A possible role for the immune system in adult neurogenesis: new insights from an invertebrate model

• Recent findings in an invertebrate model provide evidence for a recruitment of neuronal precursors from a non-neuronal source.
• The notion of long-term self-renewal of mammalian neuronal stem cells also has to be reevaluated.
• A tight link seems to exist between the immune system and the system driving adult neurogenesis.

Persistent neurogenesis in the adult brain of both vertebrates and invertebrates was previously considered to be driven by self-renewing neuronal stem cells of ectodermal origin. Recent findings in an invertebrate model challenge this view and instead provide evidence for a recruitment of neuronal precursors from a non-neuronal source. In the brain of adult crayfish, a neurogenic niche was identified that contributes progeny to the adult central olfactory pathway. The niche may function in attracting cells from the hemolymph and transforming them into cells with a neuronal fate. This finding implies that the first-generation neuronal precursors located in the crayfish neurogenic niche are not self-renewing. Evidence is summarized in support of a critical re-evaluation of long-term self-renewal of mammalian neuronal stem cells. Latest findings suggest that a tight link between the immune system and the system driving adult neurogenesis may not only exist in the crayfish but also in mammals.