Somatostatin localization in tissues.

The development of specific antibodies to somatostatin has enabled investigations on the distribution (radioimmunoassay) and precise tissue localization (immunocytochemistry) of somatostatin-immunoreactive (IR) material. Somatostatin immunoreactivity is broadly distributed both in the central nervous system and in many peripheral organs, including the gastrointestinal tract, pancreas, genitourinary system, heart, eye, thyroid, thymus, and skin. Somatostatin-IR cells display characteristic morphological features, including cytoplasmic elongations, which lend support to the postulated local or paracrine role for somatostatin. The intracytoplasmic electron-dense secretory granules in somatostatin-IR endocrine cells are characterized by their round shape, flocculent matrix, and closely apposed limiting membrane. Somatostatin-IR nerves are abundant in the gut and contain large, dense, P-type neurosecretory granules, which are distinct from those storing other peptidergic neurotransmitters. Somatostatin immunoreactivity is found frequently in neuroendocrine tumours, but the existence of the 'somatostatinoma syndrome' has recently been questioned.