How many receptor genes are expressed in each sensory neuron?
Quantitative analyses from in situ hybridization indicate that approximately 0.1% of cells express a given receptor gene.3,4 If the receptor genes are equally represented in the olfactory epithelium and there are one thousand genes, these data imply that one receptor gene is expressed in each cell. Catherine Dulac and Axel have recently shown that amplification of a single-cell cDNA library with degenerate primers to conserved regions of the receptor genes gives a single PCR product, further supporting the notion of one gene per cell.5 Given that one odorant receptor gene is expressed in each sensory neuron, how are sensory neurons organized in the nose? In other sensory systems, the spatial segregation of neurons or their projections is used to indicate the quality of the stimulus. In the auditory system, for example, the positions of hair cells along the basilar membrane are used to represent different tones. In the somatosensory system, the submodalities of pain and temperature are
