How hair actually grows, and why it falls out.

Quick answer: Each hair follicle goes through a cycle of growing, changing and resting, and most of your hair is growing at any one time. In male pattern hair loss, a hormone called DHT slowly shrinks certain follicles until the hair is too fine to see. The follicles at the back and sides of your head aren't affected by DHT, and they stay that way when they're moved. That's why hair transplants work, and also why you only have so much donor hair.

You don't need to understand any of this to have a hair transplant. But I think it really helps to know why surgery works, why there's a limit to what it can do, and why shedding (the thing people worry about most) usually isn't anything to worry about.

What a follicle actually is

The hair you can see is actually dead. It's made of keratin, which is why cutting it doesn't 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 a lot of people, and it matters when it comes to 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 isn't 3,000 hairs. It's often around two hairs per graft on average, although it varies from person to person. It's worth knowing so you can compare quotes properly, because clinics talk in grafts but most of us think in hairs.

The growth cycle

Every follicle runs its own cycle, separate from the ones around it. That's why you don't lose all your hair at once.

Losing around a hundred hairs a day is completely normal. Finding hairs on your pillow doesn't mean anything on its own.

What male pattern loss actually does

Male pattern hair loss (androgenetic alopecia) isn't really about hair falling out. It's more 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.

This is called miniaturisation. With each cycle, the follicle grows a slightly finer, shorter and lighter hair than before. Normal hair becomes wispy, then turns into fine fluff you can barely see, and eventually stops growing. It doesn't happen overnight, it happens slowly over years.

Why the back of your head is different

This is the part that makes hair transplants possible.

For most men, the follicles at the back and sides of the head aren't affected by DHT, because of their genetics. That's why someone can be almost completely bald on top but still have a full band of hair around the back.

And that stays with them when they're moved. If you move one of those follicles to the top of your head, it carries on growing just like it did at the back. This is called donor dominance, and it's the reason hair transplants work.

Not everything is male pattern loss

It's worth knowing that other things can cause hair loss too. Some are only temporary, and a transplant isn't the right answer for 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 hair loss usually affects the temples and crown, happens slowly over years, and often runs in the family. If your hair is thinning all over, falling out suddenly, or coming out in patches, please speak to a doctor before you speak to a clinic.

What this means practically

• Shedding after a transplant is normal. The transplanted follicles go into a resting phase and drop their hairs around week three. The follicle stays put and the hair grows back.
• Twelve months really is how long it takes. That's roughly how long the new hair needs to grow to a length you can see.
• A transplant doesn't stop hair loss. It moves the strong follicles, but the ones around them can keep thinning. That's why a good surgeon plans for the future, not just how you look today.
• Your donor hair is the limit. How much good hair you have at the back decides what's possible, which is why an honest assessment matters more than a good price.

If you'd like the practical side 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.