Human Olfactory Epithelium: Normal Patterns and Types of Lesions Found in The General Population

Abstract

Immunocytochemical studies of tissue sections and whole-mount preparations have been used to analyze human olfactory epithelium in autopsies obtained from cases ranging from 2 days to 91 yr of age. Olfactory epithelium in humans is very uniform in the neonate and young child, but sensory areas and scattered neurons become interspersed with nonsensory areas in the adult. A few olfactory neurons may remain in areas that include respiratory cells, goblet cells, or secretory glands; conversion to squamous epithelium occurs in some cases. Islands of densely packed olfactory neurons may remain even when most of the region has been converted to respiratory epithelium, suggesting that neuronal stem cells cannot effectively migrate laterally to reconstitute the sensory epithelium. Deep pits lined with olfactory neurons are seen at all ages and have not been observed in other animals, except for monkeys. Their function is unknown. Large invaginations of the epithelium into the lamina propria are common in older individuals. In some cases, including Alzheimer’s disease patients, misdirected neurites accumulate within the epithelium in masses, and these neurites, as well as some swollen and degenerate cells, show abnormal expression of neurofilament proteins and tau. This phenotype of ectopic neurite outgrowth and altered expression of cytoskeletal proteins may represent a necrotic neurodegenerative pathway that differs from the more frequent route of neuronal degeneration triggered by most damaging events, which may be through programmed cell death. Elucidation of the cellular pathways and signals that regulate differentiation of the assorted cell types that repopulate the epithelium will help identify the targets of various xenobiotic and pathogenic agents and the mechanisms that lead to an altered epithelial phenotype.