IMMUNOCYTOCHEMICAL AND ULTRASTRUCTURAL CHARACTERIZATION OF PROLACTIN,GROWTH-HORMONE, AND SOMATOLACTIN CELLS FROM THE MEDITERRANEAN YELLOWTAIL (SERIOLA-DUMERILII, RISSO 1810)

Background: Prolactin (PRL), growth hormone (GH), and somatolactin (SL ) are structurally related pituitary hormones that belong to a peptide family, Whereas growth hormone and prolactin are present in the hypop hysis of all vertebrates, somatolactin, a recently discovered hormone, has been found only in fish. It has been demonstrated immunocytochemi cally in a few teleost species; ultrastructurally, cells producing thi s hormone have been characterized only in one species of salmon. In th is paper, we identify and characterize ultrastructurally the cells pro ducing these three hormones in Mediterranean yellowtail (Seriola dumer ilii). Methods: Pituitaries from adult specimens were dissected out an d processed for electron microscopy. The immunogold technique was perf ormed in some ultrathin sections using fish primary antibodies. Result s: PRL cells had round, peripherally distributed, very electron-dense, homogeneous secretory granules of variable size. GH cells had dense, round secretory granules with a conspicuous scalloped membrane, which were grouped in the cell area near the neurohypophysis. SL cells had r ound, polymorphic, or very irregularly shaped secretory granules, the last seeming to arise from the fusion of various secretory granules. T he population of secretory granules varied greatly from one cell to an other. In all cases, immunogold labeling was seen exclusively in the s ecretory granules. Exocytosis was observed in all cell types. Some of the PRL, GH, and SL cells showed involutive features. Conclusions: PRL , GH, and SL, although structurally and functionally related, are secr eted by ultrastructurally different cells in the pituitary of M. yello wtail. (C) 1997 Wiley-Liss, Inc.