AN IMMUNOCHEMICAL, ULTRASTRUCTURAL, AND DEVELOPMENTAL CHARACTERIZATION OF THE HORIZONTAL BASAL CELLS OF RAT OLFACTORY EPITHELIUM

The olfactory epithelium, which retains a capacity for neurogenesis th roughout life, contains two categories of basal cells, dark/horizontal and light/globose, neither of which is fully characterized with respe ct to their function during the processes of neurogenesis and epitheli al reconstitution after injury. The aim of this study was to define th e potential biological role(s) of dark/horizontal basal cells (D/HBCs) in the epithelium by performing immunochemical, electron microscopic, and developmental analyses of this cell population. The D/HBCs expres s several specific immunochemical characteristics, which include the r at homologues of human cytokeratins 5 and 14, which were identified on the basis of staining with subunit-specific monoclonal antibodies and two-dimensional immunoblot analysis of the immunoreactive proteins. I ndeed, the D/HBCs are the only cells in the olfactory mucosa that expr ess these specific cytokeratins. The D/HBCs also express an alpha-gala ctose or alpha-N-acetyl galactosamine moiety to which the I beta(4) is olectin from Bandeiraea simplicifolia binds. Moreover, the D/HBCs are heavily labeled by two different antibodies against the EGF receptor a nd by a monoclonal antibody that binds to phosphotyrosine. These chara cteristics are also common to the basal cells of respiratory epitheliu m. The electron microscopic analysis of the basal region of the olfact ory epithelium and the light microscopic immunofluorescence observatio ns demonstrate that the D/HBCs provide a bridge between the basal proc esses of some sustentacular cells and the basal lamina. The most strik ing ultrastructural feature of the D/HBCs is their enfolding of virtua lly all bundles of olfactory axons within tunnels formed where D/HBCs arch over the basal lamina. The intimacy of the arrangement between D/ HBCs and olfactory axons suggests that signals may pass from axons to D/HBCs or vice-versa. With respect to the development of D/HBCs, cells that express cytokeratins 5 and 14 and the EGF receptor first appear near the boundary with respiratory epithelium late in development, but do not extend throughout the olfactory epithelium until the middle of the first postnatal week. Taken together, the present findings and pr eviously published data suggest that D/HBCs help to maintain the struc tural integrity of the olfactory epithelium, participate in its recove ry from injury, and may also function to signal the status of the neur onal population of the epithelium. (C) 1995 Wiley-Liss, Inc.