Biology 202
2002 First Paper
On Serendip

Neurogenesis, the production of new nerve cells, has been a revolutionary finding as nerve formation has always been thought to end with adulthood. It has not been until recently that such dogma has been contradicted as research findings report that neurogenesis continues in the hippocampus throughout most of the adult life of mammals and primates (1). Recent correlations have been further made between neurogenesis and depression as the latter depletes neuron cells in the brain while antidepressive drugs have demonstrated to increase neuronal growth
(2).

Neurons are the building blocks of the nervous system as they are responsible for the input, processing and transmission of information. Neurons are derived from stem cells as the latter differentiate into specialized cells and make progenitor cells which are responsible for the formation of neuron and glial cells. Although the majority of neurons are formed during the pre-natal and perinatal stage of development, neuron formation continues in the dentate gyrus of the hippocampus (2). The hippocampus, which lies beneath the cortex is a major factor of learning and memory formation and can indirectly influence emotion. Progenitor cells which are present in the sub ventricular zone of the hippocampus are responsible for such growth as they produce daughter neuron cells through division (2).

In the 1960s, Joseph Altman from MIT reported that new neurons were being produced in the dentate gyrus of the hippocampus of adult rat brains. Although such findings were groundbreaking, they failed to create the stir that Elizabeth Gould's 1998 study caused . The Princeton University neurobiologist demonstrated that the marmoset brain generated neurons (1). BrdU or bromodeoxyuridine served as a marker when incorporated into the DNA of the progenitor cells and through autoradiographic techniques, the labeled progenitor cells were identified to produce daughter neuron cells (4). The neurons have "processes which go inward and follow paths to other structures within the hippocampus and get incorporated into basic circuitry of the brain" (2).

In 2000, Fred Gage of the Salk Institute reported that neurogenesis can take place in the hippocampus of adult animal brains. Progenitor cells were isolated from the adult brains of five subjects with tumors and analyzed in culture dishes. Brdu, was once again used as a marker while the ultimate division of the extracted progenitor cells to neurons observed (4).

Although these findings are not only revolutionary and exciting, there are many questions which have yet to be answered by neurogenesis. What is the functionality of these replicating neurons? When questioned, Gage stated that "we have theories as to the significance of their function but no proof yet" (1). Furthermore, as laboratories around the country are finding evidence of these new nerve cells in the hippocampus, the "so what?" question must be asked. How can such findings be applied to everyday life?

Studies on clinical depression demonstrate that patients with bouts of clinical depression have smaller volume sized hippocampus than their normal sized counterparts who do not suffer from it. There is also constant decrease in hippocampal volume due to a lifetime duration of depression (2). In 2001, Ron Dunman's research brought not only association of the neurogenesis/depression phenomenon but possible application when demonstrating that treating rats with Prozac and ECT resulted in newly formed neurons in the hippocampus (3).

Progenitor cells which are inhibited from dividing throughout most of the brain, do so in the dentate gyrus of the hippocampus and the olfactory bulb as they develop into daughter neurons. Constant renewal of such neurons is thought to be necessary for learning processing and storage of new information (2). If the mechanisms which inhibit most progenitor brain cells from developing into neurons are found, there is the possibility of repair for damaged brain tissue due to illness, trauma or age. There are also exciting possibilities of stem cell development to diseases such as Parkinson's and Alzheimer's, all which are neurodegenerative(5).

Other studies have found that neuronal growth not fixed to the hippocampus. Elizabeth Gould has also published findings which support the neuron formation in the neocortex., as the latter is the most evolved and complex part of the brain which controls cognitive function and language. Pasko Rakic, who is chairman of Yale's neurobiology department has consistently contradicted her findings, however. In December 2001 publication, he concurred with current research that adult primate produce neurons in the hippocampus but could not support the same in the neocortex. The politics revolving around such scientific rivalry and differences must be ignored as future research is done on the neocortex (6).

Constant neuronal generation in the hippocampus can have many purposes. Throughout its lifespan, an adult has new experiences which require the storage and processing of new information. Since such demand is constant, new neurons may be required to store such information. However, when taking such perspective, there are many new questions which arise. If new neurons may needed for new information, then what happens to the old neurons in the hippocampus? Does lack of usage lead to their degeneration or do they continue to be functional? Stored memories are often recalled in adults thus leading to the possibility that older neurons may needed for the processing and transmission of such needed information.

Neurogenesis is a complex issue with an array of possibilities for the future. Further investigations will have to answer some of the questions and issued raised.

WWW Resources
(1)The Scientist Magazine , Human neurogenesis. Group demonstrates that adult human brains grow new cells after all.
(2)American Scientist Magazine, Depression and the Birth and Death of Brain Cells.
(3)National Depressive and Manic Depressive Association, Antidepressants and Brain Cell Growth.
(4) Neuroscience for Kids, New Neurons in Neocortex? New Study Says NO!
(5)National Institute of Mental Health, Learning From Songbirds About Adult Brain Generation.
(6) The Scientist Magazine..

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