Freeze-fracture characterization of cell types at the surface of the taste organ of the frog, Rana esculenta.

The ultrastructure and distribution of intramembrane particles in the chemoreceptor surface of the frog taste organ have been studied by means of freeze-fracture. Sustentacular, wing, mucous cells and two different types of putative taste cells were found to reach the free surface of this chemoreceptor. Each of these cell types was characterized by a different pattern and density of intramembrane particles in the free surface. Wing cells displayed a relatively low number of large intramembrane particles (11.1 +/- 1.4 nm in diameter). Particles of similar size were also present in a much higher concentration in the membrane of cylinder-ending putative taste cells. In microvilli-ending putative taste cells, mucous cells, and sustentacular cells, small intramembrane particles were observed (6.8 +/- 0.78, 6.9 +/- 1.3, 7.2 +/- 0.7 nm in diameter, respectively). The density of these particles was higher in the sustentacular cells than in the other two cell types. These data provide evidence that there are two morphologically distinct types of putative taste cells in the frog taste organ, demonstrating that they are characterized by different pattern of intramembrane particles in their free surface. Furthermore, the present results support previous findings indicating that wing and sustentacular elements represent two different cell types.