Mixed Signals and Interspecies Variation in the Plasticity of Adult Mammal Brains

Despite the growing interest in brain structural plasticity and the substantial body of knowledge that has accumulated over recent decades, some issues remain poorly defined, leading to confusion in the interpretation of results. In addition to stem cell-driven neurogenesis in adult neurogenic niches (adult neurogenesis), neuronal precursors in a state of arrested maturation have also been described, representing a form of neurogenesis without division based on so-called "immature" or late-maturing neurons. These processes occur in different brain regions yet share certain molecular markers and temporal windows. Recent advances in comparative neuroplasticity have further complicated our understanding. Studies reveal a reduction in adult neurogenesis in the olfactory bulb and hippocampus of large-brained, gyrencephalic mammals compared with small-brained species such as rodents. Conversely, a higher prevalence of immature neurons has been reported in the neocortex and amygdala of larger-brained mammals. It is becoming evident that evolutionary trade-offs took place in distinct plastic processes, resulting in the predominance of certain forms in particular species, while others coexist and share overlapping markers. Regardless of the approach employed (neuroanatomical, immunocytochemical, phylogenetic, or transcriptional), current evidence indicates substantial heterogeneity in cell types with different origins and fates across diverse mammalian species. These patterns appear to be sculpted by evolutionary pressures yet unified by shared transient maturational states.