Restricted distribution of S-phase cells in the anterior subventricular zone of the postnatal mouse forebrain.

The anterior subventricular zone (SVZa) is the site for postnatal neurogenesis of interneurons of the olfactory bulb (OB). Concurrently or after proliferation, neuronal precursors therein migrate within it to reach the OB, an event known as the rostral migratory stream (RMS). We used bromodeoxyuridine (BrdU) incorporation with short survival times to investigate the distribution of S-phase nuclei in the SVZa/RMS of the postnatal mouse. We observed that they were distributed along a radial, outside-in, decreasing gradient that persisted until postnatal day 10 (P10), then faded away to finally disappear by P16. After longer post-injection survival times labeled cell distribution became homogeneous. GFAP-positive glia are present at the periphery but not at the core of the SVZa. Our results represent the first evidence of a discrete spatial organization of a cell cycle phase within the SVZ, and also suggests a segregation of proliferating and migrating cells in the rostral migratory stream of the early postnatal mouse.