Merkel Cells, Normal and Neoplastic: An Update

Abstract

Merkel cells (MC) occur in the basal epidermal layer, hair follicles, and oral mucosa, as complexes with sensory axons. The axons transduce slowly adapting type I mechanoreception, and MC modulate their sensitivity. MC also determine and maintain the 3-dimensional epidermal structure. They have neuroendocrine granules, rigid spinous processes, and desmosomal junctions with each other and with keratinocytes. Rare MC are dermaWl. Current evidence supports a basal cell origin. Merkel cell carcinomas (MCC) occur mostly in sun-exposed skin in old age. Trabecular, intermediate, or small cell in pattern, MCC have neuroendocrine granules, intercellular junctions, rigid spinous processes, and a paranuclear collection of intermediate filaments staining for cytokeratin 20. Most MCC behave indolently, but those with the small cell pattern, and some with the intermediate pattern, are aggressive and rapidly fatal.

Human Polyomaviruses and Other Human Viruses in Neuroendocrine Tumors

“…NETs show distinct biologic and clinical characteristics, however, share some common histologic features, such as neuroendocrine granules, the paranuclear ball of intermediate filaments, and intercellular junctions (3,4). With the potential exception of Merkel cell carcinomas (MCC), NETs have a common embryonic origin from the neuroectoderm.…”

Plasticity of neuropeptidergic neoplasm cells in the primary and metastatic Merkel cell carcinoma

“…The small cell growth pattern is a characteristic feature of a one subtype of MCC invasion. The small cell morphology was found to be associated with high biological malignancy, fast growth, as well as early formation of metastases [20]. Neuropeptides can function as autocrine growth factors in cancer cells.…”

Merkel Cell Carcinoma: Recent Progress and Current Priorities on Etiology, Pathogenesis, and Clinical Management

“…Electron microscopy has vividly illustrated that Merkel cells contain whorls in the cytoplasm and dense granules at the cytoplasmic membrane. 26 Focal degranulation has been observed, but its significance is unknown. The granules label with antibodies to chromogranin and other neuroendocrine markers.…”

“…Reported histologic variants include a large-cell type that often has a trabecular pattern and reportedly has the ultrastructural finding of a high density of granules, and a small-cell type that has an observed ultrastructural paucity of granules and a resemblance to skin metastases of small-cell lung carcinoma. 26,28 It is not clear whether prognosis differs for these histologic variants in comparison with the intermediate-cell type that accounts for the majority of MCC tumors.…”

“…23,26,28 Nonetheless, immunohistochemistry exploits these unusual properties to distinguish MCC from morphologically similar malignancies, including some lymphomas, Ewing sarcoma, and especially small-cell carcinoma of the lung. In particular, distinguishing MCC from small-cell lung carcinoma is based on MCC staining with CK20, often visualized as conspicuous paranuclear dots, and the absence of reactivity to thyroid transcription factor.…”