Fluorescence histochemistry of RNA in human pituitary adenomas

Acridine orange (AO)-RNA fluorescence was studied histochemically in 9 normal human pituitary glands, in 26 secretory and nonsecretory pituitary adenomas, and in a dysgerminoma. Six adenomas showing immunoreactivity for prolactin showed intense orange-red cytoplasmic fluorescence; 6 other tumours exhibiting immunoperoxidase activity of growth hormone showed less intense AO-RNA fluorescence, and 5 adrenocorticotropic hormone-containing adenomas showed still weaker orange fluorescence. Among the chromophobe adenomas without immunoreactivity for secretory products, 5 had uniformly very weak AO-RNA fluorescence, while in 4 other a few scattered cells with strong AO-RNA fluorescence were detected among the majority of weakly fluorescent cells. The pituitary dysgerminoma contained many cells with strong AO-RNA fluorescence. Electron microscopy of these cases showed good correlation of cytoplasmic concentration of ribosomes with AO-RNA fluorescence of the adenomas. AO is useful as an inexpensive, simple supplementary stain for frozen or paraffin sections of pituitary tumours to infer secretory activity as a correlate of RNA concentration and to demonstrate a secretory potential in some cells of hormonally inactive tumours.