https://phys.org/news/2018-05-immune-cell-cradle-mammary-stem.html

"Learning more about the factors that keep mammary stem cells alive and healthy may provide insights into the development of breast cancer when such regulation goes awry," said Yibin Kang, the Warner-Lambert/Parke-Davis Professor of Molecular Biology at Princeton University and an associate director of the Rutgers Cancer Institute of New Jersey, who led the international team that made the discovery. "Our study establishes macrophages as important components of the mammary stem cell niche," he said.

Stem cells are cells that have the potential to mature, or differentiate, into several different types of cells. Mammary stem cells shepherd breast development during puberty and are active during pregnancy and lactation.