The Truth About Neurogenesis

By: Sheila Bhavsar

Exactly how the brain works is still a mystery and there is much to discover. Fortunately, a large amount has been learned about the brain in the past few decades. It was once commonly believed that we are born with a finite number of brain cells. Therefore, if brain cells are destroyed, the cells can never be replaced. This scientific myth was taught and believed by the science community, which caused the myth to spread. However, disputing of the myth regarding neurogenesis began in the 1960’s.

Joseph Altman presented the first evidence favoring neurogenesis. He produced lesions in the lateral geniculate nucleus (LGN) of the thalamus in 10 rats. Altman also injected radioactive thymidine, a specific precursor of chromosomal DNA, into the damaged area, which served as a marker for active cell division. Altman then took coronal sections of the LGN in increments of one day, one week, two weeks, one month, and two months after damaging the LGN. Numerous glial cells, which are cells in the nervous system that support neurons, showed uptake of the radioactive starting at the one-week time point. The radioactive material was localized to nucleus of the glial cells, and the majority of the glial cells that showed uptake of radioactive thymidine were found in areas of the lesions.

In 1998 Fred H. Gage published results that further disputed the myth that neurons cannot be replaced in the adult brain. Gage examined human brain tissue from the hippocampus. He specifically examined the subgranular zone, the hillus and the granular cell layer of the hippocampus. The tissue was obtained from cancer patients who had been treated with the thymidine analog, bromodeoxyuridine (BrdU), which labels DNA during DNA replication. Using immunofluorescent labeling for BrdU and labeling for other neuronal markers, such as calbindin, Gage found that BrdU labeled cells co-expressed neuronal markers. Gage then used unbiased counting techniques to quantify cells that co-expressed both neuronal markers and BrdU. Gage found that approximately 22 percent of granule cell layer co-expressed a neuronal marker and BrdU. Therefore, Gage concluded that new neurons are generated from dividing progenitor cells in the hippocampus.

In addition, regions known to have a connection with lateral geniculate bodies showed uptake of radioactive material, such as the lower layers of the visual cortex and the optic tract. Labeled neuroblasts and neurons were also found in coronal sections of the thalamus, which is where the LGN is located. Altman concluded that his evidence strongly indicates that new brain cells may develop, especially when the brain is injured. However, his results were widely ignored because it conflicted with the popular belief that neurogenesis only occurs during prenatal development.

There are many other researchers that conducted studies supporting the theory that neurons can regenerate. For example, Elizabeth Gould’s studies at Princeton supported neurogenesis in regions of the adult primate brain, which occurred shortly after Gage’s experiments. Innovative research specifically conducted by Altman and Gage helped solve one of the many mysteries of the brain, and they changed the way many other scientists think. Because of these scientists, the myth regarding neurogenesis has officially been busted.