Hair Anatomy

About Hair
Follicular Unit
FU vs. Hair
Histology

All about Hair

The Origins of Hair

Begin to develop by week 8 in fetal life. By week 22, all hair follicles formed.

At birth there are 5 million body hair follicles - one million in the head, 100,000 being scalp hair. No new hair follicles after births. Density of scalp hair reduced when the scalps expand on growing.

A scalp follicle may go through 10-20 hair cycles in a life time. Each scalp hair follicle is thus expected to last 85 years.

The Functions of Hair

• The primary function is to communicate information about a person's age, gender, state of sexual maturity, general health and vitality. Hair styles are used to identify the individual society status and to attract a mate.

• Protection against heat loss.

• Transmit sensory, tactile information from the environment.

• Form a tough barrier to protect the scalp from scratches and ultra violet light.

• Eyebrows and eyelashes protect the eyes by channeling or sweeping away fluids, dust and debris.

• Nasal hair traps air borne foreign particles before reaching the lungs.

• Increases surface area for sweat evaporation.

• Spreads secretion from sweat and sebaceous glands.

Androgen-Dependent Hair

Androgens (male hormone) regulate changes in human hair growth. In puberty the increase production of androgen changes the childhood vellus follicles into pigmented terminal follicles in many areas, including pubic and axillary hair, beard and moustache. These are called Androgen dependent hairs.

Unfortunately androgens have the opposite effect on scalp hairs. They reverse large, terminal, scalp follicles back to small vellus pale hairs and cause balding. Scalp hairs are therefore not androgen dependent.

Follicular Unit

The 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". They are readily visible under 2X magnification.

Human has an unorganized pattern of groupings. Each individual has his/her own characteristic pattern of follicular units to give a unique appearance.

One FU may contain:
•  one to five terminal hair follicles;
•  one or more vellus hairs follicles;
•  sebaceous gland lobules;
•  insertion of the arrector pili.

Follicular Unit Density

On average one square cm contains about 80 to 120 FUs, with individual and ethnical variation.

For the same person FU density is highest in the midoccipital region (back of head), least in the supra-auricular area (above the ears).

Though large graft contains more FUs, it's the smallest ones that give a more natural cosmetic result. In adult:
•  A 4 mm biopsy contain 12-14 FUs;
•  A 2 mm mini-graft contain 3-4 FUs
•  A 1 mm micrograft contain only 1 FU

The exclusive use of follicular unit is known as "Follicular Unit Transplant" or "FUT".

Follicular Unit Density in Various Ethnic Groups

	Caucasian	Africans	Asians
FU Density	100 / sq.cm	60 / sq.cm	90 / sq.cm

Follicular Unit vs. Hair

Application in Hair Transplant

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.

With a higher FU density and hair-graft ratio, Caucasians are more suitable for FUE.

Follicular Units - FUs

Scalp hairs emerge via the scalp skin orifice in groups, known as the Follicular Units.

1 Hair Follicular Unit or 1-Hair FU
•  if only one hair in the Follicular Unit;

2 Hair Follicular Unit or 2-Hair FU
•  if only two hairs in the Follicular Unit;

3 Hair Follicular Unit or 3-Hair FU
•  if only three hair in the Follicular Unit;

Occasionally some people may have up to 4 or 5 hairs per follicular unit (4-hair or 5-hair FU).

Ratio of Follicular Units in Normal Chinese Scalp.

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

Graft

The latest hair restoration surgery transplants Follicular Unit (FU), therefore a FU is also called a GRAFT.

Hair-Graft Ratio

Each individual will have a 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.

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

Hair-Graft Ratio in Various Ethnic Groups

	Caucasian	Africans	Asians
Hair-Graft ratio	2.3	2.3	1.8

Histology

Application in Hair Transplant

• The bulge contains stem cells, and is thus 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.

Shafts and Roots

Adult hair has 2 distinct structures – shaft outside the skin, and follicle (root) buried under the skin.

Hair Shafts

• Keratin , a hard protein formed by dead cells.

• Binding material;

• Water.

The 3 layers of shaft:

1. 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.

2. 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.

3. Medulla - a central hollow core found in terminal hair.

Hair Root or Dermal Papilla

A tunnel-like segment of the epidermis that extends down into the dermis.

Vertical Compartments

1. Upper follicle (infundibulum and isthmus)

2. Middle follicle (the bulge) - the storage area for hair follicle stem cells that are able to regenerate the hair follicles.

3. Lower follicle (the bulb or suprabulbar) - This forms the dermal papilla which looks like a healthy "pear" shape. Cells here divided 23 to 72 hours to regenerate the hair follicle. This rate of proliferation is the fastest in the body. Functions of dermal papilla are:
•  Generation of a hair follicle
•  Regulate hair cycle
•  Control the size of the hair shaft
•  Also play a role in hair growth.

Upper and middle parts are permanent. Lower follicle degenerate and regenerates with each hair follicle cycle.

Horizontal compartments

1. 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.
In "Hair Cloning", dermal sheath is injected into the skin to form a new dermal papilla and induce new hair formation

2. 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, and Amelanotic melanocytes.

3. Inner root sheath (IS) - consists of 3 layers: Henle's layer; Huxley's layer; and IRS cuticle. Functions of the IRS are:
•  protect and form the growing hair shaft
•  anchor and direct the growth of the emerging hair shaft
•  determine the racial difference in shaft cross-section (Asians – round ; Africans – flat ; European - ellipsoidal).

Surrounding Tissues

The sebaceous gland

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.

Arrector pili

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.