Neurogenesis in the Human Brain

It was once believed that the growth of new neurons – those specialized cells in the human central and peripheral nervous systems that allow for the transmission of nerve impulses – did not occur in the adult brain.  This has been shown to be substantially incorrect.  How the growth of new neurons – through a process referred to as neurogenesis – in the adult human brain was discovered is quite interesting.

Indirect evidence for neurogenesis was derived some fifteen years ago though an ingenious approach.  This study focused on the hippocampal region of the brain that is known to be involved in the storage and processing of memories.  This area of the brain would obviously be an excellent candidate for studying neurogenesis.  The result of this investigation was that neurogenesis was detected in five individuals up to 75 years of age.  To accomplish this, the investigators injected the human subjects with a label that binds permanently with the host’s DNA.  The compound used was bromoedeoxyuridine (BrdU).  Once BrdU is incorporated in the DNA, it can be detected using antibodies that specifically react with the modified DNA.  This specific binding can then be visualized by the experimenter with the proviso that the patients so injected were willing to donate their brains following death.  This type of study was eventually curtailed due to problems associated with its safety.

Recent studies conducted by Dr. Kirtsy L. Spalding and colleagues from the Department of Cell and Molecular Biology Karolinska Institutet in Stockholm, Sweden were based on more direct evidence derived from an unusual source.  It seems that during the era of extensive above-ground testing of nuclear warheads between 1945 and 1963, a considerable amount of the radioactive isotope of Carbon (C¹⁴ ) was released into the atmosphere.  The amount of C¹⁴ in the air has steadily declined since the Limited Test Ban Treaty of 1963 effectively banned above-ground testing.

 

Since dividing cells require carbon and since the source of that carbon is from the atmosphere, C¹⁴ becomes incorporated into the cellular DNA.  Furthermore, the amount of incorporated C¹⁴ directly corresponds to the amount of C¹⁴ in the atmosphere at the time of cell division.  Therefore ,to calculate the age of the cell, the investigator simply has to measure the amount of this isotope that was incorporated into its DNA.

From the analysis of the data, Spalding and his investigative group were able to show that human adult-brain neurogenesis is, in fact, confined to the hippocampus and some subpopulations actively divide while others do not.  From this data, it has been estimated that one-third of adult hippocampal neurons are actively dividing.  This activity within the hippocampus makes sense given the role of this region of the brain in creating and managing memories throughout the lifetime of the individual.

This is an important and elegant study for it answers the question whether or not the adult human brain undergoes neurogenesis unambiguously and is also able to provide a quantitative as well as a qualitative answer.