Immunocytochemical characterization of two types of microvillar cells in rodent olfactory epithelium

Microvillar cells (MCs) have been identified in the olfactory epithelium of various mammalian species from rodents to humans. Studies on properties and functions of MCs to date have yielded partially controversial results, supporting alternatively an epithelial or a neuronal nature of these cells. In the present study, single and double immunolabeling investigations were carried out using antibodies against cytoskeletal and integral membrane proteins in order to further characterize MCs in rat and mouse olfactory epithelium. Application of antibodies against ankyrin (ANK), a protein that links integral membrane proteins to the submembrane cytoskeleton, led to intense labeling of the basolateral membranes of numerous cells with characteristic MC morphology. ANK-immunoreactive (ir) cells bore an apical tuft of β-actin-ir microvilli, were filled with cytokeratin 18 (CK18)-ir filamentous network, and extended a basal process that appeared to end above the basal membrane. Immunoreactions for villin, an actin-crosslinking protein particularly prominently expressed in brush cells in the gastrointestinal and respiratory tract epithelia, and for the α-subunit of sodium-potassium ATPase (Na+, K+-ATPase), revealed that ANK-ir MCs fall into two subpopulations. The less frequent type I MCs displayed villin immunoreactivity in their apical microvilli and underneath the basolateral membranes; the more numerous type II MCs were negative for villin but possessed intense basolateral immunoreactivity for Na+, K+-ATPase. Strong reactivity for the epithelial-type integral membrane protein of adherens junctions, E-Cadherin, was localized in basolateral membranes of both types of MCs. Our results support an epithelial nature of ANK-ir MCs in rat and mouse olfactory epithelium. Type I MCs strongly resemble brush cells in their immunocytochemical characteristics, namely, their ANK reactivity, CK18 reactivity, and villin reactivity. The intense Na+, K+-ATPase reactivity of type II MCs implicates these cells in transport processes.