PUBERTAL DEVELOPMENT OF MALE AFRICAN CATFISH (CLARIAS-GARIEPINUS) - PITUITARY ULTRASTRUCTURE AND RESPONSIVENESS TO GONADOTROPIN-RELEASING-HORMONE

Pubertal development was studied in male African catfish by immunocyto chemical examination of pituitary gonadotrophs and by monitoring the r esponsiveness of gonadotropin (GTH) secretion to salmon GnRH analogue (sGnRHa) in vitro. Experiments were carried out with fish from 9 to 28 wk of age. Fish were assigned to four groups, according to the stage of spermatogenesis: I, spermatogonia alone; II, spermatogonia and sper matocytes; III, spermatogonia, spermatocytes, and spermatids; IV, all germ cell stages, including spermatozoa. Basal and sGnRHa-stimulated s ecretion of the LH-like GTH II increased 3- to 4-fold from stage I to II and from stage II to III, whereas a 15-fold increase was recorded f rom stage III to IV. The ED(50) values of sGnRHa varied between 0.08 a nd 0.49 nM, stages II and III being less sensitive. The highest dosage of sGnRHa (100 nM) led to a reduction of GTH secretion. In the first three stages, the pituitary secreted large amounts of free alpha-subun it while free GTH II beta-subunit was not detected at any stage of dev elopment. Antisera against GTH II and its alpha- and beta-subunits wer e used for immunocytochemical studies. In stages I and II, two subtype s of gonadotrophs, which differed in the size and labeling intensity o f their secretory granules, were present. Both types of granules were immunopositive for the two subunits of GTH II. In stages III and IV, o nly gonadotrophs of the subtype with the larger granules were found. G lobules and irregular, membrane-bound masses (IMs), probably arising t hrough fusion of secretory granules, appeared in the gonadotrophs in s tage III and became more prominent in stage IV. Globules and IMs were immunopositive for the beta-subunit but negative for the alpha-subunit . We conclude that the two subtypes of gonadotrophs represent differen t developmental stages of GTH Ii-producing cells, as they shared immun olabeling for the alpha-and the beta-subunits of GTH II. The scarcity of GTH II beta-subunit may be rate-limiting for the amount of intact G TH II available for secretion, particularly al early stages of develop ment. In contrast, al more advanced stages when the readily releasable pool of GTH II has greatly increased, the amount of GTH II also appea rs to be controlled by modification or elimination of the alpha-subuni t from globules and IMs.