Ultrastructure of the Frog Pars Intermedia in Relation to Hypothalamic Control of Hormone Release

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Abstract
An ultrastructural study was made of the pars intermedia of Rana catesbeiana (bullfrog) and Rana piplens subjected to varying conditions of background adaptation. The cytoplasm of the predominant cell type in all glands studied exhibited a fairly constant population of membrane-bounded granules (2000-3000 A) of varied shapes and densities. The rough ER [endoplasmlc reticulum] of all dark-adapted animals and of bullfrogs in transitional stages between light and dark adaptation shows striking whorl formation and expansion of cisternae. Numerous dense, membrane-bounded granules are associated with the Golgi region in such specimens. Glands from dark-adapted and transitional bullfrogs contain many large (1-2 microns) dense droplets within ER cisterns. These are seen infrequently or not all in dark-adapted R. pipiens. They may be some form of polypeptlde precursor to MSH (melanocyte-stimulating hormone). The appearance of the ER and Golgi regions in dark-adapted animals, as compared with light-adapted ones, suggests augmented protein synthesis In the former group. Nerve fibers and synaptic terminals containing small, clear vesicles (200-300 A) and some dense core vesicles (600-1000 A) were observed throughout the pars intermedia in every specimen. Their presence is discussed in relation to evidence for catecholamlne-mediated hypothalamic control of the pars intermedia. The finding that the cytology of glands from animals undergoing lightening is characteristic of dark-adapted pituitaries suggests that neural inhibition from the brain first occurs at the level of hormone release. It is suggested that control of synthesis is mediated by a feedback mechanism activated by amounts of hormone stored in the gland.