Spontaneous pituitary adenomas in aging rats. A light microscopic, immunocytological and fine structural study.

The structural features of 26 spontaneous pituitary tumors in aging female Long-Evans rats have been investigated by different morphologic techniques including immunoperoxidase staining and electron microscopy. By light microscopy, the tumors corresponded to chromophobic-sparsely granulated acidophilic adenomas, containing numerous pigment granules and showing congestion as well as focal hemorrhages. Positive cytoplasmic staining was obtained with Herlant's erythrosin as well as with Brookes' carmoisine methods, used to detect secretory granules of prolactin cells. Immunoperoxidase technique revealed the presence of immunoreactive prolactin in the cytoplasm of many adenoma cells. Growth hormone and TSH immuno-stainings were negative. By electron microscopy, the tumors were found to consist of prolactin cells exhibiting marked variability in subcellular morphology and differing considerably from non-tumorous resting prolactin cells. A decrease in size and number of secretory granules, proliferation of rough-surfaced endoplasmic reticulum, formation of "Nebenkerne", accumulation of free ribosomes, prominence of Golgi complex and the presence of misplaced exocytosis were characteristic features of the adenoma cells and were interpreted as indicating enhanced secretory activity. Crinophagy and transformation of secretory granules into pigment deposits were striking findings in many adenomas. Since all the adenomas seemed to derive from prolactin cells and belong to the same tumor class, it is assumed that prolactin cells in female Long-Evans rats are more susceptible to oncogenic stimuli than other hypophysial cell types.