Hair follicles
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Hair follicles (Latin: folliculi pili) are specialized anatomical structures of the skin responsible for the production, growth, and maintenance of hair. They are dynamic biological units located in the dermis that cooperate with numerous elements of the cutaneous system, such as sebaceous glands, arrector pili muscles, and a network of blood vessels and nerve endings. Hair follicles play a key role not only in aesthetic and thermoregulatory functions but also in skin regeneration processes. Their activity is regulated by genetic, hormonal, and environmental factors, which is of significant importance in the context of skin diseases and hair growth disorders.
Hair follicles - what they are
The hair follicle is a complex epithelial-mesenchymal structure whose primary function is hair production through intensive proliferation of hair matrix cells. Each hair follicle constitutes a miniature organ capable of cyclic regeneration, passing through phases of growth (anagen), regression (catagen), and rest (telogen).
Hair follicles develop already during fetal life and remain present throughout a person's life, although their activity changes with age. In the adult human body, there are approximately 2 to 5 million hair follicles, of which about 100-150 thousand are located on the scalp.
Important characteristics of hair follicles:
- exhibit high mitotic activity,
- are subject to precise hormonal regulation (especially by androgens),
- constitute a reservoir of skin stem cells,
- are susceptible to the effects of external factors (e.g., stress, toxins, nutritional deficiencies).
Disorders in the functioning of hair follicles lead to numerous pathologies, such as androgenetic alopecia, telogen effluvium, or inflammatory diseases of the scalp.
Hair follicles - structure
The structure of the hair follicle is complex and includes several layers and auxiliary structures that together form a functional unit responsible for hair growth.
Main elements of the hair follicle:
1. Hair bulb (bulbus pili)
The lower, expanded part of the follicle, containing:
- hair matrix (intensively dividing cells),
- melanocytes (responsible for hair pigmentation).
2. Hair papilla (papilla dermalis)
A structure built of connective tissue, highly vascularized and innervated, responsible for:
- supplying nutrients,
- regulation of the hair growth cycle.
3. Hair sheaths
- inner root sheath (IRS) – stabilizes the hair and guides its growth,
- outer root sheath (ORS) – contains stem cells.
4. Hair shaft
The visible part of the hair, composed of:
- medulla,
- cortex,
- cuticle (cuticula).
5. Sebaceous gland
Associated with the hair follicle, it produces sebum, which:
- moisturizes the hair,
- protects the skin from water loss.
6. Arrector pili muscle (musculus arrector pili)
Responsible for lifting the hair (so-called „goosebumps”).
Life cycle of the hair follicle
The hair follicle functions cyclically:
| Phase | Characteristics |
|---|---|
| active hair growth (2–7 years) | |
| Catagen | transition phase (follicle involution) |
| Telogen | resting phase (hair falls out) |
Disruptions of this cycle form the basis of most types of alopecia.
Hair follicles - functions
Hair follicles perform a range of important biological functions that go beyond simple hair production.
1. Hair production and growth
The primary function of the follicle is to generate hair through intensive proliferation of matrix cells. This process requires:
- high energy demand,
- adequate blood supply,
- hormonal regulation.
2. Protective function
Hair produced in follicles plays a protective role:
- protects the scalp against UV radiation,
- protects against mechanical injuries,
- limits heat loss.
3. Sensory function
Hair follicles are highly innervated and function as touch receptors. Even minimal movement of the hair activates nerve endings, which increases skin sensitivity.
4. Participation in thermoregulation
Through cooperation with the arrector pili muscles and the presence of hair, follicles participate in body temperature regulation.
5. Stem cell reservoir
Within the outer root sheath, there are stem cells that:
- participate in epidermal regeneration,
- play a role in wound healing,
- constitute a potential target for regenerative therapies.
6. Role in skin pathophysiology
Hair follicles are the site of development for many diseases:
- androgenetic alopecia (follicle miniaturization),
- telogen effluvium (premature transition to the resting phase),
- folliculitis,
- autoimmune diseases (e.g., alopecia areata).
Clinical significance
Dysfunction of hair follicles often results from:
- hormonal disorders (e.g., excess of androgens),
- chronic stress,
- micronutrient deficiencies (iron, zinc),
- systemic diseases.
Modern aesthetic and regenerative medicine utilizes the hair follicles' ability to rebuild by applying methods that stimulate their activity, such as:
- • platelet-rich plasma (PRP) therapies – utilize autologous growth factors to stimulate hair matrix cells and extend the anagen phase,
- • scalp mesotherapy (needle) – involves direct administration of active substances (including peptides, vitamins, amino acids) that support the metabolism and nourishment of hair follicles,
- • microneedle mesotherapy – induces controlled skin micro-injuries, activating regenerative processes and improving the hair growth microenvironment,
- • therapies supporting microcirculation and cellular metabolism (e.g., carboxytherapy, LED photobiomodulation) – increase blood supply and oxygenation of the hair papilla, which favors the improvement of hair follicle functioning.
The mechanism of action of these therapies is mainly based on:
- fibroblast activation,
- improvement of blood supply to the hair papilla,
- extending the anagen phase,
- stimulation of stem cells.
