Fine structure of the rostral pars distalis of the adenohypophysis of the killifish, Fundulus heteroclitus, in fresh and salt water.

A fine structural study has confirmed earlier light microscopic observations indicating that prolactin cells are the only endocrine cells present in the main body of the rostral pars distalis of the adenohypophysis of Fundulus heteroclitus, a killifish common in New England coastal waters. Some ACTH cells occurred in thin plaques applied to the neurohypophysial trunk in the posterior part of the region. In freshwater-adapted specimens the volume occupied by the prolactin cell mass was larger than in saltwater-adapted specimens and contained larger prolactin cells. A paucity of contact specializations between the parenchymal cells may facilitate their spatial rearrangement as the prolactin cell population varies with changes in ambient salinity. Many fine neurohypophysial processes penetrated deeply into the rostral pars distalis and contained Type B (aminergic) nerve fibers believed to modulate prolactin secretion. These fibers ended mainly on the basement membrane that separates the neurohypophysial processes from the parenchyma. Synaptic contacts on prolactin cells were not observed but no prolactin cell appeared to be more than five cell widths from such a nerve terminal. The results emphasize the usefulness of the rostral pars distalis of this easily obtained and maintained teleost for studies of prolactin cell function.