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Development of the olfactory system in Xenopus laevis begins during gastrulation, with the induction of olfactory placodes at the rostral edge of the prospective neural plate. Initial placodal induction appears to involve cerberus, a molecule secreted from the involuting anterior endoderm. Possible downstream genes expressed in the anterior neural ridge and sense plate include the transcription factors Pax-6, X-dll2, X-dll3, and Xotx2. Forebrain development is dependent on the presence of the placode and subsequent innervation by olfactory axons, with the extent of this dependence declining as development advances. During metamorphosis thyroid hormones initiate extensive changes in the olfactory system, including the origins of new regions of the olfactory epithelium and olfactory bulb, and a change in olfactory projection patterns.
Figure 1. Sagittal section of a juvenile frog to show the three
nasal cavities. Anterior to left. PC, principal cavity; MC,
middle cavity; VNO, vomeronasal organ. Scale = 200 µm.
Figure 2. Electron micrographs to show differing surface
specializations of receptor cells (R) and supporting cells (S)
in the three nasal cavities. (A) Principal cavity. Ciliated
receptor cells and microvillar supporting cells. (B) Middle
cavity. Both ciliated (c) and microvillar (m) receptor and
supporting cells. (C) Vomeronasal organ. Microvillar receptor
cells and ciliated supporting cells. Scale = 2 µm.
J. O. Reiss and G. D. Burd
1
Figure 3. Timeline of important events in development of
the Xenopus olfactory system. Illustrations not to scale
Figure 4. Diagram showing the location of the prospective
olfactory placodes, olfactory bulbs, cement gland, and
stomodeum/hypophyseal placode at stage 15, when placodes
are specified, and of the differentiating structures at
stage 32, when olfactory axons have just entered the brain.
Anterior views, not to scale.
Figure 5. Diagrammatic lateral view of metamorphic
changes in the olfactory system. The middle cavity is absent
prior to metamorphosis. In midmetamorphic animals the
middle cavity and principal cavity axons both project to the
ventral part of the main olfactory bulb, but in postmetamorphic
animals principal cavity axons project only to
the dorsal part of the main olfactory bulb. PC, principal
cavity; MC, middle cavity; VNO, vomeronasal organ.
Sagittal section of a juvenile frog to show the three
nasal cavities. Anterior to left. PC, principal cavity; MC,
middle cavity; VNO, vomeronasal organ. Scale = 200 âµm.
