What they found when Prozac was introduced is this: One and only one cell population was targeted by the anti-depressant. And this was enough to account for the 40 percent increase in neurons in the adult brain of the animals studied. The cells are not stem cells, but the next generation of cells made by these stem cells. ‘‘It’s like a mother cell dropping its egg,’’ Enikolopov said. That is why they are called daughter cells. And in the Prozac-induced growth state, the daughter cells go through two or more divisions, giving birth to a granddaughter cell, and then a great-granddaughter cell. These steps are completed within two or three days. Then, it’s the great-granddaughter cell that slowly starts taking on the characteristics of a neuron. The process is complete within three weeks.

This finding has immediate appeal because it may provide clues as to why it takes that long for depressed patients taking these drugs to improve. The new neurons are born in an area of the brain called the dentate gyrus, part of the hippocampus. The dentate is made of neat layers. The neurons are born in the basement and migrate to the top of the dentate. There, they send projections to nearby neurons. The hippocampus regulates learning and memory, and newer studies suggest a role in mood control. (Jamie Talan)