Divide and Define: Clues to Understanding How Stem Cells Produce Different Kinds of Cells

Newswise ANN ARBORThe human body contains trillions of cells, all derived from a single cell, or zygote, made by the fusion of an egg and a sperm. That single cell contains all the genetic information needed to develop into a human, and passes identical copies of that information to each new cell as it divides into the many diverse types of cells that make up a complex organism like a human being.

If each cell is genetically identical, however, how does it grow to be a skin, blood, nerve, bone or other type of cell? How do stem cells read the same genetic code but divide into very different types?

Researchers at the University of Michigan have found the first direct evidence that cells can distinguish between seemingly identical copies of chromosomes during stem cell division, pointing to the possibility that distinct information on the chromosome copies might underlie the diversification of cell types.

Scientists in the lab of Life Sciences Institute researcher Yukiko Yamashita explained how stem cells can distinguish between two identical copies of chromosomes and distribute them to the daughter cells in a process called nonrandom chromosome segregation. They also described the genes responsible. Their work is scheduled to be published online May 5 in Nature.

"If we can figure out how and why cells are dividing this way, we might be able to get a glimpse of how we develop into a complete human, starting from a single cell," Yamashita said. "It is very basic science, but understanding fundamental biological processes always has wide-ranging implications that could be exploited in therapeutics and drug discovery."

During the cell division cycle, the mother cell duplicates its chromosomes, generating two identical sets. When the cell divides to become two cells, each cell inherits one set of chromosome copies. In many divisions, the daughter cells are identical to the motherone skin cell becomes two, for instance.

But in a process called asymmetric division, a cell divides into two daughters that are not identicala skin stem cell divides into another skin stem cell and a regular skin cell, for example. In that case, the genetic information within the chromosome copies remains the same, but the type of cell, or "cell fate," is different.

The Yamashita lab used stem cells from the testes of the fruit fly Drosophila to study the process of cell division.

"The Drosophila germ line stem cell can be identified at a single-cell resolution, so they are an ideal model," Yamashita said.

The stem cells cluster and are easy to identify; they divide to produce another germ line stem cell and a differentiating cell called a gonialblast, which goes on to eventually become a sperm cell.

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Divide and Define: Clues to Understanding How Stem Cells Produce Different Kinds of Cells