Histogenesis of the pituitary in rainbow trout (Salmo gairdneri) in different ambient salinities with particular reference to the rostral pars distalis.

The pituitary gland is first evident in rainbow trout two weeks before hatching. Differentiation of prolactin and ACTH cells is not marked until 3-4 days after hatching when the follicular arrangement of the prolactin cells become apparent. There is no difference in the time of development of either prolactin or ACTH cells in larvae reared in different ambient salinities despite marked changes in tissue ion and water content. This suggests that prolactin and ACTH do not play the osmo- and iono-regulatory roles in larval rainbow trout that they are considered to play in adult salmonids.