NEW YORK—Following up on a hypothesis posited as far back as 1991, researchers at Mount Sinai have identified a specific mechanism, recently published in Science, within the hair growth cycle that results in hair loss. The resulting hope is that this mechanism can someday be pharmacologically controlled, thereby slowing or stopping hair loss.
The dermal sheath surrounds the outside of the hair follicle and contains progenitor cells that maintain and regenerate the dermal papilla, a key component for hair growth. This has been recognized for decades as a key component of the hair growth cycle, though not fully understood. As individual hairs proceed through the life cycle, the dermal papilla cells signal to the stem cells at the root of the hair to begin producing a new hair shaft, while also arresting growth of the existing hair. When hair stops growing and moves into its destructive phase in the absence of new growth stimulation, hair loss and baldness results.
Hairs are continually shed and renewed during the full hair growth cycle. Hair grows from the follicle, underneath the skin, with off-shooting blood vessels which nourish hair shafts. Between starting to grow and falling out years later, each hair passes through four stages: anagen (growing phase), which lasts two to seven years and determines length of hair; catagen (destructive phase), a transitional phase when follicle shrinks and detaches from the dermal papilla; telogen (resting phase), which lasts +/- three months while old hairs rest and new hairs are starting to grow; and exogen (new hair phase), a constantly cycling phase when old hairs are shed while new hair continues to grow. Every hair can be at a different stage of the growth cycle at any given time.
The average life span of growing follicles in humans varies depending on hair type, and can be five to seven years for scalp hairs.
The hair cycle is a complex biological process that depends on intricate regulation of stem cell rest and activation, as well as progenitor proliferation, cell-fate choices, differentiation and cell death. Each human follicle is a regenerative adult tissue that follows this cycle, and the total amount of hairs remains constant so long as they don’t get lost or communications in the cycle break down, leading to baldness.
“The hair-growth cycle is asynchronous,” explains Dr. Michael Rendl, the associate director of the Black Family Stem Cell Institute at the Icahn School of Medicine at Mount Sinai. “We don’t know what kicks in the destructive phase, or what underlying ‘clock’ is at work.”
“Our major discovery is a previously unknown smooth muscle that surrounds hair follicles and is called dermal sheath. Its contraction is important for moving the instructive niche, i.e. dermal papilla cells, to the stem cells for growth of a new hair in the cycle. This type of muscle cannot be controlled voluntarily, similar to the ones in blood vessels, but we can control it by drugs that can block contraction. We are excited about the possibility to develop methods for blocking sheath contraction, stopping follicle regression and preventing the loss of the existing hair before a new hair can grow. Our studies continue to make this possible in the future,” he adds.
According to Rendl, researchers will need to conduct many more studies exploring their ability to safely block the contraction of the dermal sheath to answer some resulting questions. Would halting this process result in the evergreen presence of an old hair, thus eliminating the need for haircuts? How would a hair shaft hold up under such longevity? Would old and stagnant hair be preferable to baldness? Considering the stigma associated with hair loss for both men and women, many people will be watching the future of this research with great interest.