What is hair made of? Hair is made up of three layers: medulla, cortex and cuticle. The medulla is found in some hairs and acts as a supporting structure, the cortex makes up the largest part of the hair and contains both keratin and pigment, which is where the hair’s strength and color are located. The outermost layer, the cuticle, is made up of thin, flat cells that act as a protective shell and help retain moisture and protect the hair from wear and tear.

Each hair grows from a hair follicle, also called a hair follicle, which surrounds the hair roots. This is where the actual growth takes place. Although the hair is dead when it leaves the follicle, lacking blood vessels and nerves, the hair follicle is a living structure that controls how quickly and for how long the hair grows. Hair follicles are found all over the body and are connected to sebaceous glands that secrete sebum, a natural fat that lubricates both hair and skin. In excess, the hair is perceived as greasy, and if the hair follicle becomes blocked or inflamed, you can get ingrown hairs or folliculitis. How many hairs do you have on your body? A healthy adult has about 100,000 – 150,000 hairs on their head, and a total of about 5 million hairs on their entire body.

What does a hair follicle look like?

The structure of the hair follicle is complex and consists of several different parts that work together to enable hair growth. So what is hair made of? At the bottom of the hair follicle is the papilla, a large structure consisting of connective tissue and blood vessels and this is where the hair shaft receives its nutrition and growth signals. Above the papilla is the hair matrix, where cell division occurs and where new hair cells are formed that are pushed upwards and keratinized, that is, harden and form the hair shaft itself.

Surrounding the papilla is the hair sheath, which in turn is surrounded by the root sheath. The root sheath is divided into two parts: an outer and an inner root sheath. The outer root sheath appears empty under a microscope but contains cubic cells. The inner root sheath consists of three layers: Henle’s layer, Huxley’s layer, and the inner cuticle, which helps shape and guide the growth of the hair shaft.

Adjacent to the outer root sheath is the bulge, a particularly important part that contains stem cells. These stem cells can not only renew the cells of the hair follicle but also play a major role in the skin’s healing process after injury. The bulge is located at the attachment point of the arrector pili muscle, the small muscle that allows the hair to stand up (e.g. when cold or scared).

Together, these structures make up the hair’s structure and control its life cycle, growth and quality. It is precisely disorders in these parts that are often behind hair loss or scalp diseases.

The different growth phases of hair – what is hair?

The hair life cycle consists of four main phases: an active growth phase (anagen), a short transitional phase (catagen), a resting phase (telogen) and finally a phase when the hair falls out (exogen). These phases occur asynchronously, which means that different hairs on the head are in different phases at the same time, which is why we don’t lose all our hair at once.

Anagen growth phase – when the hair is actively growing

The anagen phase is the growth phase of hair and can last anywhere from two to seven years, depending on genetics. During this phase, the hair shaft is actively growing, and approximately 85–90% of all hairs on the head are in this stage. Growth occurs in the hair follicle where cells divide rapidly, causing the hair shaft to grow longer. The length of time the hair stays in the anagen phase determines how long it can grow. People with a long anagen phase can have very long hair, while those with a shorter phase reach an earlier natural “maximum length.”

Catagen transition phase – hair prepares to rest

After growth, the hair enters the catagen phase, which is a short intermediate phase where the hair begins to detach from its nutritional supply in the hair follicle. This occurs by restricting the blood supply to the papilla and ceasing cell division. The hair follicle shrinks, and the hair prepares to leave the skin. This phase normally lasts a few weeks to a couple of months, but with age the catagen phase can be extended, which contributes to slower hair growth and the hair reaching a resting state more quickly.

Telogen resting phase – growth stops

In the telogen phase, the hair has completely stopped growing and remains in the skin without any active function. During this period, the hair does not receive a blood supply, but it still remains in the hair follicle. At the same time, a new hair is being prepared to start growing in the same follicle.

Exogenous hair loss phase – when hair falls out naturally

Finally, the hair enters the exogenous phase, the hair loss phase. Here, the old, dormant hair is released from the hair follicle and falls out, often without you noticing. It is completely normal to lose between 50 and 100 hairs per day, as they are replaced by new hairs that have already started growing in the same follicle. After the exogenous phase, the hair follicle goes back into the anagen phase and the life cycle begins again.

Why do hair follicles shrink?

Hair follicles are dynamic and sensitive structures that are affected by both internal and external factors. When a hair follicle shrinks, a process called miniaturization, it begins to produce thinner, weaker, and shorter hairs. This change often occurs gradually and can be linked to genetic predisposition, hormonal changes, or aging. A common cause of hair follicle shrinkage is increased levels of the hormone DHT (dihydrotestosterone), which in genetically susceptible individuals binds to receptors in hair follicles and impairs their function.

Hair follicles can shrink – common causes:

• Elevated levels of DHT (dihydrotestosterone)
• Genetic predisposition to hair loss (androgenetic alopecia)
• Aging: the growth capacity of hair follicles decreases over time
• Prolonged stress
• Nutritional deficiencies: especially iron, zinc and vitamin D
• Autoimmune diseases
• Certain medications or hormonal contraceptives
• Inflammatory conditions of the scalp

Can hair follicles be stimulated to produce new hairs?

Yes, in many cases it is possible to stimulate the hair follicles and promote new hair growth, especially if the hair follicles are still alive but inactive. There are several proven treatments that target the function and growth environment of the hair follicles. These treatments will not produce the same results if the hair is already dead.

One of the most widely used is Minoxidil, a medication that is applied directly to the scalp. It works by increasing blood flow to the hair follicles, providing them with more oxygen and nutrients. Another popular method is PRP treatment, in which the patient’s own blood is centrifuged to isolate growth factors. This concentrated plasma is then injected into the scalp and can help revive dormant hair follicles, improve hair quality, and slow hair loss.