Human hair growth cycle - Stages of hair growth

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Human hair follicles, deceptively simple structures, with the most obvious function of producing hair shafts, have different growth phases.

Depending upon their location, the human hair follicles individually differ in their regenerative functions to produce shafts varying in length, thickness, shape, color, curl and growth phases. Hair growth passes through anagen (rapid growth), catagen (regression), telogen (resting) and finally exogen (shedding) phases.

In humans the hair cycling is asynchronous, intrinsic and autonomous. Though all the body hair goes through this cycling, the follicles have varying duration of growth phases at different body regions and also within the same site. The human follicular growth phases are independent of seasons and circannual rhythms. However, what initiates the stimulus for hair growth phases and the origin of the stimulus remains to be clarified.

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Akin to human skin turnover, hair growth cycling in phases is an unique inherent evolutionary process. Perhaps the process had evolved to 1) keep pace with the body growth; 2) adapt to periodic changes in the environmental conditions; 3) meet certain biological needs such as reproduction; 4) protect from malformations and malignancy; or 5) adapt to social, ethnic and regional evolutionary pressures.

Follicular morphogenesis and regeneration

Prenatal follicles form from primitive epidermis and there is follicular cell differentiation to form sheath and shaft. Follicle formation occurs once in a person's lifetime. Normally a person is born with a fixed number of hair follicles. Thereafter there is only follicular cycling.

The human skin has approximately five million follicles. The follicular cells have regenerative properties. In anagen there is complete regeneration of the lower portion of the follicle. The stem cells are present at the base of the follicle bulb and in catagen, during the regression of the follicle, these primitive epithelial stem cells remain behind.

Anagen phase in hair growth

Anagen phase is the shaft growth period and approximately 80 percent to 90 percent of healthy follicles are in anagen at any given time. Anagen of each strand on the human scalp may last between two to seven years depending on the individual. Longer anagen period contributes to long hair. However, in arms, legs, eyebrows and eyelashes have shorter strands because the anagen period lasts only for a few months.

In anagen period the follicles are very sensitive and prolonged diseases, nutritional deficiencies, extreme physical stress or mental stress may end the growth phase and trigger telogen effluvium leading to massive shedding of strands. The duration of anagen appears to have genetic influence in humans as generally Asians have longer anagen and long hair.

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The anagen has six subphases. Substage-I is primarily regrowth of papilla and onset of cell division. Substage-II involves descent of evolving bulb and differentiation of cells. In substage-III, the follicle attains its full size and cells differentiate into follicular components. In substage-IV, melanocytes are reactivated and produce melanin pigment. Substage-V involves dislodging of club hair and emergence of shaft through the tip of the internal sheath. In substage-VI hair shaft emerges through the skin surface.

Catagen Phase in human hair growth

Catagen phase in humans is a short transition period between anagen and telogen and lasts for about three weeks. The outer hair root sheath shrinks during this period and at any time about 5% of the shafts are in this mode. An unknown signal brings about catagen. There is stoppage of follicular cell growth and pigment deposit. The papilla is released from the bulb. There is cessation in differentiation of the lower follicular cells. In the follicle there is apoptosis of the distal region and vertical shrinkage.

Telogen phase in human hair

Telogen phase is the resting period and may last approximately for two or three months. After telogen, anagen returns.

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About 10 to 15% of the scalp follicles will be going through this phase at any time in a healthy individual. The ratio of between the anagen and telogen decides the density of hair present in the human scalp. Telogen is prolonged in case of strands on the eyebrows, eyelashes, arms and legs.

Exogen (shedding) phase in human hair

Humans shed 50-100 strands of hair daily. When the anagen resumes, the cells in the root region start dividing rapidly and push out the earlier telogen strand (club hair) which had stopped growing. This leads to loosening of the telogen strand and its shedding.

Further research may throw light on the following areas of importance in human follicular phases. The genetic aspects of anagen, catagen, telogen and exogen have to be further investigated. The triggers of anagen, catagen, telogen, and exogen phases of hair growth in humans have to be studied. The effects of extraneous factors including endocrine and immune functions on the human pilosebaceous apparatus have to be further understood. The factors behind vellus-to-terminal follicle switch as well as the inverse terminal-to-vellus switch have to be have to be further investigated. We further need to elucidate the dynamics of cellular division as well as apoptosis in the follicles. Better understanding of the human hair growth phases may help us in dealing with alopecia, a worldwide problem of great esthetic concern.

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Reference: 1.K. S. Stenn , R. Paus. Controls of Hair Follicle Cycling. Physiol Rev. 2001; Jan;81(1):449-494. 2.Krause K1, Foitzik K. Biology of the hair follicle: the basics. Semin Cutan Med Surg. 2006; Mar;25(1):2-10.

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