Merkel cell carcinoma is a neuroendocrine tumour that often follows an aggressive course. Understanding its biology could improve the clinical outcomes for patients who present with this rare malignant disease.
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Merkel cells are named after Friedrick Sigmund Merkel who first described them in 1857.1–7 They are dispersed throughout the stratum basale of the integuments’ epidermal layer,1–4,6,8 but are found with greater frequency in the epithelium of the hands, feet, lips, hair follicles and pallet.2–4 Merkel cells can be either innervated or non-innervated; innervated Merkel cells form sensory structures called Merkel corpuscles.1–3
Microscopically, Merkel cells have a clear, elliptical shape and are 10 – 15 µm in diameter1,4 with a lobulated nucleus containing the occasional nucleoli.3,5 Ultrastructural features include cytoplasmic electron-dense secretory granules (80–140 nm in diameter), which have been found to contain species-specific neuropeptides; for example, calcitonin gene-related peptide and vasoactive intestinal polypeptide are found in humans, met-enkephalin in mice and serotonin in the rat; 1–3,5,6,8,9 microvilli-like cell membrane projections (maximum length 2.5 µm); 1–3 epithelial membrane specialisations (eg desmosomes, hemidesmosomes and intermediate junctions) and melanosomes.1–3,9
The exact functions and origin of Merkel cells remain to be elucidated, due to the confusing picture given by their ultrastructural features.3,5,9 It is postulated that they might perform a neuroendocrine and/or sensory function (mechanoreceptor and chemoreceptor) and that they could originate either from the neural crest or keratinocytes in the epidermis.1–5,7–10
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