Postnatal subventricular zone progenitors give rise not only to granular and periglomerular interneurons but also to interneurons in the external plexiform layer of the rat olfactory bulb.

Interneurons in the granule cell layer (GCL) and glomerular layer (GL) of the olfactory bulb (OB) are generated from progenitors in the subventricular zone (SVZ) of the lateral ventricle. However, little is known about the origin of interneurons in the external plexiform layer (EPL) of the OB. On the basis of the concept of corticogenesis, I hypothesized that interneurons in the EPL of the rodent OB also originate in the SVZ. In the present study, replication-incompetent retroviruses encoding a marker gene, human placental alkaline phosphatase (AP), were injected into the lateral ventricles of postnatal day 4 Wistar rats to label dividing cells in the SVZ. Two days after injection, some of the AP-labeled cells had migrated into the OB. Five weeks after injection, AP/NeuN double-labeled cells were found not only in the GCL and GL but also in the EPL of the OB. In the EPL, most AP-labeled cells were calcium-binding protein parvalbumin (PV)-immunoreactive (+) interneurons. A subset of these cells was made up of calcium-binding protein calretinin (CR)(+) interneurons. According to their structural features, AP-labeled cells in the EPL were Van Gehuchten cells, multipolar cells, and superficial short-axon cells. Thus, postnatal SVZ progenitors give rise not only to granular and periglomerular interneurons but also to interneurons in the EPL of the OB. Furthermore, these results suggest that SVZ progenitors give rise to virtually all subpopulations of interneurons in the OB. (c) 2007 Wiley-Liss, Inc.