There is an Average 100,000 hair on head. The average life of a hair is 3-5 years. In individuals who are not loosing hair, 90% hair are present most of the time on the scalp, while remaining 10% are in the process of falling and growing back. When these 10% resume the normal 3-5 year of growth phase another 10% goes through falling and growing back phase. Every year 30% hair goes through this falling and growing back phase. It takes an average three years for all the hair on the head to go through falling and growing back process. This hair falling and growing back rejuvenation continue throughout one’s life time.
The hair rejuvenating process has been divided into three phases called, 1- Anagen 2- Catagen and 3- Telogen. Each hair goes through these rejuvenating stages on an average 25 times in ones normal life expectancy.
Anagen Stage (the average growing phase – 3-5 years)
The growing phase of hair is called Anagen. Each hair grow for 3-5 year at an average rate of 0.35 mm/day, 1 cm/month or 12 cm/year. The average depth of the hair in the scalp is 4 mm. Hair grow as single hair, two hair group and three hair group and occasionally as four or more hair group. The single hair or group of two ,three or four are referred as single hair follicular unit , two hair follicular unit and three hair follicular unit . Each follicular unit has one muscle coming from skin and attached near the middle of the hair called bulge area. The hair above the muscle attachment constitute 45% of the hair height. The 45% hair above the aarector pili muscle constitute the permanent portion of the hair because it remain intact during the entire hair cycle. Lower portion of the hair go through regression, rest and regeneration. After completing the 3-5 years of growing phase, the hair goes through regression phase lasting 2-3 weeks called catagen stage. During catagen stage lower 55% of the hair below the muscle attachment disappear except the most outer covering of hair, the fibrous root sheath which collapse on itself and provide a path for hair papilla to move up and lies below the bulge zone. After Catagen stage, hair goes through a resting phase lasting 3-4 month called Telogen stage. After completing the resting stage, hair enter into growing phase. The club hair the upper 45% permanent part is shed near the end of telogen resting stage. As hair enter the growing phase, the stem cells in the follicular bulge are activated and form a population of rapidly proliferating cells. These cells descend into the vacant and collapsed fibrous root sheath along papilla. The hair matrix is reformed around and above the papilla and various layers of the hair differentiate. The new anagen hair will continue to produce a hair shaft for next average 3-5 years.
Catagen Stage (the regression phase- 2-3 weeks)
After completing an average 3-5 year of continues growth, each hair goes through a regression stage lasting 2-3 weeks called Catagen. During the Catagen phase, the entire architecture of the lower portion of hair below the insertion of aarrector pili muscle changes. The medulla, cortex and the cuticle of lower part of the hair along matrix cells, inner root sheath and outer root sheath disappear. The vitreous layer and fibrous root sheath collapse, the collapsed vitreous and fibrous root sheath is called stela, also referred as streamer. The papilla follows upward along the collapsed structure to rest underneath the bulge zone at the level of arrector pili insertion. Just above the papilla, lower part of shaft expands to form presumptive club hair.
Telogen Stage (Resting phase- 3-4 months)
Hair papilla become condense ball of spindle shape nuclei, papilla lies just below a nipple of epithelium called secondary hair germ. Just above the secondary hair germ, the telogen club expands and cornified, this progressive cornification occurs about in three month period. Near the end of telogen phase, the permanent part of the hair along cornified club base falls off. After completion of telogen phase, the growing anagen phase resumes.