Salmonid pituitary gonadotrophs. III. Chronological appearance of GTH I and other adenohypophysial hormones in the pituitary of the developing rainbow trout (Oncorhynchus mykiss irideus).

The first appearance of adenohypophysial cells in the developing rainbow trout embryo was studied by immunocytochemistry. Antibodies generated against the beta subunits of coho salmon gonadotropins (GTH I and GTH II), the beta subunit of human thyroid stimulating hormone (TSH), chum salmon prolactin (PRL), chum salmon growth hormone (GH), and synthetic alpha-MSH were used as immunocytochemical probes. The pituitary anlage was first recognized at developmental stage 21 (18 days postfertilization). At this stage, immunoreactive (ir) PRL and alpha-MSH cells were identified in the rostral and caudal regions of the pituitary, respectively. Cells containing ir-GH and ir-adrenocorticotropin (ACTH) appeared at stage 28 (35 days postfertilization). At the last stage before hatching (stage 29, 42 days postfertilization) ir-TSH cells were identified. Ir-GTH I cells were first observed in the pituitary at stage 32 (15 days after hatching), when mitosis of gonadal germ cells also was observed. At stage 35 (35 days after hatching), meiotic figures were first observed in a few germ cells of some fish indicating that gonadal sex differentiation probably begins at about this time. Cells containing ir-GTH II were not apparent in the pituitary at any stage of embryonic or larval development (up to stage 35) and were not present in pituitaries of trout at 6 months of age when the ovaries were in the perinucleolus stage and when only spermatogonia, but no spermatocytes were present in the testis. These observations suggest that GTH I, but not GTH II, may regulate initial gonadal growth and development in the embryonic and larval rainbow trout.(ABSTRACT TRUNCATED AT 250 WORDS)