Adult Hippocampal Neurogenesis in Different Taxonomic Groups: Possible Functional Similarities and Striking Controversies

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.

Keywords: adult neurogenesis; brain; cognition; hippocampus; human; learning; memory; neurogenesis; species; taxonomic level.

Figure 1

Schematic representation of hippocampal anatomic differences among species and studies showing involvement of these areas in cognitive processes (first line) and adult neurogenesis in these areas related to cognitive processes (second line). Although clear anatomical similarity only exists between the hippocampi of mammals, evidence points to a functional similarity with other species, such as fish or reptile, with less complex brain structure/function in general for cognitive processes such as memory, learning and future planning, among others (cognitive complexity). The birth of new neurons in adult mammals is mostly restricted to the hippocampal dentate gyrus. Although there are many neurogenic zones in other species, areas homologous to the hippocampus always appear among them, suggesting some similarity in the functions of these new neurons in cognitive processes in all species. Although a dense body of information points to similarities in function, it was suggested recently that the adult human brain does not retain the neurogenic capacity, raising intense debate about the possible impact on the way the human brain handles receiving, storing, and processing new information.

Figure 2

Examples of neurogenic models. The “replacement model” would replace dead neurons and prioritize new memories because older memories would be lost as older neurons lose their synaptic connection and are replaced by new ones. The “addition model” would prioritize more hard networks as newborn neurons are constantly added to the circuit and retain older memories. Although both models exist simultaneously, the two models and their possible consequences are investigated as some predominance of one or the other.