A subtype of enteroendocrine cells found in the gastrointestinal mucosa, particularly in the glands of pyloric antrum; duodenum; and ileum. These cell type secretes serotonin and some neurotransmitters including enkephalins and substance P. Their secretory granules stain readily with silver (argentaffin stain). [ http://www.ncbi.nlm.nih.gov/pubmed/9505449 ]