Occurrence of somatostatin and insulin immunoreactivities in the stomach of sea bass (Dicentrarchus labrax L.): light and electron microscopic studies

Somatostatin (SST)- and insulin (INS)-immunoreactive (ir) cells were identified in the gut of sea bass (Dicentrarchus labrax) by immunofluorescence double staining and peroxidase-antiperoxidase (PAP) techniques for light microscopy and by immunogold method for electron microscopy using antisera to mammalian and fish peptides. SST-14 and SST-25 immunoreactivities coexisted in cells mainly located among the mucous neck cells of the gastric glands. Preabsorption controls showed that some SST-25- and, possibly, some SST-14-like peptides appeared in these cells. Immunoreactivity to fish INS, but not to mammalian INS (mINS) or insulin-like growth factor I (mIGF-I), was observed in all the SST-ir cells. The preabsorption controls suggest a cross-reaction of the fish INS antisera with SST-containing or type I cells. These cells displayed ovoid or round secretory granules with fibrous, medium electron-dense or homogeneous osmiophilic materials. Some gastric cells (type II) with round secretory granules of variable electron density, which were gold immunolabeled with bonito INS but not with mINS, mIGF-I, or SST antisera, were also found. INS-related peptide in type II cells of the sea bass stomach is suggested.