Classification and general structure
Depending on localization and structure, three types of hair are distinguished: long hair (on the scalp, beard, and in the axillary regions), bristly hair (eyebrows and eyelashes), and vellus hair, which covers almost the rest of the body.
Morphologically, a hair is divided into parts along its length:
- Shaft — the visible part above the skin surface.
- Root — the part hidden within the skin down to the hair funnel opening.
- Hair bulb — the expanded base of the root responsible for growth.
In transverse cross-section of the root and shaft, three layers are identified (they are absent in the bulb itself):
- Cuticle — a thin outer layer.
- Cortex (cortex) — the main mass formed by hard keratin.
- Medulla (medulla) — the central part consisting of soft keratin, melanin granules, and air bubbles. Important: this layer is absent in vellus hairs.
Surrounding structures and hair follicle
The hair root is enclosed in a multi-layered "sleeve". Closest to the hair is the hair follicle of epithelial origin. It consists of the internal root sheath (ending at the infundibulum) and the external root sheath, which is a direct continuation of the epidermis. The external sheath is extremely important: it harbors pluripotent stem cells capable of regenerating the epidermis after severe burns.
Externally, the epithelial follicle is enveloped by the dermal sheath (hair sac) composed of collagen fibers. The base of the bulb is indented by the hair papilla — a connective tissue outgrowth with blood vessels supplying the matrix.
Additional structures of the complex include sebaceous glands (their ducts open into the hair infundibulum) and the arrector pili muscle (m. arrector pili). This bundle of smooth myocytes extends from the hair sac to the papillary layer of the dermis. Its contraction produces the "goosebumps" effect.
Hair life cycle
Hair replacement occurs continuously throughout life. The cycle is divided into three main stages:
- Anagen (growth phase). Lasts from 2 to 6 years. Active mitosis of keratinocytes takes place in the bulb matrix, and new layers of the hair are formed.
- Catagen (involution phase). Takes 2–4 weeks. The hair papilla atrophies, and blood supply declines. Melanocytes are the first to stop producing pigment, followed by the cessation of mitosis. The bulb keratinizes, turning into a hair club. The inner sheath disintegrates, and the club shifts upward along the outer sheath.
- Telogen (resting phase). Lasts 2–4 months. The old unpigmented hair in the form of a keratinized club is held in the follicle until stem cells at the base become activated. The new growing hair will eventually mechanically push out the old one.
Briefly: answers to topical questions
What layers is the internal epithelial root sheath subdivided into?
In the lower third of the root, it has a three-layered structure, whereas higher up it transforms into a single layer of cornified scales. The cells of the internal epithelial sheath contain abundant trichohyaline granules and subsequently undergo keratinization (containing soft keratin). The exact names of the three layers are not specified in the sources.
Where exactly in the hair follicle are stem cells localized (in which specialized zone)?
Pluripotent stem cells are localized in the external root sheath. For subsequent hair growth, their preservation at the base of the external sheath is crucial; at the end of the resting phase (telogen), stem cells at the base of the follicle are activated. The specialized name of this zone is not provided in the sources.
What cellular elements are part of the hair bulb matrix besides keratinocytes?
In addition to keratinocytes, the hair bulb includes the following cellular elements:
- Melanocytes — responsible for pigment synthesis.
- Intraepidermal macrophages (Langerhans cells) — perform an immune function.
- Merkel tactile cells — carry out a receptor function.
The ACT abbreviation helps to remember the cycle phases: Anagen (Active growth), Catagen (Closure of bulb activity), Telogen (Terminal stage/Quiet).
Class and exam questions
Does frequent shaving affect hair growth rate?
No, this is a common myth. Neither cutting nor shaving changes the hair growth rate or density, although it can make them feel visually stiffer at the tips.
Why is it difficult to pull out a hair in isolation, without its sheaths?
The "zipper" effect comes into play. The scales of the hair cuticle point obliquely downwards, while the scales of the surrounding internal epithelial sheath point upwards. Under tension, they interlock tightly.
Is it true that one can go gray overnight from stress?
Skeptics deny this because the shaft lacks metabolism. However, there is a well-founded biochemical hypothesis: during severe stress, hydrogen peroxide rapidly accumulates in the cells. It can diffuse along the shaft and chemically bleach existing melanin.
What else to study on this topic
- Cellular composition of the germinative zone: keratinocytes, melanocytes, Langerhans cells, and Merkel cells.
- Matrix function of the bulb and histogenesis of hair layers.
- Mechanisms of hard and soft keratin formation.
- The role of dihydrotestosterone and 5-alpha-reductase in the pathogenesis of androgenetic alopecia.
- Topographic differences in the ratio of anagen and telogen phases (on the scalp and limbs).
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