Electron microscopy of human olfactory epithelium reveals a new cell type: the microvillar cell

The olfactory epithelium of mammals is generally considered to consist of 3 cell types: basal cells, supporting (sustentacular) cells, and ciliated olfactory receptors. We have completed a detailed ultrastructural study of the fine structure of the human olfactory mucosa. In our electron microscopic observations of biopsies of human olfactory epithelium taken from normal, consenting volunteers under local anesthesia, we have consistently observed a fourth cell type, the microvillar cell, located near the epithelial surface. The apical end of these flask-shaped, electron-lucent cells gives rise to a tuft of microvilli that project into the mucus layer lining the nasal cavity. The cell body itself contains bundles of microfilaments, mitochondria, a well-developed smooth endoplasmic reticulum, a prominent Golgi complex, electron-dense vesicles that resemble lipofuscin granules, free ribosomes, and occasional cisternae of the rough endoplasmic reticulum. A thin, axon-like cytoplasmic process extends from the basal pole of the cell and travels through the epithelium toward the lamina propria. Although there is no physiological evidence that bears upon the function of the microvillar cell, its ultrastructure suggests it may be a bipolar sensory neuron. Based upon morphological and phylogenetic considerations, the authors speculate the microvillar cell represents a second morphologically distinct class of chemoreceptor in the human olfactory mucosa.