Hair structure and the causes of baldness: from the cuticle to the follicle

What determines how your hair looks

The colour, texture and shape of hair are largely set by genetics, which is why they vary so much from person to person. Straight, wavy or curly hair, fine or coarse strands, dark or fair colour: all of this is written into the way each follicle is formed and works.

Genes are not the only influence, though. Hormonal balance, age and the environment also shape hair over a lifetime. Hair often becomes finer and lighter with age, hormonal changes can alter its thickness and growth, and sun, heat styling or harsh chemical treatments can change how the surface of the hair looks and feels.

The three layers of a hair shaft

The visible part of a hair, the shaft, is made mostly of a protein called keratin. It is not living tissue, which is why cutting hair does not hurt. Inside, the shaft is built from up to three layers.

The outer layer: the cuticle

The cuticle is the outermost layer. It is made of flat, overlapping cells arranged a little like roof tiles or brickwork. These scales protect the inner layers from damage and help the hair hold on to moisture. When the cuticle is smooth, hair reflects light and looks glossy; when it is roughened by heat, bleaching or friction, hair looks dull and breaks more easily.

The middle layer: the cortex

The cortex lies beneath the cuticle and makes up most of the hair. It contains the natural pigment melanin, which gives hair its colour, and strong keratin fibres that give it strength, elasticity and shape. Most of the changes brought about by colouring, perming or straightening happen in the cortex.

The inner layer: the medulla

The medulla is a soft, porous core at the centre of some hairs. Not every hair has one: fine hairs often lack it, while thicker hairs usually do. Its exact role is still not fully understood.

The follicle: where hair is made

Every hair grows from a follicle, a small pocket in the skin of the scalp. At its base lies a group of living cells known as the matrix. These cells divide, move upwards through the follicle and gradually harden into the hair shaft. The follicle also contains specialised structures that supply the growing hair with nutrients through tiny blood vessels, and it is connected to a sebaceous gland that keeps the hair and skin supple.

Follicles do not work non-stop. Each one goes through a repeating cycle:

• growth phase (anagen): the hair grows actively; on the scalp this phase usually lasts several years;
• transition phase (catagen): growth stops and the follicle shrinks slightly over a few weeks;
• resting phase (telogen): the hair rests for a few months and is then shed, after which a new hair usually starts to grow from the same follicle.

Because follicles cycle independently, losing a certain number of hairs every day is normal. Problems start when the cycle is disrupted or when follicles gradually stop producing full, strong hair.

Why hair thins with age

Baldness, or alopecia, is the loss of hair from the scalp or other parts of the body. The most common form by far is androgenetic alopecia, often called hereditary or pattern hair loss. It is linked to a genetic predisposition and to the way certain follicles respond to male hormones (androgens).

In susceptible follicles the growth phase becomes shorter with each cycle, and the follicle itself gradually shrinks. Hairs become finer, shorter and lighter until some follicles produce only barely visible hair. This process is slow and usually takes years.

In men, thinning typically starts at the temples and on the crown and spreads from there. In women, androgenetic hair loss more often appears as general thinning over the top of the head and a loss of volume, while the front hairline tends to be preserved. In both sexes the hair at the back and sides of the head is usually less affected, which is why this area serves as the donor area in a transplant.

Other causes of hair loss

Several other factors can cause or add to hair loss:

• autoimmune conditions, such as alopecia areata, in which the immune system affects hair follicles and round patches of hair loss appear;
• hormonal changes, for example after childbirth, around menopause or with thyroid disorders;
• a lack of certain nutrients, such as iron or protein, often linked to restrictive diets;
• physical or emotional stress, including a serious illness, high fever or surgery, which can push many hairs into the resting phase at once;
• certain medicines, for which hair loss is a known side effect.

In some of these cases hair loss is temporary and hair grows back once the cause is addressed. In others, particularly when follicles are permanently damaged or scarred, the loss is lasting.

Hair loss is a complex process, and its mechanisms are still not fully understood. The level and speed of thinning differ from person to person. If hair loss worries you, the best first step is to see a doctor or dermatologist, who can examine the scalp, order tests if needed and recommend treatment for your specific situation.

What this means if you are considering a hair transplant

A hair transplant works because of the way follicles behave. In androgenetic alopecia, follicles at the back and sides of the head are usually resistant to the process that shrinks hair elsewhere. When these follicles are moved to thinning areas, they generally keep that resistance and continue to grow hair in their new place.

For androgenetic alopecia, surgeons most often use the FUE method, in which follicles are removed one by one from the donor area. A combined FUE and DHI approach may be used both for androgenetic alopecia and to add density to areas that have thinned. The choice depends on the donor area, the extent of hair loss and the surgeon’s assessment.

It is important to be realistic: a transplant moves existing hair, it does not create new follicles or treat the underlying cause of hair loss. Native hair may continue to thin over time, so a doctor may also discuss ways to support it. A transplant is not suitable for total alopecia, when there is no donor hair to move, and hair loss caused by illness, medication or nutrient deficiency should first be investigated and treated. You can read more about the procedure on the hair transplant page.

Consultation at City Medical Centre

City Medical Centre works with international patients. The first consultation is free and can be held online: send photos of your hair, the thinning areas and the back of your head, and ask any question you have. The doctor assesses your case, explains whether a transplant is a sensible option and names the cost, which is then fixed in the contract. A personal coordinator stays with you from your first message to the end of recovery, and photos and questions can be sent by WhatsApp. Find out more on the hair transplant page.

Frequently asked questions

What are the layers of a hair?

A hair shaft has up to three layers. The cuticle is the protective outer layer of overlapping cells, the cortex in the middle holds the pigment and the keratin fibres that give strength, and the medulla is a soft core that only some, usually thicker, hairs have.

Where does hair actually grow from?

Hair grows from a follicle, a small pocket in the skin. Living cells at its base divide and move upwards, gradually hardening into the hair shaft. The follicle receives nutrients through tiny blood vessels and goes through repeating cycles of growth, transition and rest.

Why do men usually lose hair at the temples and crown?

In androgenetic alopecia, follicles in these areas are genetically sensitive to male hormones. Their growth phase shortens and they shrink over time, producing finer hair. Follicles at the back and sides of the head are usually less sensitive, so hair there tends to remain.

Is hair loss always permanent?

No. Hair loss caused by stress, illness, hormonal changes, nutrient deficiencies or some medicines is often temporary, and hair may grow back once the cause is addressed. Pattern hair loss and scarring conditions are usually lasting. A doctor can help identify which type you have.

Can a hair transplant stop baldness?

A transplant moves resistant follicles to thinning areas, where they usually keep growing, but it does not treat the cause of hair loss. Native hair may continue to thin, so the doctor may suggest ways to support it. A transplant is not suitable when there is no donor hair.