How hair actually grows, and why it falls out.

Quick answer: Each follicle cycles through growth, transition and rest, and most of your hair is growing at any given moment. In male pattern loss, a hormone called DHT gradually shrinks susceptible follicles until the hair they produce is too fine to see. Follicles at the back and sides are genetically resistant to it. That resistance travels with the follicle when it is moved, which is the entire basis of hair transplantation, and the reason your donor supply is finite.

You do not need to understand any of this to have a hair transplant. But it explains why surgery works at all, why there is a hard limit on what it can do, and why the thing people worry about most, shedding, is usually nothing to worry about.

What a follicle actually is

The hair you can see is dead. It is a shaft of keratin, which is why cutting it does not hurt and why nothing you put on it can change how it grows.

The living part is underneath: a small organ set into the skin called the follicle. At the base sits the bulb, and inside that the matrix, where cells divide rapidly and push upward, hardening as they go. Feeding it is the dermal papilla, a tiny cluster of tissue carrying the blood supply and, importantly, the instructions.

Two other things attach to it. The sebaceous gland produces the oil that keeps hair and skin from drying out, and the arrector pili, a small muscle, is what makes hair stand up when you are cold.

Hair does not grow one at a time

This surprises people, and it matters for surgery.

Hairs emerge in natural clusters called follicular units, usually of one to four hairs sharing the same opening. When a surgeon talks about grafts, they mean these units, not individual hairs.

So a quote for 3,000 grafts is not 3,000 hairs. Depending on how many hairs each unit carries, it is often somewhere in the region of two hairs per graft on average, though this varies from person to person. It is worth knowing simply so you can compare quotes properly, because clinics quote in grafts and patients think in hairs.

The growth cycle

Every follicle runs its own cycle, independently of its neighbours. That independence is why you do not moult.

Losing somewhere around a hundred hairs a day is normal and is simply the telogen phase doing its job. Finding hairs on your pillow is not, by itself, evidence of anything.

What male pattern loss actually does

Androgenetic alopecia, to give it its proper name, is not really about hair falling out. It is about hair getting smaller.

Testosterone is converted by an enzyme into a more potent androgen called DHT. In follicles that are genetically susceptible to it, DHT binds to receptors and progressively shortens the growing phase.

The consequence is miniaturisation. Each cycle, the follicle produces a slightly finer, shorter, lighter hair than it did before. Terminal hair becomes wispy, then becomes the colourless down you can barely see, then stops. The follicle does not vanish overnight, it fades over years.

Why the back of your head is different

Here is the part that makes transplantation possible.

The follicles at the back and sides of the scalp are, in most men, genetically insensitive to DHT. That is why a man can be almost entirely bald on top and still have a full band of hair around the back. Those follicles are not protected by their location. They are protected by their own genetic makeup.

And that makeup travels with them. Move one of those follicles to the top of the head and it keeps behaving as it did at the back: it continues to grow, and it continues to resist DHT. The principle is known as donor dominance, and it is the single reason hair transplantation works.

Not everything is male pattern loss

It is worth knowing that several other things cause hair loss, some of them temporary, and a transplant is the wrong answer to most of them.

• Telogen effluvium. A shock to the system, illness, surgery, severe stress, sudden weight loss, pushes an unusually large number of follicles into the resting phase at once. It shows up months later as diffuse shedding and it usually recovers.
• Alopecia areata. An autoimmune condition in which the body attacks its own follicles, typically causing distinct round patches rather than a receding pattern.
• Traction alopecia. Caused by prolonged tension on the hair from tight styles. Reversible early, permanent if it goes on long enough.
• Nutritional and medical causes. Thyroid disorders, iron deficiency and some medications all affect hair.

Male pattern loss follows a recognisable pattern: temples and crown, gradually, over years, usually with family history behind it. Diffuse thinning all over, sudden onset, or defined patches are worth a conversation with a doctor before a conversation with a clinic.

What this means practically

• Shedding after a transplant is the cycle, not a failure. Transplanted follicles are shocked into telogen and drop their hairs at around week three. The follicle stays. The hair returns.
• Twelve months is not clinic caution. It is roughly how long anagen takes to produce visible length after the follicle restarts.
• A transplant does not treat the cause. It moves resistant follicles. The susceptible ones around them carry on miniaturising, which is why surgeons plan for future loss rather than today's photograph.
• Donor supply is the ceiling. Not money, not technique, not the clinic. How much resistant hair you have is what decides what is possible, and it is why an honest assessment matters more than a good price.

If you want the practical version of all this, what a successful result actually means covers what to expect and when.

Frequently asked questions

This is general educational information, not medical advice. If you are concerned about hair loss, particularly if it is sudden, patchy or diffuse, see a doctor.