Gonadotropic cells in the Atlantic salmon, Salmo salar. An experimental immunocytological, electron miscroscopical study.

The gonadotropin-producing cells (GTH-cells) in the Atlantic salmon were studied light and electron microscopically before, during and after spawning, and after injections of luteinizing hormone-releasing hormone (LH-RH). The double immunofluorescent technique was applied using rabbit anti-carp GTH as the first antibody. Numerous immunofluorescent cells were observed throughout the pars distalis, but very few in the pars intermedia. These cells are basophilic and PAS-positive, and ultrastructurally classified as globular gonadotropes. Only one gonadotropic cell type could be identified; its size, morphology and fine structure vary considerably. In the same specimen the GTH-cells can be predominantly globular or vesicular in appearance, depending on the reproductive phase of the fish. At spawning and after LH-RH injection, many GTH-cells reach a vacuolar stage; the content of the vacuoles is not immunofluorescent. Another cell type, which resembles GTH-cells in semi-thin sections, did not show gonadotropic properties; its nature and functional significance are unknown. In addition, the present study revealed an increase in the synthetic and exocytotic activity of prolactin cells after LH-RH injections. It is suggested that LH-RH mediates this effect via LH and eventually via estradiol.