What is a hair follicle? The structure and parts of the hair root

What is a hair follicle?

The hair follicle is where a hair is made: a living structure embedded in the skin that looks like a thin tube. In everyday language, we call it the “hair root” or simply the “root.” The hair that grows out of the skin, on the other hand, is not alive; it is made of hardened cells rich in keratin (the tough protein that hair and nails are made of).

A follicle in the growth phase reaches down into the deeper layers of the skin, sometimes as far as the fat tissue beneath it. The scalp has tens of thousands of hairs; this number varies from person to person. Follicles are not scattered across the scalp one by one; most of them form small groups in which one to four hairs grow out together. These groups are called follicular units (natural hair groups); the grafts taken in FUE (the method in which grafts are taken one by one) are also usually these units.

Follicles form before birth. Under normal circumstances, no new follicles form in the adult scalp, which means the number of follicles you have does not increase during your lifetime. A hair transplant does not increase this number either; it moves existing follicles to another area.

What are the parts of a follicle?

In the table below, we list the parts of the follicle from the surface downward. The follicle and the two structures next to it, the oil gland and the hair erector muscle, are together known as the pilosebaceous unit (the hair and oil gland unit).

Inside the follicle, the hair is wrapped in several layers of sheaths. The inner and outer root sheaths (the layers of cells around the hair) shape and support the hair; on the very outside is a thin sheath of connective tissue.

How does a follicle produce a hair?

Hair production starts in the bulb. Signals from the dermal papilla make the matrix cells divide, and the new cells are pushed upward. As these cells rise, they fill with keratin, harden and die; this is how a hair forms, and it grows longer as new cells keep arriving from below.

Meanwhile, pigment cells pass a coloring substance (melanin) into the hair. The drop in pigment production that comes with age is linked to hair turning gray.

A structure that works in cycles

A follicle does not produce hair all the time; it follows a cycle made up of a growth phase (anagen), a transition phase (catagen) and a resting phase (telogen). During the transition phase, the lower part of the follicle shrinks back and gets shorter. At the end of the resting phase, the follicle enters the growth phase again: the stem cells in the bulge region are activated by signals from the dermal papilla, the lower part re-forms, and a new hair grows from the same follicle. The old hair often falls out around this time.

The upper part of the follicle (the infundibulum and the isthmus) stays in place throughout this process. Losing some hair every day is also a normal part of this cycle.

What happens to the follicle in hair loss?

In androgenetic alopecia (hair loss linked to genetic predisposition and hormones), the most common type of hair loss, follicles do not disappear all at once; they first shrink (miniaturization). In male pattern hair loss, genetically sensitive follicles shrink from one cycle to the next under the influence of a hormone called dihydrotestosterone (DHT), which is derived from testosterone; in female pattern hair loss, the role of hormones is more complex. The growth phase gets shorter, and the hair the follicle produces becomes thinner, shorter and lighter in color over time. Eventually, all that is left are fine, downy hairs that do not cover the scalp.

In most people, however, the follicles at the back and sides of the head are less sensitive to this effect; this is why we usually take grafts from there, that is, from the donor area. These follicles usually keep this characteristic even after they are moved to another area; this is the basic principle a hair transplant relies on. Even so, the donor area can also thin with age or in some types of hair loss, so we assess it carefully during the examination as well.

Some medicines used for hair loss aim to slow down this shrinking process in the follicle. Only your doctor can determine, after assessing you, whether these medicines are suitable for you; do not start taking them on your own.

Not all hair loss progresses this way. If your hair is falling out in patches or heavily over a short period, or if the hair loss comes with itching, redness, scaling or burning, the cause needs to be looked into separately. In some scalp conditions, scar tissue can take the place of the follicle. In cases like these, we recommend that you see a dermatologist for a diagnosis.

Why is the follicle taken whole in a hair transplant?

In a hair transplant, we do not move a hair; we move the follicle that produces it. For the follicle to keep producing hair in its new place, it is important to take it without damage, together with its bulb, dermal papilla, bulge region and the sheaths around it.

In FUE, we take each graft by separating it from the surrounding tissue with a fine tip. Beneath the skin, the follicle does not run straight down but at an angle, and its direction can change slightly. If the tip strays from this path, it can cut across the follicle; this is called transection (cutting through the follicle). Whether a follicle that has been cut or crushed will produce hair in its new place cannot be predicted in advance.

That is why, when we take grafts, we adjust the angle and depth to the way the follicle runs beneath the skin. In Afro and very curly hair, the follicle also curves beneath the skin, so this stage needs extra care; we explain the details on the Afro hair transplant page.

A graft is still living tissue while it is outside the body; it is important that it does not dry out and that its bulb is not crushed. We place the grafts into channels opened beforehand, using forceps or an implanter pen; we describe these approaches side by side on the hair transplant methods page.

In the weeks after the transplant, some of the transplanted hair or the hair around it may fall out temporarily; this is called shock loss. Even if a transplanted hair falls out, the follicle usually stays in the skin and enters a new growth cycle. However, surrounding hairs that had already become thin may not always grow back. It takes months for new hair to grow, and how long varies from person to person. We explain what to pay attention to during this period on the hair transplant aftercare page; the written aftercare instructions we give you always come first.

Doesn’t a graft carry stem cells too?

A graft that has been taken intact also contains the bulge region and the stem cells in it. However, these cells help their own follicle continue its cycle; they are not expected to renew the thinned follicles around them or to create new follicles. That is why a hair transplant is not a stem cell treatment. We explain what is known today about stem cell procedures on the autologous stem cell application page.

What do we not know yet?

The structure of the follicle is well understood; however, some questions are still open:

• Why are some follicles sensitive and others not? Genetic predisposition and differences in hormone sensitivity between areas of the scalp are known to play a role; where these differences come from, however, has not been fully explained.
• How long does a shrinking follicle keep producing hair? It is not possible to say for certain in advance how long a shrunken follicle will keep working.
• Can new follicles be made? Research on creating new follicles in the laboratory is ongoing; however, it is still at the experimental stage and is not a treatment that can be offered today.
• What do supporting procedures do to the follicle? The evidence on how procedures such as PRP or mesotherapy affect the follicle is limited. Most of the studies are small, and their methods differ from one another.

How do we look at your follicles during the examination?

A hair transplant plan is based on the current condition of your follicles. Examining the scalp with a magnifying device (trichoscopy) can also help with this assessment. Together, we look at:

• How dense the follicular units are in the donor area, and how many hairs a unit usually contains
• The thickness and curl of your hair, and the contrast between your hair color and skin tone
• Whether there are follicles in the recipient area that have thinned but are still producing hair
• Whether there are signs of thinning in the donor area as well
• Whether the cause of the hair loss is clear, and how it is progressing

If there are follicles in the recipient area that are still producing hair, we plan to place the grafts between them, taking care not to damage them. This hair may continue to thin after the transplant; the transplant does not prevent this. That is why, when we make the plan, we also take into account how the hair loss may progress in the future.

This information helps us determine which method suits you and how far we can draw on your donor follicles, which are limited in number.

Whether you are suitable for a hair transplant is determined during the examination, and results vary from person to person.