Freeze-fracture study of the plasma membranes of the septal olfactory organ of Masera.

The olfactory border and the apical cell contacts of the organ of Masera (MO) of the mouse were investigated by freeze-fracture electron microscopy. The olfactory border is mainly composed of the terminals of receptor and supporting cells. Cells with thick microvillus-like projections, though less frequent than the other two cell types, also contribute to the border. Olfactory knobs show transitions between those displaying numerous cilia and those characterized by few or no cilia. The olfactory cilia have a typical necklace of 6-9 rows of particles. The eruption of developing cilia seems to be preceded by the formation of circular arrays of particles. The density of intramembranous particles (IMP) per micron2 in P- and E-faces of the ciliary membranes is 1095 +/- 190 and 205 +/- 65, respectively. In the microvilli of supporting cells, the density of IMP per micron2 is 1800 +/- 270 for the P-face and 570 +/- 135 for the E-face. At the base of the supporting cell microvilli, rod-shaped particles are observed. The lateral plasma membranes of these cells bear orthogonal arrays of particles. In the apical region of the MO neuroepithelium, extensive zonulae occludentes are present which seal the intercellular cleft. The zonulae occludentes between supporting and receptor cells are composed of 5-13 junctional strands, usually arranged in an elongate network. Zonulae occludentes between supporting cells are, in addition to the elongate network, also arranged in a mesh-like pattern. Gap junctions, both associated with the zonulae occludentes and independent of them, are occasionally found between supporting cells. The results obtained indicate that important similarities exist between the neuroepithelium of the MO and the olfactory epithelium proper, whereas remarkable differences exist between the MO and the vomeronasal neuroepithelium.