Histochemical localization of neurohumors in a sponge

The histochemical localizations of acetylcholinesterase, monoamine oxidase, epinephrine, norepinephrine, 5‐hydroxytryptamine, and neurosecretory substance were investigated in Sycon. All reactions that were most intense and sensitive to inhibitors were localized in spindle‐shaped bipolar cells and multipolar cells situated in the mesenchyme. Some spindle‐shaped cells formed a ring around the osculum and others contained branching and interconnecting processes. Multipolar cells were situated singly and possessed up to ten short processes. Activity was most commonly localized to small granules, vesicles, or large droplets within the cells and their processes. These results demonstrate the presence of cells specialized for the secretion of neurohumors and neurosecretory substance in sponges.