Hair follicles — FU1, FU2, FU3, FU4

1. What is a hair follicle?

A hair follicle is the anatomical unit in which a hair grows — comparable to a tiny biological organ. It extends about 3–5 mm deep into the scalp and contains the hair papilla (the growth center), the hair matrix (where new cells are formed), as well as sebaceous glands and an arrector pili muscle.

Humans have an average of 100,000 to 150,000 hair follicles on their scalp. This number is fixed from birth — no new follicles are formed during a person's life. What can change is their size, their activity, and whether they miniaturize (shrink) and eventually stop producing hair.

Crucial for hair transplantation: Follicles do not grow individually, but in natural groups — the follicular units.

Anatomy of a hair follicle — in detail

The hair follicle is anatomically one of the most complex mini-organs of the body. It consists of several functional structures that together produce, nourish, and control hair.

The Hair Papilla

Sits at the base of the follicle (3–5 mm deep) and contains the blood vessels that supply the hair with nutrients. If the hair papilla is damaged, the follicle can no longer produce new hair — no matter how many nutrients are supplied.

The Hair Matrix

The area directly above the papilla where cell division takes place. Here, new hair cells are produced, which extend the hair from bottom to top. An intact matrix is a prerequisite for growth.

The Bulge Region

At the level of the sebaceous gland — contains the stem cells of the follicle. New growth cycles originate from this region. If the bulge is destroyed, the follicle is definitively lost.

The Sebaceous Gland

Produces sebum, which protects the hair and scalp. Hyperactive sebaceous glands can lead to problems such as seborrheic dermatitis or oily scalp.

The Arrector Pili Muscle

Small smooth muscle that can make the hair stand up ("goosebumps"). It plays no direct role in transplantation but is important for follicle vitality.

The Layers of the Hair Shaft

The actual hair consists, from outside to inside, of:

• Cuticle — Outer layer, scale-like
• Cortex — Fibrous layer, gives the hair strength and color (due to melanin)
• Medulla — Inner canal, often absent in fine hair

2. FU1 to FU4 — the Follicular Units

When a doctor speaks of "grafts," they mean follicular units. Each unit produces 1 to 4 hairs simultaneously — and it is precisely this variation that makes a natural appearance possible.

FU1 — Single Follicle

Produces exactly one hair. Accounts for about 15–20% of all follicular units. Used specifically for the hairline because it creates the most natural, "softest" transition — a hairline composed exclusively of FU2 or FU3 would look artificial.

FU2 — Double Follicle

Two hairs from one unit. The most common type — about 40% of all follicular units. Forms the transition zone behind the hairline and ensures a gentle increase in density.

FU3 — Triple Follicle

Three hairs. About 25–30% of units. Used in denser areas further back — where volume is more important than fine transition aesthetics.

FU4 — Quadruple Follicle

Four hairs from one unit. Rare — only about 5–10%. Very valuable for areas where maximum density is required (e.g., crown, swirl area).

In the planning of a transplant, we not only determine the total number of grafts but also their distribution by FU type. It is precisely this distribution that determines whether the result looks natural or "transplanted."

3. The Hair Growth Cycle

Every follicle goes through a cycle of three phases throughout its life. These occur asynchronously — which is why you lose 50 to 100 hairs daily without creating a gap.

Anagen — Growth Phase

2 to 7 years. 85–90% of all scalp hairs are simultaneously in this phase. The hair grows about 1 cm per month. The longer your individual anagen phase, the longer your hair can grow.

Catagen — Transition Phase

2 to 4 weeks. The follicle shrinks, the root detaches from the supply. About 1% of hairs are constantly in this phase.

Telogen — Resting Phase

2 to 4 months. The hair remains in the follicle, eventually falling out. About 10–15% of hairs are permanently resting in this phase. In telogen effluvium, this proportion is significantly increased — which manifests as diffuse hair loss.

After the telogen phase, the cycle restarts: a new hair grows from the same follicle. In hair loss (e.g., androgenetic), the anagen phase shortens progressively, the new hair becomes thinner, shorter, and eventually is no longer produced.

DHT — the biological trigger for hair loss

Hereditary hair loss has a clear biological mechanism. Understanding this mechanism will help you understand why certain therapies work — and why others don't.

The enzyme 5-alpha-reductase

Testosterone is converted into DHT (dihydrotestosterone) in the skin and scalp by the enzyme 5-alpha-reductase. DHT is a significantly more potent androgen than testosterone itself.

Binding to the androgen receptor

DHT binds to the androgen receptor (AR) in the cell membrane of hair follicles. In genetically sensitive follicles, this binding triggers a cascade that has the following effects:

• Shortening of the anagen phase (growth phase)
• Lengthening of the telogen phase (resting phase)
• Gradual shrinking of the follicle (miniaturization)
• Reduction of hair matrix activity
• Reduction of the hair papilla

Why exactly these follicles?

DHT sensitivity is genetically determined and follicle-specific. Follicles on the forehead, temples, crown, and vertex carry many androgen receptors and react strongly to DHT. Follicles on the back of the head between the ears carry hardly any AR and are DHT-resistant — they retain this resistance for life. This property is precisely what makes hair transplantation possible.

Therapeutic consequences

• Finasteride inhibits 5-alpha-reductase and thus reduces DHT formation by up to 70%.
• Minoxidil does not act via DHT, but directly prolongs the anagen phase.
• Transplantation relocates DHT-resistant follicles from the back of the head to the DHT-sensitive areas — the resistance is maintained.

Miniaturization — how follicles slowly die

Hereditary hair loss is not a sudden loss. It is a slow process of shrinking — miniaturization. It progresses in understandable stages over years.

Stage 1 — Terminal Hair

Healthy, full hair — thick, pigmented, with a complete anagen phase of 2–7 years. Growth rate approx. 1 cm per month. Hair diameter 70–100 µm.

Stage 2 — Intermediate Hair

First DHT effect. Anagen phase shortens, hair becomes thinner (50–70 µm), pigment begins to fade. This transition is not yet visible from the outside.

Stage 3 — Vellus-like Hair

Anagen phase shortens to a few weeks, hair reaches only a few millimeters in length before falling out. Diameter under 30 µm. Recognizable trichoscopically — from the outside, the area appears "thinner."

Stage 4 — Vellus Hair (Peach Fuzz)

The hair is just a thin, unpigmented fuzz — like baby hair. It does not contribute to hair density. The follicle is still active but no longer produces visible hair.

Stage 5 — Fibrosis

Endpoint. The follicle is atrophied, the bulge region (with the stem cells) is lost, the hair papilla is fibrotic. From this point, the follicle is definitively dead — no therapy can revive it.

Therapeutic Window

Conservative therapies (Finasteride, Minoxidil, PRP) only work in stages 1 to 3 — as long as the follicle is still alive. From stage 4, only a transplant is an option. Therefore: early diagnosis with trichoscopy is crucial.

4. Donor vs. Recipient Area

A hair transplant only works because not all hairs react equally to the hormone DHT (dihydrotestosterone). This fact forms the biological basis of the procedure.

The Donor Area

Refers to the area at the back of the head between the ears, extending down to the nape of the neck. The follicles here are genetically resistant to DHT — they do not fall out, even if the rest of the head thins. They retain this characteristic even after transplantation to another region.

The capacity of the donor area is limited — typically 4,000 to 8,000 extractable grafts, depending on hair density, hair structure, and skin elasticity. If too many are removed, a visible thinning occurs there.

The Recipient Area

Refers to the region where the follicles are implanted — usually the hairline, temples, crown, or vertex. In the recipient area, microchannels are created beforehand (or in parallel with DHI) in a precise growth direction.

The healing capacity of the scalp and blood circulation are crucial here for the take-up rate — typically 90–95% with correct execution.

5. What this means for a transplant

The anatomy of the follicular units determines the entire operation plan. Three consequences are particularly important:

FU distribution is outcome quality

A good surgeon sorts the extracted follicles by FU type and implants them strategically: FU1 into the front row of the hairline, FU2 into the transition zone, FU3 and FU4 into the denser areas further back. Anyone who simply sells "4,000 grafts" says nothing about whether this distribution is observed.

Donor area is finite

Every transplanted graft is missing from the donor area. If too many are removed early, there will be no reserves left for follow-up treatments later if hair loss continues — which it usually does with androgenetic alopecia.

The take-up rate is measurable

Not all implanted follicles survive. A good take-up rate is 90–95%. We document the number of extracted vs. visibly growing follicles after 12 months — as a quality control for our work.

Frequently Asked Questions

What is a hair follicle?

The anatomical growth unit of a hair — extends 3 to 5 mm deep into the scalp. Contains hair papilla, hair matrix, stem cells, sebaceous gland, and arrector pili muscle. A person has 100,000 to 150,000 follicles on the scalp.

Do hair follicles grow back?

No. The number of follicles is determined at birth. What changes: their size, their activity, and whether they miniaturize and eventually stop producing. Lost follicles cannot be regenerated.

What is the difference between FU1, FU2, FU3, and FU4?

The number indicates how many hairs grow from a follicular unit. FU1: one hair (~15–20% of all follicles). FU2: two hairs (~40%). FU3: three hairs (~25–30%). FU4: four hairs (~5–10%). The distribution during implantation determines the aesthetic result.

How long does a single hair grow?

The anagen phase lasts 2 to 7 years — your individual anagen length is genetically determined. With a monthly growth rate of approx. 1 cm, this means a maximum hair length of 24 to 84 cm.

Why do I lose hair every day?

10 to 15% of all scalp hairs are constantly in the telogen phase (resting phase). At the end of this phase, the hair falls out, and a new one grows from the follicle. With 100,000–150,000 follicles, this means a daily loss of 50–100 hairs — completely normal.

What is DHT and why does it cause hair loss?

DHT (dihydrotestosterone) is formed from testosterone by the enzyme 5-alpha-reductase. It binds to the androgen receptor of genetically sensitive hair follicles — typically on the forehead, temples, and crown — and triggers their miniaturization. Follicles on the back of the head are DHT-resistant and remain intact.

Can miniaturized follicles be revived?

In early miniaturization stages (1 to 3): yes, through Finasteride, Minoxidil, PRP, or mesotherapy. From stage 4 (vellus hair), it becomes significantly more difficult. From stage 5 (fibrosis), the follicle is definitively lost — no therapy can reactivate it then.