Mucous domains: microchemical heterogeneity in the mucociliary complex of the olfactory epithelium

Access to and clearance of odorants from binding sites on olfactory cilia are regulated by a complex interplay of molecular, physical and cellular factors. These perireceptor events occur primarily in the mucociliary complex. The use of gold-labelled lectinoprobes, one from Limax flavus (LFA) which is specific for terminal sialic acid residues, and one from Datura stramonium (DSA) specific for N-acetylglucosamine residues, demonstrated intricate patterns of binding in mucous domains of the olfactory mucus and ectodomains of the glycocalyx of olfactory cilia. In electron micrographs of Lowicryl-embedded salamander olfactory mucosa, the mucus consisted of an electron-dense domain that lay superficial to an electron-lucent domain; the interface between the two was irregular. A significantly higher density of binding sites for both lectins was present in the superficial than in the deeper domain. The two domains were not homogeneous: there were small electron-lucent domains (hsL) within the superficial electron-dense domain (hsD) that bound a 4.8-fold lower density of gold-labelled DSA than the surrounding matrix, and the olfactory cilia, which project into hsD, were surrounded by an electron-lucent sheath that appeared to be continuous with the deeper domain. Ectodomains of the glycocalyx associated with olfactory cilia exhibited a higher density of binding sites for both LFA and DSA than did either microvilli of sustentacular cells or respiratory cilia. Specificity of the lectinoprobes was confirmed by inhibition of binding with specific sugars or enzymic removal of specific sugar residues. These results demonstrated microchemical heterogeneity of the non-homogeneous mucous domains in olfactory mucus and in the attendant glycocalyx of olfactory cilia based on the differential localization of sialic acid and N-acetylglucosamine sugar residues.