Ultrastructure of the pars intermedia of pigmented rats: Observations of melanin-like granules within secretory cells

Ultrastructure of the pituitary intermediate lobe in aging rats

The pituitary intermediate lobe, a source of pro-opiomelanocortin (POMC) peptides, was examined with light and electron microscopic techniques in Sprague-Dawley rats aged 5-18 months and Zucker rats aged 18 months. Cysts were common in the intermediate lobe in the Zucker animals, a finding also noted in human pituitary glands by other investigations. The nuclear envelopes were often indented in cells of aging rats, while those of young animals were generally smooth. Lipid droplets and lysosomes, rarely seen in tissue from young animals, were frequently observed in endocrine cells of older rats. Most cells had an abundance of secretory granules, suggestive of enhanced storage of peptides in the cytoplasm. Nerve terminals which were present among endocrine cells contained myelin figures in some of the old rats, and may indicate degenerative changes, while other terminals appeared normal. These morphologic findings suggest that the aging phenomena in intermediate lobe tissue have characteristics in common with nervous tissue, and may also reflect a diminished inhibitory neuroregulation.

Effects of reserpine administration on the fine structure of the rat pars intermedia

Hormone release from the pars intermedia is under inhibitory control of the hypothalamus. Control may be mediated via dopaminergic fibers which directly contact secretory cells. Administration of reserpine in vivo to adult male rats for four consecutive days results in major alternations in pars intermedia cytology. Cells show expanded areas and whorls of rough endoplasmic reticulum, as well as extensive Golgi zones with numerous dense granules. Some nerve fibers exhibit alterations in vesicle content, while others retain a more normal appearance. Freeze-fracture of glands from reserpine-treated animals provides evidence for exocytosis of granules, although such phenomena are not observed in thin sections. The ultrastructural findings suggest that reserpine alters the content of local inhibitory neurotransmitters in the pars intermedia, leading to unrestrained hormone release, followed by renewed granule synthesis.