The diffuse endocrine system is composed of hormone-secreting cells that synthesize peptides and amines which act as hormones or neurotransmitters. These neuroendocrine cells have common enzyme systems for amine handling and peptide production. They have been noted as APUD (amine precursor uptake and decarboxylation) cells, and may have a common origin from neural crests, though this has not been confirmed. APUD cells can act on nearby or distant cells in paracrine or endocrine function respectively, and are present in several endocrine glands.