ULTRASTRUCTURAL CHARACTERISTICS OF 2 DISTINCT GONADOTROPES (GTH I-CELLS AND GTH II-CELLS) IN THE PITUITARY OF RAINBOW-TROUT ONCORHYNCHUS-MYKISS

The salmonid pituitary produces two chemically distinct gonadotropins (GTHI and GTHII). Ultrastructural characteristics of GTHI- and GTHII-producing cells were studied in the trout pituitary with electron-microscopic immunocytochemistry using antisera against salmon GTHIbeta- and IIbeta-subunits. In pituitaries from vitellogenic fish, GTHI-cells were characterized by numerous dilated cisternae of the granular endoplasmic reticulum (GER) and a small number of Ibeta-positive granules (diameter, 100-300 nm), whereas GTHIIbeta-immunoreactivity was found on granules (diameter, 200-400 nm) and large globules (diameter, 500-4000 nm) in apparently different cells (GTHII-cells). Distinct cellular distributions of GTHI and GTHII were maintained during gametogenesis, although morphological characteristics of GTHI- and GTHII-cells over-lapped each other due to changes in number and size of the granules, globules and cisternae of the GER. Interestingly, the globules in the GTHI-cells were immunonegative for GTHIbeta, although in the GTHII-cells they were always stained with GTHIIbeta-antiserum. These results confirm that GTHIbeta and GTHIIbeta are synthesized in distinctly different cell-types in the salmonid pituitary and indicate that morphological characteristics cannot be used to distinguish these two cell-types.