Adult subventricular zone neuronal precursors continue to proliferate and migrate in the absence of the olfactory bulb

Neurons continue to be born in the subventricular zone (SVZ) of the lateral ventricles of adult mice. These cells migrate as a network of chains throu gh the SVZ and the rostral migratory stream (RMS) into the olfactory bulb ( OB), where they differentiate into mature neurons. The OB is the only known target for these neuronal precursors. Here, we show that, after eliminatio n of the OB, the SVZ and RMS persist and become dramatically larger. The pr oportion of dividing [bromodeoxyuridine (BrdU)-labeled] or dying (pyknotic or terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end -labeled) cells in the RMS was not significantly affected at 3 d or 3 weeks after bulbectomy (OBX). However, by 3 months after OBX, the percentage of BrdU-labeled cells in the RMS decreased by half and that of dying cells dou bled. Surprisingly, the rostral migration of precursors continued along the RMS after OBX. This was demonstrated by focal microinjections of BrdU and grafts of SVZ cells carrying LacZ under the control of a neuron-specific pr omoter gene. Results indicate that the OB is not essential for proliferatio n and the directional migration of SVZ precursors.