Hyperplasia of the lymphoepithelium of NALT in rats but not in mice upon 28-day exposure to 15 ppm formaldehyde vapor

To investigate if local lymphoid tissues are a target of FA, nasopharynx-associated lymphoid tissues (NALT) and upper-respiratory tract-draining lymph nodes were examined in a 28-day inhalation study with FA vapor in Fischer-344 rats and B6C3F1 mice.Paraffin-embedded tissues were sectioned and stained with H&E or stained immunohistochemically for cell proliferation (BrdU incorporation).Light microscopy revealed simple hyperplasia of NALT lymphoepithelium of rats exposed to 15 ppm and an increased proliferation rate of the epithelial cells. Principal component (discriminant) analysis of rat NALT and lymph nodes data did not reveal other effects or effects at lower exposure levels. Mice tissues were not affected.It was concluded that hyperplasia of the lymphoepithelium of NALT of rats exposed to 15 ppm was the only distinct effect of FA vapor on local lymphoid tissues (NALT and lymph nodes) of Fischer-344 rats and B3C3F1 mice.