Structure and function of the transplanted pituitary in the seawater goby, Gillichthys mirabilis: I. The rostral pars distalis

Abstract

Auto- and homotransplanted rostral pars distalis (RPD) of the pituitary of a euryhaline seawater fish, Gillichthys mirabilis, was studied by electron microscopy. In intact seawater fish, the RPD contains prolactin, ACTH, and stellate cells, as well as a portion of the neurohypophysis. Type “B” fibers make direct contact with prolactin cells and ACTH cells; some type “A” fibers are adjacent to ACTH cells. After transplantation into the connective tissue under the “tongue,” the pituitary became revascularized rapidly; it was clearly visible under the transparent mucosa even 50 days after transplantation. The prolactin cells of the graft exhibited cytologic signs of active synthesis and release of secretion. Plasma sodium concentrations were significantly increased after autotransplantation. In homotransplanted fish, the prolactin cells within the gland in situ became cytologically inactive after 40–50 days. When fish bearing an autografted pituitary were transferred directly to fresh water, exocytosis of the secretory granules from the prolactin cells was frequently observed. ACTH cells showed no clear cytological changes in response to experimental manipulations. These observations indicate that in Gillichthys the prolactin cells may be regulated by at least the following three factors: (1) inhibitory control from the hypothalamus presumably mediated by type “B” fibers, (2) direct stimulation due to decreasing plasma sodium level, and (3) negative feedback control by prolactin or increasing plasma sodium. The ACTH cells seem to be comparatively independent from hypothalamic control.