11.   Though mature cells and even adult stem cells eventually age, a special group of human cells never grows old.

12.   Though the body is made up almost entirely of mature cells, its continued existence depends on a handful of stem cells.

13.   Mice that lacked mature B cells were protected, indicating that the B cells play some crucial role in getting prions into the brain.

14.   Researchers have made neurons from embryonic stem cells and have shown in mice that the mature cells will migrate throughout the brain and make new nerve connections.

15.   Scientists believe they can learn to direct the development of embryonic stem cells in order to grow mature cells or tissues that could be used to treat disease.

16.   Scientists believe they can learn to direct the development of embryonic stem cells to grow mature cells or tissues that could be used to treat disease.

17.   The development of immature cells into particular kinds of mature cells is guided by chemical signals from their surroundings.

18.   This is called somatic gene modification and involves changing the genes in existing mature cells.