Abstract
Adult neurogenesis occurs in many species, from fish to mammals, with an apparent reduction in the number of both neurogenic zones and new neurons inserted into established circuits with increasing brain complexity. Although the absolute number of new neurons is high in some species, the ratio of these cells to those already existing in the circuit is low. Continuous replacement/addition plays a role in spatial navigation (migration) and other cognitive processes in birds and rodents, but none of the literature relates adult neurogenesis to spatial navigation and memory in primates and humans. Some models developed by computational neuroscience attribute a high weight to hippocampal adult neurogenesis in learning and memory processes, with greater relevance to pattern separation. In contrast to theories involving neurogenesis in cognitive processes, absence/rarity of neurogenesis in the hippocampus of primates and adult humans was recently suggested and is under intense debate. Although the learning process is supported by plasticity, the retention of memories requires a certain degree of consolidated circuitry structures, otherwise the consolidation process would be hampered. Here, we compare and discuss hippocampal adult neurogenesis in different species and the inherent paradoxical aspects.
Highlights
Adult neurogenesis was claimed by Joseph Altman in the early 1960s
Additional evidence has revealed a circuit in the pallium of the electric fish (Apteronotus leptorhynchus) between the dorsal pallium and dorsolateral pallium, similar to the hippocampal circuit in mammals [53]; this circuit can perform the same separation and completion pattern ascribed to the dentate gyrus and cornu ammonis 3 (CA3) circuits, tasks that are closely associated with adult hippocampal neurogenesis in rodents [54]
Adult neurogenesis has been shown to occur in many species from fish to mammals, with the generation of new neurons in the hippocampus and homologous areas pointing to functional similarities at all taxonomic levels
Summary
Adult neurogenesis was claimed by Joseph Altman in the early 1960s. After intracranial injection of [H3 ]-thymidine, an exogenous marker of cell proliferation, Altman found marked cells with the characteristics of neurons [1] that were able to differentiate from dividing precursors during adulthood. Based on the later findings in rodents, adult neurogenesis is currently considered to play a relevant role in cognitive ability, especially in learning and memory. A better approach to the phenomenon of adult neurogenesis has been possible with developments in technology and immunostaining techniques, the use of retrograde tracers, and 14 C-labeling This labeling methodology consists of the prior separation of cell nuclei immunolabeled for neuronal nuclei (NeuN), a mature neuronal marker, with subsequent analysis of DNA-14 C, providing a precise estimation of the “age” of the neurons taking into consideration the exposure of people to environmental 14 C released by nuclear bomb testing [13]. We discuss the recent conflicting findings regarding the presence or absence (or extreme rarity) of human adult hippocampal neurogenesis, raising methodological and conceptual issues
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