How Does Hair Grow, And What Causes Age-Related Hair Loss?

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One concern when it comes to growing older is hair loss, especially if you are male, though up to two thirds of women also experience hair loss after the menopause. Why does this happen, and why does hair loss occur in some places before others? Is the hair of our eyebrows short because it has stopped growing, or is it possible to grow beard eyebrows? These are some of the pressing questions that this article will attempt to answer.

What Is Hair Made Of?

Hair is 95% keratin – a strong, fibrous, helix-shaped protein that is also abundant in the outer layer of the skin and in nails. However, hair is more complex than just a bundle of proteins, and comes with three distinct layers:

• The medulla is the innermost layer and is made of a soft, oily substance.
• The cortex is the middle layer and forms the bulk of the hair shaft. It contains long keratin chains that grant the hair its strength and elasticity.
• The cuticle is a highly keratinised outer layer. It has a ‘scaly’ appearance and serves to protect the hair shaft.

In addition to keratin, the cortex also contains most of the hair’s melanin – the pigment that determines hair colour. Two types of melanin are responsible for hair pigmentation: eumelanin, which makes the hair more black, and pheomelanin, which makes it more red.

How Does Hair Grow?

Hairs grow within hair follicles, organs that reside in the middle layer of the skin – the dermis. The very bottom of the hair shaft, called the hair bulb, contains dividing keratinocytes that will later die to form the bulk of the hair, as well as melanin-producing cells called melanocytes. These cells are supplied with nutrients and oxygen via blood vessels.

While it may seem like some hair stops growing after reaching a certain length, this is actually because hairs are replaced after a certain growth period. This growth cycle consists of three main phases.

In the anagen phase, cells at the base of the hair shaft divide rapidly and add to its length. The growth rate during the anagen phase varies for hair on different parts of the body – on the scalp, hair generally grows about 1cm per month. The duration of the anagen phase also varies in different locations and between individuals, but it is always the longest of the three phases. On the scalp, the anagen phase usually lasts 2-8 years, and around 90% of scalp follicles are in this phase at any one time.

The catagen phases is a transition phase and is the shortest of the three phases. The hair follicle shrinks and the hair separates from the bottom of the follicle, cutting it off from its blood supply and causing it to stop growing.

During the telogen phase, the hair follicle is in a resting state. The hair shaft is no longer growing but remains anchored in place. Towards the end of the telogen phase, the hair follicle will begin to grow again, loosening the anchor point and allowing the hair to be released from the follicle and shed. This is sometimes considered to be a separate phase called the exogen phase. It is normal to lose 50 to 100 hairs a day in this way.

Why Is Some Hair Longer Than Other Hair?

The length of hair depends on how quickly it grows, and the length of time it grows before it falls out. The timing of the growth phases varies according to genetics and also in response to environmental factors such as stress, which can cause hair follicles to enter the telogen phase early. Generally, however, a full growth cycle takes 2-8 years for scalp hair, and around a year to a year and a half for most other types of hair. This means that, while it may not seem like it, your eyebrows are not the same as the ones you had two years ago, as each hair has already fallen out and been replaced since then.

This is why eyebrows do not grow beyond a certain length, just as scalp hair would also eventually reach a maximum length if you never cut it. This means that, unfortunately, you cannot grow your eyebrows into beards (though they may get longer with age – more on that later). On the plus side, this also means that eyebrows will grow back if you go overboard with a razor, though plucking is a different matter, as that may damage the hair follicle itself. Indeed, since hair growth occurs at the base of the hair shaft, there’s not much you can do to the hair itself to affect its growth. Shaving, for example, will not make hair grow faster, since the cells at the base of the hair shaft have no way of ‘knowing’ that it’s been cut.

What Causes Hair Loss During Ageing?

Age-related hair loss usually appears as a gradual thinning. In men this usually begins on the forehead, causing the hairline to recede, while in women it is more common for hair loss to be diffuse, and is usually first noticed as a broadening of the central parting of the hair. Some people can also lose hair in circular or patchy bald spots.

Age-related hair loss happens because the anagen phase, the phase in which the hair is growing, becomes shorter, while the telogen phase lengthens. This means that fewer hairs are growing at any given time. Hair follicles also shrink, resulting in thinner hair, and eventually die, preventing new hair from being grown at all.

We still don’t know exactly what causes these changes, but hormones called androgens (male sex hormones) seem to play a central role in both sexes. 95% of hair loss in males is caused by androgenic alopecia, the result of an androgen called dihydrotestosterone, or DHT. DHT is produced from testosterone by an enzyme called 5-alpha reductase, and plays similar roles to testosterone within the body. However, DHT is the more potent of the two. DHT actually stimulates the growth of body hair, but for unknown reasons, has the opposite effect in hair follicles of the scalp, causing them to atrophy. The sensitivity of hair follicles to DHT is genetic, and tends to increase with age, making body hair grow longer while scalp hair is reduced.

Although produced in much smaller quantities than in males, females still need some testosterone and DHT. However, the relationship between androgens and female hair loss is less straightforward, and may have more to do with a decline in female sex hormones following the menopause. For example, it has been suggested that hair loss in females may be partly caused by a loss of aromatase, an enzyme produced by hair follicles and that can convert androgens to oestrogens, which lengthen the growing phase. This is also why women sometimes lose hair at a higher rate following pregnancy, as oestrogen elongates the anagen phase during the pregnancy period, resulting in a large number of hairs entering the telogen phase at once when oestrogen returns to normal.