Ultrastructural demonstration of exocytosis of neural, neuroendocrine and endocrine secretions with an in vitro tannic acid (TARI-) method.

The release of neural, neuroendocrine, and endocrine secretory products by exocytosis was ultrastructurally studied by means of tissue incubation in Ringer containing tannic acid (Tannic Acid Ringer Incubation-method; TARI -method), followed by conventional fixation. Tannic acid strongly enhances the electron density of extracellular (secretory) substances. During TARI -treatment of tissues exocytosis proceeds, but the exteriorized contents of the secretory granules are immediately fixed by tannic acid and do not diffuse away into the extracellular space. In this way detection of exocytosis is markedly facilitated since the number of exocytosis phenomena visible at the ultrastructural level considerably increases with progressing incubation time. Studies of the central nervous system of the mollusc Lymnaea stagnalis show that the occurrence of exocytosis during TARI -treatment is calcium-dependent. With the TARI -method exocytosis has been clearly demonstrated in a variety of structures (endocrine cells, neurohaemal axon terminals, synapses) of L. stagnalis, the insect Locusta migratoria, and the rat, including cell types where exocytotic release had not been shown before.