Every person, every mouse, every dog has a definite sign of aging: hair loss. But why does this happen?
Rui Yi, professor of pathology at Northwestern University, decided to answer the question.
A generally accepted hypothesis about stem cells is that they replenish tissues and organs, including hair, but they will eventually become depleted and then die on the spot. This process is considered to be an integral part of aging.
Instead, Dr Yi and his colleagues made a surprising discovery that, at least in the hair of aging animals, stem cells escape from the structures that house them.
“It’s a new way of thinking about aging,” said Dr. Cheng-Ming Chuong, a skin cell researcher and pathology professor at the University of Southern California, who was not involved in the study by Dr. Yi, published Monday in the journal Natural aging.
The study also identifies two genes involved in aging hair, opening up new possibilities to stop the process by preventing stem cells from escaping.
Charles KF Chan, a stem cell researcher at Stanford University, called the article “very important”, noting that “in science, everything about aging seems so complicated that we don’t know where to start. “. By showing a way and a mechanism to explain hair aging, Dr Yi and his colleagues may have provided a boost.
Stem cells play a crucial role in hair growth in mice and humans. Hair follicles, the miniature tunnel-shaped organs from which hair grows, go through cyclical periods of growth during which a population of stem cells living in a specialized region called the bulge divides and becomes growing hair cells. fast.
Sarah Millar, director of the Black Family Stem Cell Institute at Icahn School of Medicine in Mount Sinai, who was not involved in Dr Yi’s article, explained that these cells give rise to the hair shaft and its sheath. . Then after a period of time, which is short for the hair on the human body and much longer for the hair on a person’s head, the follicle becomes inactive and its lower part degenerates. The hair shaft stops growing and falls out, to be replaced by a new lock of hair as the cycle repeats.
But while the rest of the follicle dies, a collection of stem cells remains in the bulge, ready to start turning into hair cells to grow a new strand of hair.
Dr Yi, like most scientists, had assumed that with age, stem cells die in a process known as stem cell depletion. He expected the death of stem cells from a hair follicle to mean the hair would turn white, and when enough stem cells were lost, the strand of hair would die. But this hypothesis had not been fully verified.
Together with graduate student Chi Zhang, Dr. Yi decided that in order to understand the process of aging hair, he needed to observe individual strands of hair as they grew and aged.
Usually, researchers who study aging take pieces of tissue from animals of different ages and look at the changes. There are two drawbacks to this approach, said Dr Yi. First, the tissue is already dead. And it is not clear what led to the observed changes or what will follow.
He decided his team would use a different method. They observed the growth of individual hair follicles in the ears of mice using a long-wavelength laser that can penetrate deep into tissue. They tagged the hair follicles with a green fluorescent protein, anesthetized the animals so they wouldn’t move, put their ears under the microscope, and turned over and over to observe what was happening to the same hair follicle.
What they saw was a surprise: When the animals started to age and turn gray and shed their hair, their stem cells started leaking out of their tiny homes in the bulge. The cells changed shape from round to amoeba and were expelled from tiny holes in the follicle. Then they resumed their normal forms and rushed forward.
Sometimes the escaping stem cells jumped long distances, in cellular terms, from the niche where they lived.
“If I hadn’t seen it for myself, I wouldn’t have believed it,” Dr Yi said. “It’s almost crazy in my mind.”
The stem cells then disappeared, possibly consumed by the immune system.
Dr Chan compared the body of an animal to a car. “If you keep it running long enough and don’t replace the parts, things wear out,” he said. In the body, stem cells are like a mechanic, providing spare parts, and in some organs such as hair, blood and bones, the replacement is continual.
But with the hair, it now looks like the mechanic – the stem cells – just quits work one day.
But why? The next step for Dr Yi and his colleagues was to ask if the genes were controlling the process. They found two – FOXC1 and NFATC1 – that were less active in older hair follicle cells. Their role was to trap the stem cells in the bulge. The researchers therefore selected mice lacking these genes to see if they were the main controllers.
At the age of 4 to 5 months, the mice began to shed their hair. By 16 months of age, when the animals were middle-aged, they looked old: they had lost a lot of hair, and the remaining scattered strands were gray.
Now, researchers want to save hair stem cells in aging mice.
This story of the discovery of a completely unexpected natural process makes Dr. Chuong wonder what remains to be learned about living creatures.
“Nature has endless surprises in store for us,” he said. “You can see some fantastic things.”