Unraveling the Secrets of the Hair Cycle

The Hair Cycle

We look in the mirror to comb and brush our hair in the morning, to rearrange mussed-up hair, to check on hair styling-and perhaps to regret the loss of hair. When we comb and brush we find strands of hair in the comb and brush, on the vanity stand under the mirror, and on our clothing.

We may or may not give any thought to the probability that most of this "lost" hair is also "dead" hair from follicles that have lived through the normal cycle of hair development (anagen), hair death (catagen) and follicle rest (telogen) before the cycle starts all over again. (See About Hair Loss and The Hair Growth Cycle).

The "dead" hairs have been pushed out of follicles by new hair developing from renewal of the anagen (growth) phase of the hair follicle.

The Hair Follicle Enigma

Every hair follicle transitions through a complete anagen-catagen-telogen-anagen cycle 10 to 20 times in a normal human lifetime. A normal anagen phase lasts 3 to 10 years, a normal catagen phase 2 to 3 weeks, and a normal telogen phase 3 to 4 months. At any given time, about 1% of hair follicles are in catagen phase, and about 15% in telogen phase.

A question that science is still striving to answer is: Why do hair follicles undergo the cycling process?

Hair follicles are one of he few organs or tissues of the human body that undergo cyclic degeneration and regeneration throughout the human lifetime. The cycle is not affected by season of the year-unlike the case with many animals that have seasonal changes in hair growth and color. So, what causes hair follicles to cycle, and what benefit-if any-does cycling have for human comfort and survival?

The causes of hair follicle cycling are buried deeply in human development. Hair follicles are appendages of skin, and the development of skin and its appendages begins very early in development of the human embryo-after the process called gastrulation when the embryo surface emerges as a single layer that will become the nervous system and skin (epithelium).

As the embryo matures, gene-derived molecular signals guide the development and maturation of structurally complex hair follicles in the skin. A number of gene-derived signaling molecules have been identified that guide the development of hair that the hair follicles produce.

The full-term infant is born with mature hair follicles. What causes the follicles to begin their life-long pattern of anagen-catagen-telogen cycling? A number of gene-derived signaling molecules have been identified as regulators of the cycle. However, not yet understood is how these molecular regulators coordinate their activities to initiate and terminate the anagen-catagen-telogen cycling phases.

Over the last decade, investigators began to understand the central role played by stem cells in hair follicle cycling. Every hair follicle contains a permanent reservoir of multipotent stem cells (stem cells that have the capacity to participate in growth, development and repair of tissues).

As the anagen cycle begins anew after catagen, the hair-producing capacity of a hair follicle is regenerated by the stem cells under the guidance of gene-derived molecular signaling molecules. The effort depletes the reservoir of follicular stem cells, but the reservoir is replenished by processes that are still not well understood. The eventual understanding of this process of stem cell replenishment may contribute to molecular-level treatments to revive hair follicles that have stopped producing mature hair.

Still to be answered is the question: Why do human hair follicles have the capacity for cycling degeneration and regeneration? So far, the only answer we have is: We don't know.

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