Figure 1: Different ways for achieving structural plasticity in the adult mammalian brain. Stem cell-driven genesis of new neurons (adult neurogenesis) and synaptic/axonal plasticity (A, top) represent two extremes of plastic events in the brain. Non-newly generated “immature” neurons (A, bottom), as a form of delayed neurogenesis without division, might be considered as an intermediate form of plasticity providing new elements for the pre-existing neural circuits in the absence of active stem cell niches/neural progenitors. Note that a similar outcome (the addition of a new neuron in the circuits) can be obtained through different plastic processes, not all of which involve stem/progenitor cells (high top: color code indicating different maturational states of neurons; dark blue indicates newly formed elements). (B) Specific types of plasticity such as “classic” adult neurogenesis (top) or “immature” neurons (bottom) can coexist, yet, with highly different distributions and amounts. The numbers of immature neurons can vary remarkably in mammals, with phylogenetic variation between small-brained and large-brained species (La Rosa et al., 2020b). Asterisk, the reduction in adult neurogenesis rates across mammalian species has not yet been assessed through systematic, comparable approaches.