According to a study conducted by researchers at the Dalhousie University in Nova Scotia and the National Research Council of Canada, teenagers with major depression may have small hippocampuses. The hippocampus, the part of the brain associated with memory, is apparently significantly smaller in teenagers with depression than in teenagers that don't have depression.

Frank MacMaster studied 34 teens ages 13 to 18 for this study.

"Stress hormones can cause a chain reaction of events that lead to decreased dendritic branching and reduction in number of neurons [brain cells] in the dentate gyrus of the hippocampus," he said. "With the loss of neurons and reduction in their size, an overall change in volume, like the one noted in this study, can happen."

The hippocampus is most affected because it is very sensitive to certain hormones and neurotransmitters.

"Think of it as the 'hot spot' for any kind of damage or insult to the brain. It plays a central role in controlling the body's response to stress as well," MacMaster said. "If that is adversely affected, then the stress response can run unchecked and damage the hippocampus further."

Dr. Stuart Kaplan, director of child and adolescent psychiatry at the College of Medicine, specializes in child and teen depression.

Kaplan said the hippocampus is a highly specialized region of the brain -- one of the only parts capable of routinely reproducing new brain cells.

People who are depressed for a long period of time most likely have a lower production of new brain cells in the hippocampus, which

Kaplan believes can be a cause of depression.

He also said that studies on mice confirm similar results exhibited by human behavior, acknowledging brain cell production as a main factor in determining depression.

"Stress produces higher levels of cortisone, which the hippocampus is very sensitive to. This sensitivity to high levels of cortisone, which are most present in depressed people, can lead to cell death," Kaplan said.

"Antidepressant effectiveness is directly related to the ability of the hippocampus to produce new brain cells. Sometimes it takes a few weeks for the antidepressants to kick in because it may take that long for the brain cells to develop," he said.

MacMaster said that to find out the relationship between smaller hippocampus volume and depression, various studies needed to be carried out.

"The true cause of depression is likely a combination of genetics and environment," MacMaster said.

"The neurobiology of depression is relatively understudied in children and adolescents. The prevalence of depression in children and adolescents has been found to be as high as 8 percent -- that would be two kids in a classroom of 25 students," MacMaster said. "Half of adults with major depression report having their first episode of depression by the age of 20. An understanding of the biology behind the disorder is needed in order to advance our understanding of the disorder and develop new treatments."

Brain Imaging and MR scans aid in diagnosing and treating psychiatric illnesses. In his study, MacMaster found an enlarged pituitary gland in cases of pediatric depression.

"This may be indicative of dysfunction of the body's response to stress -- via the hypothalamic pituitary adrenal axis. The hormones that the pituitary controls are the same ones that ultimately affect the health of the hippocampus," he said.

Currently, there is no treatment that will grow new viable connections or brain cells in the hippocampus.

"It is one thing to grow a new neuron [brain cell] and quite another to have it connect with other neurons in meaningful ways. People should not lose hope that once this biological change has manifested itself, they shall remain depressed," MacMaster said. "The brain is a very pliable organ, and with the aid of medication or talk therapy, many people do experience wellness again."

MacMaster is continuing his research on teen depression and its relation to brain structure.

"We are going to scan children of parents with mood disorders and follow them for as long as possible. We hope to find out if the smaller hippocampus comes first [prior to the depression developing] or not," he said.