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AT LAST researchers may have found the long-sought stem cells that form the milk-producing tissue within the human breast. The finding raises the possibility of women growing breast tissue to replace what they鈥檝e lost to cancer or surgery.

鈥淭hey regenerate the different cell types that make up the ductal unit of the breast,鈥� says Mina Bissell of the Lawrence Berkeley National Laboratory in California, who found the cells with Ole William Petersen of the University of Copenhagen. The ductal units are tree-like structures that produce milk, which is supplied to the nipple via ducts. 鈥淭hey鈥檙e what makes the breast what it is,鈥� says Bissell. They are also where cancer is thought to begin, so Bissell hopes that the stem cells will lead to new discoveries about how breast tumours form.

Petersen and Bissell isolated the stem cells from tissue left over from breast reductions. First they isolated a group of cells carrying particular surface proteins. Among these, they found a distinct subgroup that formed a tree-like structure when grown on a gelatinous scaffold, or when they were transplanted into the mammary region of a mouse. The team then established cell lines by adding a virus to the cells, a common procedure to make cells grow indefinitely in culture. The cell lines are still going strong after two years, says Bissell. They look healthy, and have normal numbers of chromosomes.

But could this kind of stem cell be isolated from individuals and used to regrow functional breast tissue? 鈥淭hat was the first thing that occurred to me,鈥� says Bissell. But this is many years away, she says, since you鈥檇 not only have to grow ductal units, but also figure out how to connect them to a nipple. Another problem is that the stem cells don鈥檛 give rise to the connective tissue and fat that contribute to the structure of the breast. 鈥淲e will have to see,鈥� says Bissell. 鈥淭his would be the first step.鈥�