Olfactory bulb interneurons - The developmental timeline and targeting defined by embryonic neurogenesis.

The generation of mouse olfactory bulb (OB) interneurons (INs) is initiated in the embryo but continues throughout life. It is generally agreed that OB INs generated postnatally affect the connectivity of the OB, depending on the timeline of neurogenesis. Here, we focused on OB INs generated embryonically, which have generally received less attention than those generated in the adult. Birthdates of embryonic INs were differentiated by maternal injections of thymidine analogs and their final destinations and phenotypes in the OB analyzed by immunohistochemistry. We found that the first embryonic INs were generated at embryonic day 10 (E10) and continued through the entire embryonic development. Analysis in adult tissues showed that embryonic INs were retained and were distributed across all layers of the OB. Interestingly, an initial lateral preference in cell density was seen in INs generated during E11-E13. Although INs are broadly distributed in the OB, we found that within the granule cell layer (GCL), OB INs distributed mostly in the superficial GCL. Immunostaining for calbindin, parvalbumin, tyrosine hydroxylase, 5T4 and calretinin were lacking co-expression with thymidine analogs labeled cells, suggesting that maturation of embryonic INs occurred slowly following birth. We studied the embryonic neuroblasts migration and differentiation by labeling IN progenitor cells in the lateral ganglionic eminence using in utero electroporation. We found that IN neuroblasts reached the primordial OB as early as E13 and began to differentiate apical dendrites by E15, which extended into the developing external plexiform layer. We established E16 as the embryonic stage at which the prototypical chain of migrating neuroblasts denoting the embryonic rostral migratory stream (RMS) was visible. Collectively, our data highlight the importance of studying OB INs in isolated time windows to better understand the formation of circuits that define the olfactory system function.