Each strand of hair grows out of a hair follicle, also called a hair follicle, that encloses the hair roots. This is where the growth itself takes place. Although the hair follicle is dead by the time it leaves the hair follicle, by lacking blood vessels and nerves, the hair follicle is a living structure that controls how fast and 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 fat, and if the hair follicle becomes clogged or inflamed, ingrown hairs or follicular inflammation can occur. How many hairs do you have on your body? An adult healthy human has about 100,000 to 150,000 hairs on the head, and a total of about 5 million hairs on the 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 allow hair growth. So what does hair consist of? At the bottom of the hair follicle is the papilla, a large structure made up 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.Around 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 looks 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 direct the growth of the hair.Adjacent to the outer root sheath is the bulge, a particularly important part containing stem cells. These stem cells are not only able to renew the cells of the hair follicle, but also play a large role in the healing process of the skin after injuries. The bulge is located at the attachment point of the arrector pili muscle, the small muscle that allows the hair to rise (e.g., in case of cold or fear) .Together, these structures make up the build-up of the hair and control the life cycle, growth and quality of the hair. It is precisely disorders of these parts that often underlie hair loss or diseases of the scalp.
The different growth phases of hair - what is hair?
The hair follicle 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 detaches (exogenous). These phases occur asynchronously, which means that different hairs on the head are in different phases at the same time and that is why we do not lose all the hair at once.
Anagen growth phase — when the hair is actively growing
The anagen phase is the growth phase of the hair and can last anywhere from two to seven years, depending on genetic prerequisites. 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 the cells divide rapidly, causing the hair shaft to lengthen. How long the hair stays in the anagen phase determines how long it can get. People with long anagen phase can get very long hair, while those with shorter phase reach a previously natural “maximum length”.
Catagenic transition phase — hair is preparing to rest
After growth, the hair transitions into the catagenic phase, which is a short intermediate phase in which the hair shaft begins to detach from its nutrient supply in the hair follicle. This is done by strangling the blood supply to the papilla and stopping cell division. The hair follicle shrinks, and the hair shaft prepares to leave the skin. This phase normally lasts for a few weeks up to a couple of months, but with age, the catagen phase can be prolonged, which contributes to a slower hair growth and that the hair shaft more quickly reaches a state of rest.
Telogen resting phase — growth stops
In the telogen phase, the hair shaft has completely stopped growing and remains in the skin without having any active function. During this period, the hair shaft receives no blood supply, but it still remains in the hair follicle. At the same time, a new strand of hair is being prepared to start growing in the same follicle.
Exogenous hair loss phase — when hair falls off naturally
Finally, the hair shaft enters the exogenous phase, the hair loss phase. Here, the old, dormant hair detaches from the hair follicle and falls off, often without being noticed. It is perfectly normal to lose between 50 and 100 hairs per day, as they are replaced by new hairs that have already begun to grow in the same follicle. After the exogenous phase, the hair follicle goes back into the anagen phase and the life cycle starts over.
Why do hair follicles shrink?
Hair follicles are dynamic and delicate structures that are influenced 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 may be linked to genetic predisposition, hormonal changes or aging. A common cause of hair follicles shrinking is increased levels of the hormone DHT (dihydrotestosterone), which in genetically susceptible people binds to receptors in the 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 ability of hair follicles decreases over time
- Prolonged stress
- Nutritional deficiencies: especially iron, zinc and vitamin D
- Autoimmune diseases
- Certain drugs 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 do not give the same results if the hair is already dead. One of the most used is Minoxidil, a drug that is applied directly to the scalp. It works by increasing blood circulation to the hair follicles, providing them with more oxygen and nutrition. 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 bring dormant hair follicles to life, improve hair quality and slow down hair loss.
