Cellular and molecular interactions in the development of the Xenopus olfactory system.

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.