Is the expression of receptor genes also random in the olfactory epithelium of mammals?
Linda Buck, now at Harvard University, and Robert Vassar, a postdoctoral fellow in Richard Axel’s laboratory, have independently shown by in situ hybridization that there are four zones of receptor gene expression in the olfactory epithelium of mammals, each similar in complexity to the entire epithelium of the fish (Figure 3).3,4 Although the expression of each receptor gene is restricted to a single zone, sensory neurons expressing different receptor genes are randomly distributed in a punctate pattern within each zone. Clearly, the limited segregation in the epithelium is inadequate to explain the extreme discriminatory power of the olfactory system. From these data Model 1 could be excluded from consideration. To distinguish between Model 2 and Model 3, biologists had to analyze the next level of the sensory system. Previous experiments by Gordon Shepherd, John Kauer, and their colleagues at Yale University using voltage-sensitive dyes and metabolic labeling indicated that exposure