Changes in the Cells of the Adenohypophysis Associated with the Diadromous Migration of the Threespine Stickleback, Gasterosteus aculeatus L

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Abstract
The adenohypophysis of the threespine stickleback was studied light-microscopically to determine and estimate the cell types and their function. For these purposes, the adult specimens obtained during the period from migration to spawning were examined. Further, the juveniles caught in the spawning bed were subjected artificially to sea water. The rostral pars distalis consists mainly of 2 types of cells: dorsally shifted lead hematoxylin-positive cells bordering the neurohypophysis which correspond to corticotrophs, and antero-ventrally shifted acidophil cells identified as prolactin cells. The latter undergo marked hypertrophy and active state just at the time of entering the river (Feb.), while no detectable change was seen in the former throughout anadromous migration. The role of prolactin on the osmoregulation in the fresh water environment is suggested. The proximal pars distalis consists mainly of 2 cell types: the basophil cells in round shape, regarded as the gonadotrophs, and the acidophil cells in ellipsoid shape, considered to be somatotrophs. The size of the gonadotrophs reaches the maximum at the time of spawning. A few aldehyde fuchsin-positive cells of elongate shape occur in the dorsal region and are identified as thyrotrophs. In the pars intermedia, 2 types of cells are discernible: PAS[periodic acid Schiff]-positive and PAS-negative cells. The latter attained their maximal size in the earliest time of anadromous migration.