Neurogenesis persists in the subependymal layer of the adult mouse brain

Neurogenesis of olfactory bulb granule cells is known to persist in adult rats where, in some strains, the bulbs grow throughout life. In mice, bulb growth ceases early in adulthood and here we ask if granule cell neurogenesis persists after the bulbs have stopped growing. By injecting adult mice with bromodeoxyuridine (BrdU) and allowing short and long survival times, we found that new cells form in the subependymal layer and that they migrate subsequently into the olfactory bulbs where they acquire the nuclear morphology of granule cells and express neuron-specific markers. Using [3H]thymidine, we found that most of these adult-generated granule neurons persist within the bulbs for at least 16 weeks. This shows the persistence of neurogenesis and neuronal migration in adult animals in which the olfactory bulbs have stopped growing.