WHY HAIR GROWS
THE HAIR FOLLICLE
ANATOMY OF THE HAIR FOLLICLE
HAIR GROWTH CYCLE
HORMONAL CONTROLS OF HAIR GROWTH
TYPES OF HAIR
 
MEDICAL HAIR RESTORATION
INTERNATIONAL SOCIETY OF HAIR RESTORATION SURGERY
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WHY HAIR GROWS
 
Why does hair grow on your scalp? The answer is very simple: you inherit it from your mother and your father.... that is to say; it is in your genes. The genes involved in the formation of your hair follicles are present when your mother's egg is fertilised by your father's sperm.

After the hair follicle is formed in the embryo, it continues to acquire the signalling pathways it will need to produce, maintain and cycle hair for its lifetime. Production of hair requires synthesis of proteins (keratins) and other molecules, and these will determine the pattern of your hair (ie: will it be straight, wavy, curly, or frizzy) and the colour of your hair (blond, red, brown, black).

DNA variations that alter this protein and molecular synthesis will result in hair abnormalities. Although a number of hair abnormalities are recognised, very little is known regarding their actual cause and/or their genetic transmission (inheritability). Some congenital hair abnormalities are associated with congenital physical and mental developmental anomalies that have been mapped onto specific chromosomes. Genes are defined as autosomal dominant or autosomal recessive.

The dominant gene is all-powerful, which means that a person with autosomal dominant conditions will have one affected parent, and the condition is transmitted to future generations. Males and females are affected equally, and both males and females can transmit the condition. When a person carrying the autosomal dominant gene(s) that causes the condition has children with a person who does not carry the gene(s), the condition will appear in about 50% of their children.

The recessive gene can be dominated by other genes, and therefore both parents must carry the gene(s) that cause a condition, although neither parent necessarily exhibits the condition. The child inherits the gene(s) from both parents, and the condition appears in the child.

X-linked recessive: this occurs only in males, but the gene is actually transmitted by carrier females who carry one gene for the specific condition. An affected male cannot transmit the condition to his sons, but all of his daughters will be carriers of the gene.

Unfortunately the human condition cannot be explained by these simple genetic terms. DNA is one of the most complex and complicated things to describe. Medical scientists have been trying to decipher DNA for the last 50 years, and will probably still be trying to understand what is going on with the DNA for the next 500 years. Even someone who is so-called normal, his/her DNA will have up to 10 million variants (also known as mutations). All of this results in the variations that will occur between various family members.
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THE HAIR FOLLICLE
 
The hair follicle has a complex anatomical structure and a complex physiological function that is conducted under the influence of genetic and hormonal controls. It has a limited effective lifetime: each follicle completes 10 to 20 anagen-catagen-telogen cycles in its lifetime. In other words, each follicle produces 10 to 20 hairs in its lifetime. Completion of each scalp hair cycle can take from 2 to 8 years.
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ANATOMY OF THE HAIR FOLLICLE
 
The hair follicle, irrespective of whether it is on the scalp or elsewhere on the body, has a characteristic anatomical structure.

The follicle is a slim, hollow shaft extending from the surface of the skin into layers of underlying skin and fat cells. There are about 5 million hair follicles on a human, of which about 100,000 to 150,000 occur on the scalp.

In a cross-section view, the follicle is seen to be organized in concentric compartments. The outermost layer of cells is the outer root sheath, beneath it the inner root sheath and then the hair shaft. The hair shaft itself has three concentric compartments: the outermost cuticle, the underlying cortex and the innermost medulla.

Longitudinally, the follicle is anatomically organized from top to bottom as (1) the hair canal, which becomes indistinct after birth, (2) infundibulum that extends to the opening of the sebaceous (oil) gland duct, (3) sebaceous gland that produces the oil that keeps hair lubricated, (4) the isthmus that begins at the sebaceous gland and ends at (5) the bulge, where the arrector pili muscle inserts [the little muscle that makes hair stand on end in response to cold or fear] and where follicular epithelial stem cells are located [stem cells that may be capable of generating new matrix cells and initiating new follicles], (6) the keratinization zone where the hair shaft acquires its tough coating of keratinocytes, and (7) the hair bulb that contains the hair matrix where a new hair is generated, and melanocytes that contribute to hair colour.
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HAIR GROWTH CYCLE
 
The normal hair cycle is associated with the temporary loss of hair and the growth of new hair from the same follicles. The hair growth cycle in humans occurs in three phases:

- Anagen-growth phase
- Catagen-degradation phase
- Telogen-resting phase


There is no break or discontinuity between phases; the hair cycle is a continuous process that occurs during the life of the hair follicle. A complete scalp hair cycle stretches over 2 to 8 years:

- Anagen (growth)-2 to 8 years
- Catagen (degradation)-2 to 4 weeks
- Telogen (rest)-2 to 4 months


The "dead" hair that was degraded during the catagen (degradation) phase is pushed out of the follicle when a new hair emerges in anagen (growth) phase. A healthy person with a full head of hair will shed on average 50 to 100 "dead" hairs per day. Shedding can be influenced by internal factors such as age, change in hormonal or nutritional status, skin disease and stress, and by external factors such as cancer chemotherapy, ionizing radiation, and exposure to some types of industrial chemicals.

All follicles, everywhere on the body, undergo hair cycles. The duration of each cycle may vary by body site; on the same person, scalp hair cycling may be slightly different in duration from body hair cycling.
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HORMONAL CONTROLS OF HAIR GROWTH
 
Androgenic (male) hormones are essential for the growth of human hair everywhere on the body to a variable extent. In boys, especially, the appearance of body and facial hair occurs in parallel with increase in levels of androgenic hormones as the body matures sexually. The relative absence of heavy hair growth on the bodies and faces of girls is an external indication of the relatively lower levels of androgenic hormones in girls.
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TYPES OF HAIR
 
Under genetic control, hair follicles are formed and develop to produce different kinds of hair for different purposes. Hair can be body-site specific: nostril hair, eyelashes, eyebrows, hair in the genital area, torso hair and scalp hair have different physical qualities specific to their function. The physical characteristics of hair may change over time or under the influence of hormonal changes in the body.

Genetic ancestry influences the physical qualities of hair-heavy, fine, straight, curly, colour. For example, Asian ancestry is associated with black, large-diameter, straight hair; African ancestry is associated with black, tightly curled hair; Scandinavian ancestry is associated with red or blond hair of fine texture. The number of genes and the specific genes involved in determination of hair qualities is not known with certainty.

Hair is not "alive" like other body tissues. Hair is a non-living fibre made up of biological components. The components are assembled into a hair in the hair follicle. The major components of a hair shaft are the cortex and the cuticle.

The cortex constitutes the bulk of the hair fibre. It is made up largely of keratins, a family of proteins that also provide the tough outer sheath of skin

cells. The cuticle is the armour of the hair shaft. Made up of thin scales of dense keratin, it protects the cortex from physical and chemical damage. Seen under a microscope, the layered scales of cuticle resemble the armoured scales of a reptile. When the cuticle is damaged by chemicals or physical trauma, the cortex is exposed and open to damage. A typical appearance of such damage is broken hair and "split ends".

The colour of hair is determined by follicular melanocytes (cells containing the colouring pigment melanin). The melanocytes are located in the matrix area of the follicle.
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