Neuroendocrine cells are found in almost every organ of the body and are also dispersed among other epithelial cells throughout the gastrointestinal and bronchopulmonary systems (the diffuse neuroendocrine system). The cells contain dense, core secretory granules, which synthesise, store and secrete serotonin and numerous other hormones/bioactive peptides with various functions in the gut, including nutrient sensing, gut secretions and motility, hormone release, and energy balance and appetite.
Neuroendocrine neoplasms (NENs) are uncommon tumours that derive from these neuroendocrine cells. They range in phenotype from poorly differentiated neuroendocrine carcinomas (NECs) to well-differentiated neuroendocrine tumours (NETs).
NENs (but mostly NETs) uniquely express cell-surface peptide hormone receptors, including somatostatin (SST) receptors (SSTRs). SST is a regulatory peptide hormone with a predominantly inhibitory function, inhibiting hormone secretion and cell proliferation and/or growth through five SSTR subtypes (SSTR1–5). The presence of SSTRs on NETs can be exploited to facilitate imaging and treatment of these tumours with somatostatin analogues (SSAs) and peptide-receptor radionuclide therapy (PRRT); and allow more detailed imaging output via SSTR PET.