In old mice, an inflammatory signal from skin fat keeps hair follicles asleep

Hair does not grow continuously. Each follicle cycles between a growth phase, called anagen, and a resting phase, called telogen, before starting again. With age the resting phase stretches out, the growth phase gets shorter and follicles shrink, which is one reason hair thins. Hair also loses pigment.

Wrapped around the follicles sits a thin layer of fat, the dermal white adipose tissue. It is not just padding: in mice it sends signals that help decide when follicles wake up. Earlier work had shown that this fat becomes more inflamed with age. A team at Southern Medical University in China asked which of its inflammatory signals matters for hair, and how.

What they did

All the experiments were in mice: old males aged 18 to 24 months and young males aged 6 to 7 weeks, usually three to six animals per group. The researchers worked on the back skin, where a shaved patch shows clearly when follicles switch from resting to growing.

They took the question apart in steps. First they gave old mice daily injections of pioglitazone, a prescription diabetes drug that calms inflammation in fat tissue, for 30 days, and measured which signalling molecules in the skin fat changed. Then they focused on one molecule, MCP-1, a chemical "come here" signal for macrophages, the immune cells that patrol tissues and clear debris. They blocked MCP-1 with an antibody in old skin, injected extra MCP-1 into young skin, wiped out skin macrophages with a research tool called clodronate liposomes, and transplanted lab-grown macrophages of different types into the skin.

What they found

• The drug woke follicles and lowered MCP-1. Old mice given pioglitazone entered the growth phase earlier than untreated old mice. In their skin fat, MCP-1 showed the clearest drop of all the signals measured, along with two other inflammatory molecules. Young mice showed no comparable change. The effect did not depend on the drug making the fat layer thicker.
• Blocking MCP-1 restarted growth in old skin. Injecting the blocking antibody every two days for two weeks pushed resting follicles in old mice into growth and cut the number of macrophages. In young mice it did nothing noticeable.
• Adding MCP-1 held young follicles back. Extra MCP-1 delayed the growth phase in young mice and produced shorter follicles with narrower bulbs. When macrophages were removed at the same time, the delay disappeared, which points to macrophages as the messengers.
• Old skin is crowded with inflammatory macrophages. Macrophages made up 7.9% of the analysed skin cells in old mice against 1.1% in young ones. Among them, the inflammatory "M1-like" type made up 88.3% in old skin and 34.1% in young skin. Most of the MCP-1 came from mature fat cells, with some from the macrophages themselves, a possible vicious circle.
• Removing macrophages restarted hair. In old mice whose skin macrophages were depleted, follicles entered the growth phase around day 11, while in controls they were still resting at day 15. Sixty days later the treated patches had more hairs, and the hairs scored higher on a pigmentation index. Genes for hair proteins (keratins) and for melanin production were switched up, and p16, a protein that marks aged cells, went down.
• The type of macrophage mattered. Transplanted inflammatory M1 macrophages kept follicles resting in old and young mice, while unpolarized macrophages and the "repair-type" M2 cells set off growth.

What to keep in mind

The authors point out themselves that mouse and human skin fat are built differently. In mice it forms a continuous sheet and follicles cycle in waves together; in people each follicle has its own small cone of fat and cycles on its own schedule. They say the findings still need to be checked in human scalp tissue before any conclusion about people can be drawn. They also note that sorting macrophages into M1 and M2 is a simplification: real macrophages come in more varieties.

Beyond that, the groups were small, often three to five mice, and only male mice were used. Removing macrophages from skin is a laboratory technique, not a treatment, and macrophages have many useful jobs, including fighting infection and healing wounds. Pioglitazone is a prescription drug for diabetes with known side effects; here it was injected into mice as a research tool. The pigment results are mostly gene and protein measurements plus one pigmentation score: the study did not show grey hair turning dark again.

Why it matters

Aging tissues tend to carry a low, persistent level of inflammation, and this study traces one concrete route by which that inflammation could reach a specific organ. Fat cells next to the follicle send out MCP-1, inflammatory macrophages gather, and the follicle stays at rest. Each link was tested by turning it up or down, not only observed.

The work fits a broader idea in aging research: that the surroundings of a stem cell, not only the stem cell itself, can decide whether it keeps working. Whether the same chain runs in the human scalp is an open question, and the team plans to look at human skin samples next.