Hairs are made by modified skin cells, and cover the whole of the skin with the exception of the palms of the hands, the soles of the feet and certain parts of the genitalia.
Normal skin has a thickened protective outer layer containing a substance called keratin. The modified skin cells that produce hair, together called hair follicles, make keratin-containing cells that grow as a long shaft. The structure, length and shape of this varies depending on the part of the body; on the back of the eyelid, for example, there are hair follicles, but the shafts do not reach the surface of the skin. The hair in the pubic region is much coarser than that covering the rest of the body – and often curlier. There is also variability throughout the course of life: the foetus at about 5-months gestation is covered by a fine hair called lunago (and it’s actually hairier than a gorilla at this stage of development). This is shed shortly after birth, often together with most of the scalp hair, which re-grows from the age of 3-6 months.
At puberty, with the growth spurt, there is increased hair growth as testosterone-sensitive hair follicles become activated, producing body hair. The colour of hair is determined by cells called melanocytes. As we age these become less active and the hair colour fades. This, together with the incorporation of minute air bubbles in the shafts of the hairs, causes us to become grey or white haired, and more of it falls out as we get older. The style of our scalp hair – straight or curly – is determined by genetic factors that affect the shape of the hair and the way it grows. Straight hair is oval or round in cross section and takes a fairly straight route from bulb (see below) to surface, while curly hair is flatter in cross-section and takes a more curved route.
Hair Follicles
In general terms, a hair follicle consists of a group of cells at the bottom – the hair bulb, from which the hair grows – surrounding a structure called the papilla, in which there are blood capillaries that supply nutrients. There are also nerve endings (which is why it hurts when our hair is pulled). From the bulb to the surface is the shaft of the follicle. Opening into this are one or more sebaceous glands, which supply lubrication to the growing hair. Some hair shafts have a muscular attachment, called the arrecta pili. Under conditions of cold or emotional stress these shorten, causing the hair to ‘stand on end’. It’s this that causes the appearance of ‘goose pimples’ when we’re chilly or frightened (in some animals, such as the cat, this can have the effect of making it look twice as big as it is).
From the middle outwards, hairs consist of three layers, called the medulla, the cortex and the cuticle.
- The medulla may be absent from the fine hair covering the body (and also from some scalp hair).
- The cortex constitutes the main part of the hair – and contains the pigment in coloured hair or air bubbles in white hair.
- The cuticle is a delicate single layer of cells around the cortex.
The Hair Cycle
Normal hair follicles have a lifecycle consisting of three phases: a growing phase called anagen, a resting phase called telogen, and a regressing phase called catagen, where the hair bulb shrinks and the hair falls out. Unlike many other mammals, the human scalp has hairs that are in different phases of the cycle – most growing in anagen, about 15% in telogen, and 1% in catagen (in other species the hairs cycle in synchrony and fall out together, producing the ‘moult’). The whole cycle for scalp hair takes about 3 years (for others, such as eye lashes, it’s much shorter).
One consequence of our continuous variation in hair cycles is that we lose hairs every day -between 50 and 300. There are various conditions that can affect the hair cycle – leading to increased hair loss – described below.
Healthy Hair
The outer layer of hair – the cuticle – consists of overlapping cells which contain a lot of keratin. This will be covered by a thin layer of sebum produced by sebaceous glands in the hair follicle. This reflects light, and causes healthy hair to shine. It also contributes to the trapping of air between the shafts of hair – giving what hairdressers call ‘body’ or ‘bounce’.
Not all of us, of course, make the same sort of hair or even the same sort of sebum, so some have dryer hair than others. Lifestyle also affects the amount of scalp sweat, which affects how frequently we need to wash our hair. The use of harsh shampoos will cause the hair to dry at the ends and become greasy at the bottom, because the follicles will make more sebum to compensate. Those who start with dry hair may need to add oils in the form of conditioner. Over-drying, excessive crimping or perming or powerful colouring agents can damage the cuticle, making the hair appear lifeless. You may need to wait until the dye or the perm has grown out before it regains its former vigour – which may take longer than you think.
Disorders Affecting Hair
Hair Loss
When abnormal, is called alopecia. In males, this may present as genetically determined familial baldness – termed androgenetic alopecia, because it’s linked to the effects of the male androgenic hormone testosterone. This causes so-called typical male-pattern baldness: thinning and regression of the hair at the temples and at the crown of the head. The rate at which this occurs is dependent on the family history; that is on the genes we’ve inherited. It may be hastened by other factors affecting hair loss (see below). The underlying mechanism is thought to involve changes in the hair cycle, with more hairs entering telogen and catagen, and a shortened growth phase. Eventually the follicles shrink (a process called miniaturisation).
There are a number of possible therapies involving topical applications of chemicals that increase blood flow and reverse the process of miniaturisation, or agents that oppose the effect of testosterone. None has universal success.
In females androgenetic alopecia causes diffuse thinning of hair over the whole of the scalp rather than the temples and crown. This is more common after the menopause, and is also thought to be genetically determined though possibly in a different way to the male. In younger women, it may be a sign of an underlying medical disorder or hormonal imbalance, and may be amenable to treatment (see hair loss and related disorders below).
Telogen Effluvium
Means your hair falls out because an excessive proportion of hairs have entered the telogen phase followed by the catagen phase all at once. This is the explanation for the phenomenon of severe hair loss following extreme emotional stress. It commonly occurs some 3 months after childbirth, and may follow any severe illness producing a high fever, surgery, liver or kidney failure, or exposure to toxic chemicals. Usually hair growth recovers once the precipitating event is past, though it may take 3-6 months.
Hair Loss and Related Disorders
Iron deficiency is commonly associated with increased diffuse hair loss, and may be present (as reduced iron stores) even in those who are not anaemic. Iron supplements help once the reason for the deficiency is known.
Hypothyroidism where the thyroid gland is under active – causes diffuse hair loss, thought to be due to under-stimulation of hair follicles. Treatment can reverse the hair loss provided it was started soon enough – if delayed it may be ineffective.
Renal failure where the kidneys fail to function sufficiently to allow excretion of waste products, causing elevation of such chemicals as urea, can be associated with diffuse hair loss which is usually irreversible.
Nutritional deficiency of protein, vitamins, amino acids and zinc all cause diffuse hair loss which is usually reversed by correction of the deficiency.
Drugs are widely associated with hair loss. Besides the cytotoxic agents used in the treatment of cancers – where the hair is lost because the agents affect the hair cells as well as the cancer cells – there are a number of other medicines in more general use, such as beta blockers used in the treatment of high blood pressure or angina. Stopping the drug might stop the hair loss.
Alopecia areata
Is the name given to the disorder resulting in patchy hair loss. There is the development of irregular areas of complete baldness anywhere on the scalp (or elsewhere on the body) varying in size from a 50-pence piece to totality (called alopecia universalis). Its course is variable. It is thought to be an auto-immune disease where for some reason our body’s defence mechanism (antibodies) attack hair follicles in the state of anagen. There may be other associated auto-immune disorders (thyroid disease, for example). Non-pigmented hairs are relatively spared – which may explain why some people appear to ‘turn white overnight’: all the coloured hairs have fallen out.
The outcome is unpredictable. The prognosis is much worse if it occurs before puberty (approximately 50% will be completely permanently bald). Because it is an auto-immune disease, treatments (such as oral steroids) that suppress the immune response may be of benefit. What triggers the auto-immune attack is unknown, though stressful events may play a part.
Hirsutes
Is too much, rather than too little, hair – usually in the wrong place. This is a disorder affecting women – since men mostly don’t mind moustaches and beards. It is described as the growth of coarse – or so called ‘terminal’ hair, in a pattern usually found in males – on the face, chest and abdomen.
Some cases of hirsutism are caused by an underlying hormonal defect where the woman is producing high levels of the androgenic hormones testosterone or aldosterone – either from the ovaries or the adrenal glands. Often it occurs in association with polycystic ovary syndrome, involving irregular periods and infertility. There may be associated obesity and acne, together with a male escucheon – where the pubic hair grows up towards the umbilicus rather than ending in a straight line above the pubis – called the Stein-Leventhal syndrome. It may also be a feature of insulin resistance, or metabolic syndrome X – described in the WIS Family Health Encyclopaedia. There are other, rarer causes, and the condition is worsened by medications such as steroids or some components of the oral contraceptive, which can have androgenic effects in some women.
The degree of distress – and therefore the need for treatment – depends both on the degree of hirsutes and also on social factors as well as hair colour, since dark facial hair is much more pronounced and noticeable than the fairer variety.
There are many different treatments – shaving (which doesn’t encourage greater growth), waxing, or the use of hair-dissolving creams. Electrolysis or lasers may effect a longer cure but electrolysis is painful, and not all hair colours respond equally well to laser therapy.
Dandruff
Is a condition characterised by excess shedding of scales from the skin of the scalp. This is very common and is usually caused by the presence of a small round microbe of the yeast family called Pityrosporum Ovale – giving rise to the medical term for dandruff – Pityriasis Capitis. In severe cases this can produce an inflammatory, eczematous reaction in the scalp and cause increased hair loss.
There are many effective treatments involving shampoos containing agents such as selenium or anti-fungal drugs. You do, of course, still have to use the shampoos.
Head Lice
Can affect anybody at any age except the completely bald. They are spread from one person to another when the heads are in contact – long enough for them to walk across – so more than a minute or two. Having settled in, the female then lays about eight eggs per night, each attached to the shaft of a hair. When hatched the empty shells – called nits – have a bright white appearance. Each louse feeds about six times a day by piercing the scalp and sucking blood. Sometimes – but not always – this causes itching. Many people with lice do not know they have them – so they go around giving them to people who don’t know they’re getting them.
It is said that they prefer clean hair to dirty hair – but there is no evidence for this. Besides, it’s not the hair they are after.
Treatment involves both application of either lotion or shampoo, and vigorous combing with a fine-toothed comb. Then it’s necessary to find where you got them from before you get them again (pubic lice, affecting the pubic hair, are also transmitted during contact).
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