The generation of new neurons in the human hippocampus strongly decreases from childhood to almost undetectable levels in adults. It can therefore be stated that neurogenesis in the human hippocampus is almost non-existent, as opposed to what has been thought in recent decades. Research in this field was published by the Nature journal back in March, and has been confirmed by a literature review recently published in Cell Stem Cell.
The new work developed by the team of Universitat de València neurobiologist José Manuel García Verdugo (Institut Cavanilles), in international collaboration with laboratories of scientists Arturo Alvarez-Buylla (University of California, San Francisco) and Zhengang Yang (University of Fudan, Shanghai), backs this theory and is thus reflected in Cell Stem Cell by way of a review project of the Cavanilles Institute for Biodiversity and Evolutionary Biology.
For years, numerous pieces of research have shown that the subventricular area of the dentate gyrus of the hippocampus of certain mammals - an area linked to memory and learning - continues generating new neurons in the adult brain. Although there are studies that extrapolate this data to the human adult hippocampus, the work that is now backed by Cell Stem Cell, months after being published in Nature, shows very different results.
After conducting a broad study of the neurogenetic capability of the human hippocampus in samples belonging to different ages - from fetal development to adult stages -, the results show that the number of proliferator progenitors and new neurons in the dentate gyrus decreases drastically during the first year of life.
Something similar occurs, according to the study, with the proliferative capability of the gyrus dentate of the non-human Macaca mulatta primate. Analyses show that during the first post-birth stages there is a generation of new neurons, which is then noticeably diminished during childhood development.
"The generation of new neurons in the human and non-human primate hippocampus mainly takes place in embryonic stages and then scarcely in post-birth periods, for the first few months of life," says José Manuel García Verdugo, principal co-researcher of the project and researcher of the Cavanille Institute for Biodiversity and Evolutionary Biology, of the Parc Científic of the Universitat de València. "We have barely observed some new isolated neurons at ages 7 and 13, and none in the case of studies conducted on adult patients suffering from epilepsy." The analyses have been carried out in collaboration with the Functional Neurosurgery team of the Hospital Universitario y Politècnico La Fe of Valencia, and the person in charge, neurosurgeon Antonio Gutiérrez.
This data opposes the extensive neurogenesis that takes place in the hippocampus of other more primitive mammals, rodents, the study model for thousands of research results.
"In opposition to rodents, us humans are born with practically all the neurons needed for normal development, and this work suggests that future modifications and treatments must be based on plasticity," explains García Verdugo. Neuroplasticity or synaptic plasticity refers to the communication that is established among neurons, a property that intervenes to module the perception of the surrounding stimuli, which until not had also been linked to neurogenesis.
It is not the first time that they appear differentiated in the development of the human brain in comparison to other species of mammals. "One of the great evolutionary leaps of the human being is the existence of currents of new neurons towards the prefrontal cortex of infants, which does not happen with m ice and which, as well as being linked to memory and learning, is related to social human activity."
In the case of the hippocampus, the results of this work now suggest a possible involvement of other plasticity mechanisms of the adult human dentate gyrus in the processes of memory and learning. "More so than new neurons, we must start speaking of new neural circuits," concludes the scientist.
The results of this research were first published in Nature last March, and the publication is on the list of the 100 publications that generated the most worldwide impact in 2018, according to Almetric. This annual ranking is based both on mentions in media outlets and blogs as well as the dissemination of scientific papers on social network sites such as Twitter, Facebook and Google+.