Embryonically generated interneurons (EGINs) exhibit a distinctive developmental program and lamination in the olfactory bulb

Interneurons (INs) in the mouse olfactory bulb (OB) develop during an extensive period of time that begins in the embryo and continues throughout the lifetime of an individual. The connectivity and location of these neurons is profoundly affected by the time of generation, making these developmental windows critical to understand the circuitry of the OB. Here, we focus on the OB embryonic generated interneurons (or EGINs), which have generally received less attention than those generated in the adult. Birthdates of EGINs were differentiated by embryonic injections of thymidine analogs and their final destinations and phenotypes analyzed by immunohistochemistry. We found that EGINs were retained in the adult and were distributed across all layers of the OB. However, a lateral-to-medial neurogenic gradient is seen only in the earliest generated EGINs. Within the granule cell layer (GCL), EGINs predominantly accumulated in the superficial region at almost all ages and remained there during adulthood. Immunostaining for calbindin, parvalbumin, tyrosine hydroxylase, and calretinin were largely negative suggesting that EGINs may represent subpopulations of OB INs that are not yet fully characterized. Using in utero electroporation to label EGIN progenitors, we demonstrated that they reach the primordial OB as early as E13 and begin to differentiate apical dendrites by E15. At E16, EGIN progenitors clustered into a chain of migrating neuroblasts denoting the embryonic rostral migratory stream (RMS). 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 responses.