Because they reside in the skin, the cells are easily accessible. But purifying and characterizing them has taken years of research. "We had to learn enough about the properties of the different cells within the skin in order to come up with a strategy to isolate pure populations of these cells, and determine that these cells are in fact stem cells," said Fuchs.

In 2004, the group first published a strategy for labeling and monitoring the cells in the journal Science. "Since then, we and others have devised several different ways to isolate these cells, and this has accelerated the rate at which we are learning about the properties and potential of skin stem cells."

To clone the mice, researchers removed the nucleus from an unfertilized egg cell, called an oocyte and replaced it with the nucleus from an adult keratinocyte stem cell. They cultured these hybrid cells in the laboratory to grow them to the blastocyst stage, when the embryo is a tiny hollow ball of cells. At this point, the cultured blastocysts were implanted in a mouse's uterus and allowed to develop into a cloned fetus. This is the cloning technique known as nuclear transfer.

Typically, only about one to two percent of transferred mouse blastocysts result in a live birth. Furthermore, cloned mice that do survive to birth are often not healthy. In Fuchs' and Mombaerts' study, the success rates were 1.6 percent when using skin stem cells from female mice. When the stem cells came from male mice, however, 5.4 percent of the transferred blastocytes developed into mice. The oldest of these animals is now nearly two years, which is old age for a mouse. In addition, many of these mice were fertile and healthy.

The difference in cloning rates of male and female stem cells seems likely to involve epigenetics," Fuchs said. Epigenetic modifications are those that affect a gene's function without altering its DNA se
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Contact: Jennifer Michalowski
michalow@hhmi.org
301-215-8576
Howard Hughes Medical Institute
12-Feb-2007