Those who kill themselves and those who are born
Most tissues in the body are able to recover after trauma, illness or injury and restore themselves. The dead cells are replaced by neighboring cells that divide in an accelerated manner. Another way to restore the tissue is carried out by the tissue's parent cells, which are able to differentiate into all the types of cells that inhabit it. These cells, found in most tissues, are called stem cells. Young cells are created from the stem cells every day to replace the old cells that have died. In emergency situations, when cells in the tissue die in an accelerated manner, the stem cells enter into activity, dividing rapidly and from them the missing cells are formed.
Of all the body's tissues and organs, the brain has the lowest regeneration capacity, and most injuries to it - injury, disease or stroke - cannot be repaired. For years, scientists thought that at the end of embryonic development, the role and final number of brain cells were determined, and these cannot be changed throughout life. Dead brain cells, the researchers believed, are not replaced by new cells.
But a growing line of research indicates that the brain is more dynamic than we have tended to think so far. The study that discovered that in certain areas of the brains of developed creatures new brain cells are born every day was crowned as one of the ten biggest discoveries of the past year. He joined a relatively old discovery that indicated the existence of stem cells in the brain.
From the discovery of neural stem cells, the researchers learned that the brain has the ability to regenerate. But most areas of the brain are not endowed with the ability to produce new cells. An example of such an area is the cerebral cortex, the area where higher brain functions such as learning and memory occur.
"We knew from previous work that the death of nerve cells can change the fate of the neural stem cells, but these findings were only discovered in those areas of the brain where there is a regeneration process," the researchers write. From these works it became clear that the death of cells triggers accelerated division and the creation of new cells from the stem cells. These facts motivated the authors of the study to find out if it is indeed possible to stimulate such steps in the cerebral cortex as well.
The researchers, who published the results of their work last month in the journal "Nature", showed that the deliberate destruction of cells in the cerebral cortex of mice stimulated the creation of new nerve cells, which replaced the missing cells and formed connections with the surrounding nerve cells. Studying the mechanisms leading to the creation of new brain cells in humans will open a door to the restoration of memory in Alzheimer's disease and the restoration of a damaged spine.
The cerebral cortex in mice is located near the area that has neural stem cells (in humans it is about five centimeters away from it). The researchers chose a group of cells in the cerebral cortex of the mice and injected it with a chemical that causes their death by activating a "suicide" program in them. This process of cell suicide (apoptosis) occurs routinely in the body. It usually happens when a cell is unnecessary and does not bring any benefit, or when changes occur in it that can cause diseases.
Two weeks after the injection it became clear that in those areas where cell death was caused, new brain cells were formed. Did these cells actually replace the dead cells? To test this, the researchers followed the stages of development of the new nerve cells. They injected the mice with a chemical substance that binds to newly formed DNA molecules and in this way identifies the cells that are in a state of division. It turned out that in the cerebral cortex, where cell divisions do not normally occur, nerve cells appeared in different degrees of development. These were the new cells that developed from the stem cells. It turned out that the new cells appeared only in the area of the cerebral cortex that was destroyed, and settled in place of the missing cells. These cells, it is hypothesized, were recruited from afar by signals generated by cell death. They were located in the cerebral cortex and integrated into the neural activity.
The researchers showed that the nerve extensions that these cells developed reach the same areas as the extensions of the cells that died. "We have evidence that by soaking the suicide process in the brain cells, we reactivated genes that participated in the development of the brain when the mice were embryos," said Geoffrey McCleese, one of the members of the research team to the journal "Science". "In order to know how to use such a reaction, we must learn what the molecular signals are that stimulate the migration of cells, their differentiation, their settlement in the right place and the creation of connections with the rest of the brain. Understanding these mechanisms will open the door to repairing brain damage."
{Appeared in the Haaretz newspaper, 13/7/2000} - The Hidan website was until 2002 part of the IOL portal from the Haaretz group
