X-Gal-labelled P2 glomeruli in TAP+/+ and TAP−/− mice. High-magnification images of cryosections from the main olfactory bulb. These sections were cut parallel to the long axis of the bulb. The external plexiform layer is labelled epl. The olfactory nerve layer is labelled nl. Axons from P2 olfactory sensory neurons are labelled with the blue X-gal dye. (a) Two representative cryosections from a P2iTLZ animal with labelled P2 glomeruli in the ventrolateral and ventromedial domains of each olfactory bulb. The black, vertical line extending from dorsal to ventral mitral cell layer is the dorsoventral axis, as described in §5. The dashed blue lines outline the glomerular layer (gl) in each cryosection. As is typical for P2iTLZ animals, the left, more rostral image contains a ventral P2-lateral glomerulus at 150°, whereas the right, more caudal image contains a ventral P2-medial glomerulus at 205°. (b) A singlet P2-medial glomerulus in a TAP+/+ animal. Left (TAP+/+): typical singlet glomerular cross section from a TAP+/+ animal. The scale bar indicates 100 µm. Right (TAP−/−): a set of triplet P2-medial glomeruli in a TAP−/− animal. Although these glomeruli have a normal morphology, the arrow heads in the right panel point to two exceptionally small glomeruli. (c) Two distinct, abnormally fused P2 glomeruli seen in TAP−/− animals. (d) Serial montage of glomerular cross sections from a TAP+/+ doublet (top panel) with one intervening, unlabelled glomerulus and a fused TAP−/− doublet (bottom panel) with zero intervening glomeruli.