Hair

Hair grows in follicles which are in the dermis, a layer of skin beneath the epidermis. The dermis is an inner layer of skin beneath the epidermis, composed of connective tissue, blood and lymph vessels, sweat glands, sebaceous glands and hair follicles, and an elaborate sensory nerve network    Function  
Hair is an important accessory structure of the skin and one of its main functions is physical protection: eyebrows and eyelashes protect the eyes, hairs lining the ears and nose collect dust and bacteria. 
Body hair also helps with temperature control. The erector pili muscles attached to each hair pull the air up right and trap a layer of air – goose pimples to keep the body warm. 
Hair also facilitates the evaporation of perspiration and also act as sense organs

  
Structure of Hair

Hairs are formed by a down- growth of epidermal cells called hair follicles.  
At the base of the follicle is a cluster of cells called the bulb which contains the growing part – the matrix. 
The part of the hair above the skin is called the shaft and the rest is the root.  
Hair is nourished via a rich blood supply provided by the dermal papilla which surround the bulb.  
Around the hair follicle are sebaceous glands secreting the oily, waterproofing substance sebum. 
Also found here is the arrector pili muscle which when it contracts in response to cold or fright pulls the hair up straight trapping air and causing goose pimples 
 

There are 3 main layers to hair:

1. outer layer - the cuticle 
- is made of transparent, overlapping scales which help to protect the other layers as well as providing elasticity 
2. middle layer - the cortex  
- consists of keratinised cells containing the pigment melanin. This layer gives the hair its strength as well as the colour variations between individuals. 
3. inner layer - the medulla 
– is also made of keratinised cells but they are more loosely arranged than in the cortex and contain air spaces. This layer helps reflect the light and gives hair its sheen 
Keratin is the waxy protein that develops in the end stages of hair, nails and epidermal skin growth  
 
The layers of the follicle consist of the following  
The Inner Route sheath  
It is an important structure of the lower part of the hair follicle that surrounds and protects the growing hair. It lies between the hair in the center and the outer root sheath.  The inner root sheath is made up of three distinct layers: the cuticle which interlocks with the cuticle of the hair, Huxley's layer which is two or more cells thick and Henle’s layer which is one cell thick. The Outer Root Sheath  
Forms the follicle wall and encloses the inner root sheath and hair shaft. It is a continuation of the growing payer of the epidermis of the skin. 
 
Vitreous Membrane  
This separates the connective tissue from the outer root sheath  

 
Types of hair

There are 3 main types of hair: 
1. Terminal hair 
is thick and coarse and protects the delicate areas of the body such as the head, eyes, underarms and genital areas. 
2. Finer vellus hair  
  covers the rest of the body providing a downy protective layer. 
3. Lanugo hair  
which is very fine and soft and unpigmented is present in a foetus and usually shed just before birth but can often be present in new borns especially premature babies. (Interestingly, lanugo hair often replaces normal body hair in anorexics and is considered a common diagnostic sign of the eating disorder)

Stages of hair growth

Hair growth begins in the foetus around 3 months into the pregnancy. 
Unlike the skin, the hair does not grow continuously but goes through a growth cycle of 3 different stages, a growing stage, a changing stage and a resting stage. 
1. growing stage – or anagen phase – is due to cell division ( mitosis ) at the bulb: old cells are pushed upwards to form the hair.As they push upwards the cells are filled with the protein keratin and this then forms the hair shaft . This stage can last months to several years.It is during this stage that the pigment melanin develops.

2. changing stage – or catagen phase – is when the shaft is fully grown and the follicle begins to shrink and causes the hair bulb to detach itself from the dermis.This stage lasts around 2-4 weeks.

3. The resting stage – the telogen phase – is when the hair shaft eventually falls out of the hair follicle as part of a natural shedding process. About 25 –100 hairs a day can be shed in this phase. The telegen stage lasts about three months  
The whole cycle will then begin again. Each hair has its own growth cycle and different types of hair have different growth patterns all of which is dependent on genetics and hormone levels so each one of your hairs will be at the various different growing phases. 
 
  

Factors that affect hair growth cycle

A range of factors can affect hair growth including 

• Hormonal. As the state of the endocrine system changes as an individual grows, and varies depending on stress, menstrual cycle etc the hair growth cycle is also affected. Changes in androgen (Male hormone ) levels can affect the rate of hair growth and the thickness of the hairs. Female hormone levels (oestrogen) slow growth and extend the growing phase of the hair cycle. Hair growth can be affected in several ways by the balance of androgen and oestrogen in the blood e.g. at puberty the rise in androgen levels is responsible for pubic and underarm hair and high levels of oestrogen during pregnancy can affect hair growth in women. 

Certain female hormonal (oestrogen) imbalances can affect hair distribution such as in PCOS (Polycystic Ovary Syndrome)       where higher than normal levels of male hormones increase the growth of facial and body hair

• Nutrition. Specific dietary deficiencies can lead to hair loss eg not enough protein, iron, vitamins, omega -3 fatty acids.  
• Medication   Side effects of some drugs can contribute to excess hair growth, changes in hair colour or texture  or even hair loss
• As well as smoking, pollution, stress.   

Pathologies of hair 

Alopecia aureate 
The commonest hair disorder is alopecia aureata which is partial or complete hair loss (which can also include body hair as well as eyebrows and eyelashes).This disease is known as an auto immune disease which means for some reason the body is attacking its own cells. 
Hair loss may also occur due to severe “traction “on the hair such as prolonged wearing of very tight ponytails or similar hairstyles where the hair is pulled into shape. 
 

Androgenic Alopecia 
This is a common form of hair loss in both men and women. In men, this condition is also known as male-pattern baldness the most common type of hair loss in men. Male pattern baldness usually includes either a receding hairline, hair loss at the crown, or both. In women, hair loss usually includes uniform thinning across the scalp, with a preserved hairline. The crown may be affected, but hair loss rarely proceeds to baldness as in men.  
 

Hirsutism
A condition in which women have excessive hair growth. The hair is normally thick and dark and grows on the face – for example upper lip and chin / Chest / Lower back and buttocks Hirsutism is caused by an excess of male sex hormones called androgens. It may affect one to three women in every 20 who have not yet started the menopause  
Ingrown hairs 
These are hairs that have curled around and grown back into your skin instead of rising up from it and may result in inflammation. Sometimes dead skin can clog up a hair follicle. That forces the hair inside it to grow sideways under the skin, rather than upward and outward. Ingrown hairs tend to be more common in areas with coarse hairs, like the bikini area in women and the beard and neck in men and where the hair is growing in a pit or groove, for example in a natural skin fold or in scarred skin. 
Pediculosis Capitis  
Highly contagious infestation of human head lice. Tiny insects that live on the scalp and feed on blood.. The female louse attaches her eggs (nits) to the base of the hair near the scalp, and the nits hatch 7–10 days later. While the adult louse cannot survive for more than 2 days off the human head, a nit can stay alive for up to 10 days off the body (for example, on clothes, hairbrushes, or carpets). Lice can be spread  by close contact and by sharing belongings that are infested with lice eg combs, hats, clothing and linen.  

Sycosis barbae 
An inflammation of hair follicles of skin that has been shaved. 

Folliculitis  
This is a bacterial infection of the skin – mainly affecting men who shave and commonly known as “barber’s itch “but which can also cause patchy hair loss. 

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