GTH-CELLS IN THE PITUITARY OF THE AFRICAN CATFISH, CLARIAS-GARIEPINUS, DURING GONADAL MATURATION - AN IMMUNOELECTRON MICROSCOPIC STUDY

In an ultrastructural immunocytochemical study we investigated the dev elopment of the gonadotropic cells in the pituitary of two to six mont hs old male African catfish in relation to testicular development. In this period, pituitary and testicular tissue samples were collected on five occasions (groups I - V). Blood samples could only be taken from the fish in groups III-V. The testicular development was divided in t hree stages i.e., immature (only spermatogonia, group I), early (sperm atogonia and spermatocytes, groups II and III) and advanced (all germ cell stages including spermatozoa, groups IV and V) spermatogenesis. 1 1-Ketotestosterone blood levels were low, except for the last group. A ntisera were raised against the complete catfish alpha,betaGTH-II, as well as to the separate alpha- and beta-subunits of catfish GTH-II. In the proximal pars distalis of immature fish, undifferentiated cells, somatotrops, putative thyrotrops (pTSH) and putative gonadotrops (pGTH ) were found. In the two latter, secretory granules were labeled with anti-alphaGTH, but not with anti-betaGTH-II. pTSH- and pGTH-cells were distinguished on the basis of the size of their secretory granules. D uring early spermatogenesis, two classes of putative gonadotrops could be distinguished. One type had the same immunocytochemical and ultras tructural characteristics as in immature fish; the secretory granules in the second cell type, which was more abundant, were also immunoposi tive for anti-betaGTH-II. The mean volume of the secretory granules in these GTH-II cells was three times larger than that in the early appe aring pGTH-cells. In addition, the later appearing GTH-II cells contai ned large inclusions, known as globules. These structures labeled with anti-alphabetaGTH-II and with anti-betaGTH-II, but not with anti-alph aGTH. It is assumed that the globules are involved in a differential s torage and/or breakdown of the GTH-II subunits. During advanced sperma togenesis the two gonadotropic cell types could still be distinguished , but the early appearing pGTH-cell type was scarce. The present obser vations permit the conclusion that the early appearing cells may be GT H-I cells. However, definitive proof about their identity depends on t he availability of antibodies or cDNA probes specific for GTH-I.