In a recent study, scientists at University of California–San Francisco (UCSF) found that adult neurogenesis, which is seen in other species, does not occur in human beings.
In the study, which was recently studied in Nature, researchers found that in human hippocampus neurogenesis gradually declines all through childhood and becomes unnoticeable in adults.
The hippocampus was is crucial for learning and memory and one of the areas where the proliferation of new neurons was previously thought to take place.
The study was based on the careful analysis of 59 human hippocampus samples from epileptic patients, including 37 postmortem tissue samples and 22 surgically excised samples, which suggested that neurogenesis does not occur in adult human brain.
We find that if neurogenesis occurs in the adult hippocampus in humans, it is an extremely rare phenomenon, raising questions about its contribution to brain repair or normal brain function."
Dr. Arturo Alvarez-Buylla, Senior Author and Professor at UCSF
The research findings present a challenge to many researchers who have proposed that boosting neurogenesis could enhance the treatment for several brain diseases, including depression and Alzheimer's disease.
In a former research paper, scientists at MIT carried out experiments on rodents and concluded that neurogenesis does occur in the adult mammalian brain.
The findings were widely disregarded until the researchers at Rockefeller University found that new neurons are born and put to use throughout lifetime in several parts of the songbird brain.
They evaluated the difference in the number of newborn neurons and neural stem cells present in these samples, from prenatal to adulthood, using various types of antibodies to identify cells of different types and states of maturity, including neural stem cells and progenitors, newborn and mature neurons, and non-neuronal glial cells.
Researchers also assessed the cells they labeled based on their structure and shape–including imaging with high-resolution electron microscopy for a subset of tissue samples–so as to confirm their identity as neurons, glial cells, or neuronal stem cells.
They found several evidences of neurogenesis in the DG during prenatal brain development and in infants. They found that the number of newcells significantly declines during early infancy.
DG samples from infants contained fivefold reduced number of new neurons, compared to samples from newborn infants.
The observed decline prevailed well into childhood, with the number of new neurons reduced by 23-fold between the age group of 1–7 years, followed by an additional fivefold decrease by 13 years, at which point neurons also appeared more mature than those seen in samples from younger brains.
Researchers observed only about 2.4 new cells per square millimeter of DG tissue in early adolescence, and observed no evidence of newborn neurons in any of the 17 adult post-mortem DG samples or in surgically extracted tissue samples from 12 adult patients with epilepsy.
“The fact that we could compare newborn brains, where new neurons were clearly present, to the adult, where we saw no evidence for young neurons, gave us added confidence that what we were seeing was correct," said Dr. Paredes.
According to researchers, absence of neurogenesis in the human brain may not be a bad thing; however, they emphasized the need to understand the distinctive features of human brain and recommend other researchers to focus on a better way of developing treatments for human brain diseases.
In young children, we were able to see that substantial numbers of new neurons continue to be made and integrated into the dentate gyrus, but neurogenesis fades away completely by early adolescence."
Dr. Mercedes Paredes, Assistant Professor, UCSF