Figure 2.

Schematic representation of the spatio-temporal neurogenic pattern and its connections within the olfactory systems during embryonic development in the mouse. (A) At approximately embryonic day 9.5 (E9.5), the olfactory placode starts to form with the first signs of neurogenesis in the presumptive OE, while the first neurons are generated by E11 at the OB and by E10 at the OC. (B) The first axons emerging from the olfactory placode are identified before E10.5, and they form clear olfactory and vomeronasal nerves between E11 and E12, which form synapses in the OB and AOB around E13.5 (dotted lines suggest the first axons from the VNO arriving into the AOB). (A) First LOT axons (AOB) are observed between E11.5 and E12 and (B) LOT covers the OC surface by E13.5. (C) Axon collaterals emerge from the LOT in gross amount E15.5, colonizing the OC structures. (D) OSN axons, diffusely innervating the OB at E13.5, reorganize topographically innervating protoglomeruli by E17.5 in a process which extends until postnatal stages (not illustrated). The maturation of synapses between OSN and OB neurons occurs by E17–E18, while AOB synapses mature by the end of the first postnatal week (not illustrated). The definitive OB and AOB layering depends on the arrival of the prospective interneurons generated in their neurogenic niche at the forebrain SVZ (see text for details).