(A) Olfactory epithelium consists of supporting cells, basal cells, and olfactory sensory neurons. The basal cells are the stem cells for production of the olfactory neurons. Six to eight modified cilia project from the apex of the olfactory neuron and contain the odorant receptors. (B) This micrograph shows olfactory neurons in the nose of a genetically modified mouse in which the gene encoding LacZ has been inserted into an odorant receptor locus, so that all the cells that would normally express that particular receptor now make the enzyme LacZ in addition. The LacZ is detected through the blue product of the enzymatic reaction that it catalyzes. The cell bodies (dark blue) of the marked olfactory neurons, lying scattered in the olfactory epithelium, send their axons (light blue) toward the brain (out of the picture to the right). (C) A cross section of the left and right olfactory bulbs, stained for LacZ. Axons of all the olfactory neurons expressing the same odorant receptor converge on the same glomeruli (red arrows) symmetrically placed within the bulbs on the right and left sides of the brain. Other glomeruli (unstained) receive their inputs from olfactory neurons expressing other odorant receptors. (B and C, from P. Mombaerts et al., Cell 87:675–686, 1996. © Elsevier.)