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The gastrointestinal tract ofrepresentatives ofall the vertebrate classes was investigated for the occurrence of serotonin-like immunoreactive cells using both polyclonal and monoclonal antibodies. In the alimentary canal of all the gnathostomes, but not in cyclostomes (Agnatha), sertotonin-like immunoreactive cells were detected. These cells showed a wide distribution and were found more or less throughout the gut ofaH the species, indicating that serotonin-like immunoreactive cells were established in the gut at the early stage of vertebrate evolution.

The enterochromaffin (EC) cells have been described in the gastric mucosa of mammals as early as 1870 (13). However, the function ofthese cells is still unclear. In mammals EC cells are generally identified by the chromaffin reaction (21), by their ability to reduce silver salts (argentaffin reaction)(14), or by the formaldehydeinduced fluorescence (FIF) method (8). Whereas the chromalfin reaction detects specifically serotonin-containing EC cells (6), it has not been used for the diagnosis of carcinoid tumours and is seldom employed for the demonstration ofEC cells in the gastrointestinal tract (17). This may be due to the fact that this technique is comparatively time-consuming and that the formalin-fixed material seldom provides satisfactory results (21). Using the argentaffin reaction, EC cells have been demonstrated in the gastrointestinal tract of several species belonging to various classes of vertebrates (2, 3, 12). The argentaffin reaction in the lower vertebrates depends on the fixative used and on the way by which the tissue is fixed, and is in some cases unreliable (3-) 5). The other technique, FIF …