3D atlas describing the ontogenic evolution of the primary olfactory projections in the olfactory bulb of Xenopus laevis

The adult Xenopus presents the unique capability to smell odors both in water and air thanks to two different olfactory pathways. Nevertheless, the tadpole can initially perceive only water-borne odorants, as the olfactory receptor neurons (ORN) that will detect air-borne odorants develop later. Such a phenomenon requires major reorganization processes. Here we focused on the precise description of the neuroanatomical modifications occurring in the olfactory bulb (OB) of the tadpole throughout metamorphosis. Using both carbocyanine dyes and lectin staining, we investigated the evolution of ORN projection patterns into the OB from Stages 47 to 66, thus covering the period of time when all the modifications take place. Although our results confirm previous works (Reiss and Burd [1997] Semin Cell Dev Biol 8:171-179), we showed for the first time that the main olfactory bulb (MOB) is subdivided into seven zones at Stage 47 plus the accessory olfactory bulb (AOB). These seven zones receive fibers dedicated to aquatic olfaction (aquatic fibers) and are conserved until Stage 66. At Stage 48 the first fibers dedicated to the aerial olfaction constitute a new dorsomedial zone that grows steadily, pushing the seven original zones ventrolaterally. Only the part of the OB receiving aquatic fibers is fragmented, reminiscent of the organization described in fish. This raises the question of whether such an organization in zones constitutes a plesiomorphy or is linked to aquatic olfaction. We generated a 3D atlas at several stages which are representative of the reorganization process. This will be a useful tool for future studies of development and function.