Background: Gonadotropins GTH I and GTH II from the pituitary of Mediterranean (M.) yellowtail (Seriola dumerilii) were isolated and characterized, and antisera to the whole GTH II molecule (anti-My ␣,GTH II) and to its -subunit (anti-My GTH II) were obtained. At the light microscopic level, anti-My ␣,GTH II reacted with My GTH II-immunoreactive cells (GTH II cells), thyroid-stimulating hormone (TSH) cells, and a third cell population, which could have been GTH I cells. The aim of this study was the ultrastructural characterization of GTH and TSH cells in M. yellowtail using the immunogold method in order to provide a basis for future research into reproduction of this species.Methods: Pituitaries from mature male and female specimens reared in captivity were disected out and processed for electron microscopy. The immunogold method was carried out by using anti-My ␣,GTH II, anti-My ␣,GTH II preabsorbed with the ␣ subunit of the M. yellowtail GTH (My ␣GTH-subunit), anti-My GTH II, anti-human (h) ␣,TSH, and anti-h TSH sera to reveal gonadotropic and thyrotropic cells.Results: M. yellowtail gonadotropic cells were very heterogeneous with regard to their size, shape, and ultrastructural features. Cells were found with numerous, round, variably electron-dense, secretory granules and globules; others were found with their cytoplasm occupied mostly by dilated cisternae of rough endoplasmic reticulum (RER) and scarce secretory granules; and other intermediate cell forms were found that showed varying proportions of secretory granules and dilated RER. The secretory granules and globules were immunogold labeled with anti-My ␣,GTH II, and the reaction was weaker in the latter. A similar immunogold-labeling pattern was found with anti-My GTH II and with anti-My ␣,GTH II preabsorbed with the My ␣GTH-subunit, although some cells that showed the same ultrastructural features described above were not immunogold labeled and could have been GTH I cells. Thyrotropic cells had small, round, secretory granules of medium or high electron density that were immunogold labeled with anti-My ␣,GTH II, anti-h ␣,TSH, and anti-h TSH sera, but not with anti-My GTH II or anti-My ␣,GTH II serum preabsorbed with the My ␣GTH-subunit. All of the cell forms described for gonadotropes and thyrotropes were also found in a state of involution.Conclusions: Gonadotropes that are of a single morphological type but that vary in ultrastructure are present in the pituitary of captive M. yellowtail. GTH II-and putative GTH I-producing cells were distinguishable from one another and from TSH cells by their different reactions to anti-My ␣,GTH II, anti-My GTH II, and anti-My ␣,GTH II preabsorbed with the My ␣GTH-subunit.
