ONTOGENY OF PITUITARY CELL-TYPES AND THE HYPOTHALAMOHYPOPHYSEAL RELATIONSHIP DURING EARLY DEVELOPMENT OF CHUM SALMON, ONCORHYNCHUS-KETA

The ontogeny of pituitary cell-types, hypothalamic neurons and their fibers, was studied immunocytochemically during development of chum salmon. Five weeks after fertilization (eyed stage embryos), 5 cell-types were detected in the adenohypophysial (AHP) anlage; prolactin (PRL)-, growth hormone (GH)-, adrenocorticotropin (ACTH)-, melanotropin (alpha-MSH)-, and thyrotropin (TSH)-producing cells. The PRL-, GH- and ACTH-cells were relatively well developed as compared with MSH- and TSH-cells. Gonadotropes, however, were not detected even 3 weeks after hatching (10 weeks after fertilization). The neurohypophysis (NHP), on the other hand, began to grow around the eyed stage. Neuroendocrine fibers, not only from the tuberal hypothalmus (melanin-concentrating hormone) but also from the preoptic regions (vasotocin and somatostatin), reached the NHP during the last week of embryonic life. In the developing pituitary, the ratio between the length of the boundary between the AHP and the NHP, and the area of the AHP, was stable, being approximately 1-4%. The coordinated development of the AHP and NHP in chum salmon seems to result in the development of the characteristic hypothalamo-hypophysial relationship by the time of hatching.