Well-differentiated Neuroendocrine Tumor : Microscopic
Well-differentiated neuroendocrine tumors (Grade 1; classical carcinoid tumors) are well-circumscribed and composed of cellular sheets of uniform round or polygonal tumor cells that are arranged in nests, ribbons, trabecular, glandular, gyriform, and solid growth patterns. The nuclei are uniform round to oval with punctate nucleoli and speckled chromatin pattern (salt and pepper appearance). There is no necrosis. The defining characteristics of Grade 1 NET are mitotic activity of <2/10 hpf and/or Ki-67 labeling index of <3%. Based on the neuroendocrine cell of origin, colorectal NETs can be subdivided into two main types: 1) colonic-predominant EC-cell (serotonin-producing) NETs and 2) rectum-predominant L-cell type NETs that can produce glucagon-like peptide (GLP), pancreatic polypeptide (PP), and peptide YY (PYY). The image shows a small well-differentiated neuroendocrine tumor (carcinoid) of rectum discovered incidentally on endoscopy.