Changes in the cells of the adenohypophysis associated with the diadromous migration of the threespine stickleback, Gasterosteus aculeatus L.

The adenohypophysis of the threespine stickleback, Gasterosteus aculeatus, 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 (RPD) consists mainly of two types of cells: dorsally shifted lead hematoxylin (PbH)-positive cells bordering the neurohypophysis correspond to corticotrophs, and antero-ventrally shifted acidophil cells are identified as prolactin cells. The latter undergo marked hypertrophy and active state just at the time of entering the river (February), while no detectable change was seen in the former throughout anadromous migration. The role of prolactin on the osmoregulation in freshwater environment is thus suggested. The proximal pars distalis (PPD) consists mainly of two cell types: the basophil cells in round shape are regarded as the gonadotrophs and the acidophil cells in ellipsoid shape are considered to be somatotrophs. The size of the gonadotrophs reaches the maximum at the time of spawning. A few AF-positive cells of elongate shape occur in the dorsal region and are identified as thyrotrophs. In the pars intermedia (PI), two types of cells are discernible: PAS-positive and PAS-negative cells. The latter attained their maximal size in the earliest time of anadromous migration.