The Science of Hair

Hair loss treatment

The understanding of human hair is important when studying hair loss and hair transplant. Basic hair science includes histology, anatomy, function, biochemistry, and the hair cycle. We will try to explain all these in a comprehensive way.

Application in Hair Transplant

The bulge in the middle of the hair follicle contains stem cells. It is the most important part in follicle growth and regeneration. During hair transplant this segment of the graft must be well preserved. In FUE, when too small a punch is used, the extracted follicle may deprived of bulge tissue. In "Hair Cloning", the dermal sheath of the follicle is cultured in medium. The multiplied tissue is then injected into the skin to induce new hair formation. The stem cells in the bulge is multi-purposes. Hair follicle transplant had shown to return skin color in a skin disease called "vitiligo". The average healthy hair fiber diameter is 0.08mm or over. Below 0.04mm the hair fibers are too thin to achieve visual coverage of the scalp

Application in Hair Loss Treatment

Many treatments are being developed to stimulate the stem cells: new hair follicles can be formed; even white hairs can turn black. 5 α reductase converts testosterone into DHT. This enzyme is located in the Sebaceous Glands. Dr Inaba (Japan) believes that over consumption of animal fat swells the sebaceous gland, increases the amount 5 α reductase and DHT, leading to earlier onset of androgenetic hair loss.



Human hairs do not grow all the time, but in phases or returning cycles. Different hairs would have different cycle. During a human life, the growth cycle of a scalp hair will repeat itself on average 20 times from birth on. Scalp hair cycle consists of 5 phases:

1. Anagen - the Growing Phase

•   The cells in the hair root is actively producing hair
•   Around 85% of all hairs are in the growing phase at anytime.
•   Human scalp has around 100,000 terminal hair follicles, so only 85,000 hairs are visible
•   Anagen can last for 3 to 6 years (average 3 years)
•   This cycle shortens with age
•   This phase determine the length of the hairs. Hairs grow around 10cm per year. With an average anagen phase of 3 years hairs can grow to 30cm long. However some people can have hairs longer than 1 meter

2. Catagen - the Transitional Phase

•   At the end of the growing phase, the follicles prepare themselves for the next resting period
•   The production of hair stops and the hair follicle shrinks to around 1/6th of its normal length
•   Around 1-2% of all hairs are in this phase
•   Catogen lasts for around 1 to 2 weeks

Common Myth!
Many believe hair root remains during resting phase, only to re-grow later. That's wrong! Lowest part (papilla) actually retracts upward. Only upper 2/3 remains and is called a "Club Hair".

3. Telogen - the Resting Phase

•   This phase lasts for on average 3 to 4 months
•   The hairs do not grow and in a phase of resting. The old hair shafts remain attached
•   Around 13% (vary from 4% up to 24%.) of all hairs are in the resting phase
•   More hair in this resting phase, less hair will be visible, thus giving the impression of "thinning". This condition is common after baby delivery, under stress, or on a crush diet, and is called "Telogen Effluvium"

4. Exogen - the Shedding Phase

•   The aged hair shaft will be released and shed from the follicle

Common Myth!
Many believe that the old hair is pushed out by the newly formed hair. Recent studies support a more active process that involves both active retention and active release of the club hair fibre.

5. Kenogen - the Hairless Phase

•   In this phase the old hair was shed before the new hair starts to grow
•   The follicle may remain empty for many weeks to months
•   In bald men more follicles are in Kenogen
•   This phase is undesirable as early release of the club hair without the follicle re-entering into anagen means overall loss of hair density


Follicular Unit and Graft


There are two 1-Hair-FU per graft,known as DFU ( Double Follicular Units )

What is Follicular Unit

"Follicular Unit" was first described by Dr. John Headington, a pathologist, in his 1984 paper “Transverse microscopic anatomy of the human scalp”. Scalp hair follicles were observed to bind together underneath the skin, emerging from the scalp in clusters or groups of 1-5 hairs in each pore. Each grouping of hair is called a "Follicular Unit" or "FU".

Follicular Unit Transplant (FUT) is a hair restoration technique where a patient's hair is transplanted using these naturally occurring follicular units of 1 to 4 hairs. In a properly-performed FUT, the results will mimic the way hair grows in nature and will be undetectable as a hair transplant. These small units allow the surgeon to safely transplant thousands of grafts in a single session, which maximizes the cosmetic impact of the procedure. Procedures using the larger minigrafts often produced a pluggy, unnatural look. Over the last 20 years Minigrafts transplant has been replaced by Follicular Unit Transplant.

Normal Follicular Unit Density
The FU density varies amongst different ethnic groups, being highest in Caucasians and lowest in Africans

  Caucasian Africans Asians
Hair Density 200 / sq.cm 160 / sq.cm 170 ⁄ sq.cm
FU Density 100 / sq.cm 60 / sq.cm 90 / sq.cm

Ethnical Differences in Hair Characters
In order to provide hair transplant service to patients from different ethnic groups, we have studies in depth the differences in their hair density and characters

  Caucasian Africans Asians
Hair Density 200 / sq.cm 160 / sq.cm 170 ⁄ sq.cm
FU Density 100 / sq.cm 60 / sq.cm 90 / sq.cm
Hair-Graft Ratio 2.3 2.3 1.8
Shaft Cross-Section Oval Elliptical Round
Shaft Diameter 0.07 mm 0.06 mm 0.08 mm
Hair Shape Curly Wavy Straight
Length of Follicle 4.5 mm - 5.5 mm

Follicular Unit Density in Chinese Scalps
Usually the density is highest in the mid-occipit which is at the back of the head.

Density (FU/sq.cm) Front Sides Back   Average
Normal Men 84 55 80 73
Bald Men   68  
Women 87 54 72 72
Bald Women   69  

Follicular Unit Ratio in Normal Chinese Scalps
The 2 Hair FU is the most common in Chinese, followed by the 1 Hair FU. 4- and 5- Hair FUs are very hard to find.

Ratio (%) 1 Hair FU 2 Hair FU 3 Hair FU 4 Hair FU Hair Graft Ratio
Men 29 52 17 2 1.92
Women 33 49 16 2 1.87
This Graft contains a 2-hair Follicular Unit, measured 6mm in length

What is a Graft

Confusion arises when using different units to estimate the cost of a hair transplant procedure. Over 95% of hair transplant centers worldwide use "graft" as unit to estimate procedural cost. Advance in technology renders the smallest follicular unit to be used as graft. i.e. one graft = one FU. Nowadays 2,000 graft can easily be transplanted in one session.

Some centers however, limited by technique or manpower, may not be able to achieve such numbers. In order to sound good they instead use "hair" as unit. Telling a patient he had 2,000 hairs transplanted seems impressive but that's in fact just about 1,000 grafts. To avoid confusion ask clearly the unit for calculating cost.

Hair-Graft Ratio
Each individual will have a unique combination of 1-Hair FU, 2-Hair FU, 3-Hair FU. For a certain area the average number of hair per FU is known as Hair-Graft Ratio. The Hair-Graft Ratio is different amongst various Ethnic Groups. With a higher FU density and hair-graft ratio, Caucasians are more suitable for FUE.

Hair-Graft Ratio = Total Number of Hair ÷ Total Number of FU

  Caucasian Africans Asians    
Hair-Graft Ratio 2.3 2.3 1.8

Hair Anatomy


Adult Hair Follicle Has 3 Distinct Structures

•   Hair Shaft
•   Hair Root or Dermal Papilla
•   Surrounding Tissue

hair shaft

3 Layers of Hair Shaft :

•    Cuticle - a tightly formed structure overlapping scales. A healthy cuticle is a protective layer, and controls the water content of the fiber. Much of the shine that makes healthy hair so attractive is due to the cuticle
•    Cortex - makes up most of the hair shaft, and gives hair its special qualities such as elasticity and curl. Packed with strands of keratin and pigments
•    Medulla - a central hollow core found in terminal hair

Hair Shaft Components

Component Content % Remarks
Keratin 80%-90% •    Formed from dead protein
•    Contains 20 kinds of amino acids
•    Provide strength for the shaft
Water 10%-30% Provide shaft elasticity
Lipid ( Fat ) 10%  
Trace Elements Very small Quantity Zinc, Mercury, Cobalt, Iron, Selenium ...
Melanin -- Accounts for hair color

Adult Hair Follicle Has 3 Distinct Structures

•   Hair Shaft
•   Hair Root or Dermal Papilla
•   Surrounding Tissue



Hair root, alsp known as Dermal Papilla, is a tunnel-like segment of the epidermis that extends down into the dermis. It can be divided into 2 compartments :

1. Vertical Compartments

Upper Follicle( Infundibulum and Isthmus )
This part is permanent

Middle Follicle ( the Bulge )
The storage area for hair follicle stem cells that are able to regenerate the hair follicles. This part is also permanent

Lower follicle ( the bulb or suprabulbar )
This forms the dermal papilla which looks like a healthy "pear" shape. This part degenerates and regenerates with each hair follicle cycle. Here the cells are divided 23 to 72 hours to regenerate the hair follicle. This rate of proliferation is the fastest in the body.

2. Horizontal Compartments

Outmost connective tissue ( dermal sheath )
Enveloping the hair follicle, this consists of 2 layers : an outer layer called the connective tissue sheath; and an inner layer called the hyaline membrane.

Outer root sheath ( ORS )
Protect and form the growing hair shaft. Also service as source of energy for protein synthesis during hair growth. Cells found in ORS are :
•    Merkel’s cell
•    Langerhan cells
•    Amelanotic melanocytes

Inner root sheath ( IS )
Consists of 3 layers :
•    Henle's layer
•    Huxley's layer
•    Inner root sheath cuticle

Adult Hair Follicle Has 3 Distinct Structures

•   Hair Shaft
•   Hair Root or Dermal Papilla
•   Surrounding Tissue



Components of the Surrounding Tissue

The tissue surrounding the hair root is known as Adventitous Tissue. It is consisted of several components:

1. Arrector Pil
A muscle attaches the bulge to the skin. When this muscle contracts, it causes the hair to stand up which also causes the sebaceous gland to secrete oil. This muscle must be cut in FUE to extract the follicle.

2. Melanocytes
Contain melanosomes and synthesize melanins which account for the pigmentation of hair and skin.

3. Sebaceous Glands
Opens into the isthmus. This gland secretes sebum, a semi-fluid secretion consisting chiefly of fat, keratin, and cellular material. The sebum lubricates shaft and carries away debris generated in the follicle.

4. Fatty Tissue
Scientists have discovered that stem cells are plentiful in the fat tissue, the so called Adiposed Stem Cells (ASC). ASC has been used in cardiovascular and cosmetic surgery to generate new tissues.


Chubby Grafts vs. Skinny Grafts

A study by Beehner (2010) compared the survival of chubby grafts and skinny grafts. The differences in survival is proportional to the amount of retained surrounding tissues. Stem cells are retained in chubby grafts but trimmed away in the skinny grafts.

% growth after 19 months Skinny Graft Chubby Graft
2-hair follicular units 69.3 % 88.0 %
1-hair follicular Units 48 % 98 %