, 44106, Cleveland, OH, USA e-mail: [email protected] S.M. Maricich (*) Department of Pediatrics, Division of Neurology, University of Pittsburgh School of Medicine, Children’s Hospital of Pittsburgh of UPMC, Rangos Research Building Room 8129, One Children’s Hospital Drive, Pittsburgh, 15224, PA, USA e-mail: [email protected]
(CK5, CK14) and mature (CK1, CK10) epidermal cells are absent from Merkel cells. Instead, CK8, 18, 19, and 20 constitute the building blocks of Merkel cell intermediate filaments [7, 8], and three of these cytokeratins (CK8, 18, and 20) are highly specific for Merkel cells in humans and other mammals (Fig. 1.1e) [9]. A defining feature of Merkel cells across species is the presence of electron dense granules unlike any others seen in the epidermis. These structures, 80–120 nm in size, are concentrated at the basal surface opposite to Merkel cell-associated nerve termini (see below) (Fig. 1.1a). Merkel cell granules likely derive from the Golgi apparatus classical secretory pathway and are similar in size and appearance to those found in neurosecretory and neuroepithelial cell types present in other tissues [10]. The physical location and appearance of these granules suggest that they may function as key mediators of Merkel cell– nerve interactions.
Organization and Anatomic Distribution Using basic histochemical staining methods, Merkel and others identified expanded nerve endings associated with the majority of Merkel cells (Fig. 1.2a) [11–13]. These Merkel cell–neurite complexes are present in almost all vertebrates including mammals, birds, reptiles, amphibians, and some classes of fish [11, 14, 15]. Merkel cells are innervated by neurites derived from large, heavily myelinated Ab
M. Alam et al. (eds.), Merkel Cell Carcinoma, DOI 10.1007/978-1-4614-6608-6_1, © Springer Science+Business Media New York 2013
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S.M. Ostrowski and S.M. Maricich
Fig. 1.1 Merkel cell structure. (a) Electron microscopic image of a Merkel cell (M). Note the lobulated nucleu
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