The regenerative ability of stem cells allows skin replenishment during a lifetime. But different factors can reduce their regenerative properties or promote their uncontrolled growth.
When things go wrong, this can lead to aging and disease, including skin carcinomas.
The discovery that macrophages activate skin stem cells may also have further implications beyond the possibility to develop therapeutic approaches for hair loss, but may also be relevant for cancer research.
This work emerged more than four years ago when the mice Perez-Moreno had been working with received anti-inflammatory drugs, a treatment that also reactivated hair growth.
Perez-Moreno's lab then experimented with the different types of cells involved in the body's defense system.
After years of investigation, they discovered that when stem cells are dormant, a fraction of macrophages die, due to a process known as apoptosis.
This stimulated the secretion of factors from dying and living macrophages, which in turn activated stem cells, and that is when hairs began to grow again.
Macrophages secrete a number of factors including a class of proteins called Wnt.
The discovery 'may facilitate the development of novel treatment strategies' for hair growth in humans, researchers said.
The possibility of attacking one type of cell to affect another might have broader applications that go beyond 'just' growing hair.
"Our study underlines the importance of macrophages as modulators in skin regenerative processes, going beyond their primary function as phagocytes (immune system cells)," researchers said.
The study was published in the journal PLOS Biology.

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