Neuroendocrine Tumors in Animals
Neuroendocrine cells are characterized by their ability to produce and secrete a neuromodulator, transmitter, or hormone. In addition, these cells contain dense core secretory granules, the storage site for the secreted product(s). Neuroendocrine cells can release this product in a regulated manner by classic exocytosis. Neuroendocrine cells differ from classic neurons because they lack axons and synapses. Certain molecules, particularly those of the granin family (eg, chromogranin), are synthesized and stored in neuroendocrine cells and serve as immunohistologic markers.
Previously, neuroendocrine cells were classified as amine precursor uptake and decarboxylation (APUD) cells and were believed to be solely derived from neuroectoderm. However, more recent evidence supports a more diverse embryologic origin.
Tumors that arise from this cell type comprise a family of neuroendocrine tumors. Because of the diffuse distribution of cells, particularly within the GI tract, NETs are found in a wide variety of locations. Overall, NETs are rare tumors in people and animals. Some NETs oversecrete their normal product, and the excessive levels result in the observed signs. Other NETs are nonfunctional, and clinical signs instead result from physical forces associated with expansion and/or metastasis.
Examples of Neuroendocrine Tumors
Carcinoids are a heterologous group of NETs that occur in various regions of the GI tract, liver. lung, and occasionally heart. In dogs and cats, carcinoids have been reported to occur in the stomach. The most common signs reported in affected animals include chronic vomiting, anorexia, diarrhea, and weight loss. In people (but not yet documented in companion animals), carcinoids serve as a source of serotonin or histamine. Excessive release of these transmitters can cause a syndrome of flushing, hypotension, wheezing, and diarrhea in people. In dogs and cats, carcinoids may cause obstruction in the GI tract and/or metastasize to lymph nodes and other tissues.
Insulinoma is the most common neuroendocrine tissue tumor in domestic species.
Carcinoids found in the GI tract can cause nonspecific GI signs, including vomiting, anorexia, and weight loss.
Glucagon-secreting tumors are often associated with a superficial necrolytic dermatitis affecting mucocutaneous junctions.