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Neuroendocrine cells (NECs) play important roles in normal lung development, autonomic regulation of lung function, and response to hypoxia or injury. They are located between the respiratory epithelium and the basement membrane of bronchi and bronchioles (below), often as single cells or small clusters.

NEC proliferation is a common pathologic occurrence, well established as a reactive response to chronic lung injury. Much less commonly, NEC proliferation is observed in the absence of apparent inciting factors. This neoplastic phenomenon is termed “diffuse idiopathic pulmonary neuroendocrine cell hyperplasia” (DIPNECH).

DIPNECH is recognized as a precursor lesion for carcinoid tumors. While the progression of DIPNECH to carcinoid is an accepted transformation, little is known about the genetic events that drive the pulmonary neuroendocrine cells to proliferate, penetrate the basement membrane, and sustain growth from the tumorlet stage (NEC mass <5mm) to the carcinoid stage (≥5mm). The best described genetic involvement implicates the MEN1 tumor suppressor gene on chromosome 11 as an important early event in the transformation process, but similar disease models suggest that this will prove to be just one event in the spectrum.

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