Classification and General Structure
Depending on localization and structure, three types of hair are distinguished: long hairs (scalp, beard, axillae), bristle hairs (eyebrows and eyelashes), and vellus hairs, which cover almost the rest of the body.
Morphologically, a hair is divided along its length into:
- Shaft — the visible part above the skin surface.
- Root — the part hidden in the skin up to the opening of the sebaceous gland duct (infundibulum).
- Hair bulb — the expanded base of the root responsible for hair growth.
In cross-section, the root and shaft show three layers (absent in the bulb itself):
- Cuticle — a thin outer layer.
- Cortex (cortex) — the main bulk, formed by hard keratin.
- Medulla (medulla) — the central core consisting of soft keratin, melanin granules, and air spaces. Note: this layer is absent in vellus hairs.
Surrounding Structures and the Hair Follicle
The hair root is enclosed in a multilayered sheath. Closest to the hair is the epithelial hair follicle. It consists of the internal root sheath (terminating at the infundibulum) and the external root sheath, which is a direct continuation of the epidermis. The external root sheath is critical: it contains multipotent stem cells capable of regenerating the epidermis after severe burns.
Externally, the epithelial follicle is wrapped by the dermal root sheath (hair sac) made of collagen fibers. The base of the bulb is indented by the hair papilla — a connective tissue protrusion containing blood vessels that supply the matrix.
Additional structures of the complex include sebaceous glands (their ducts open into the hair canal) and the arrector pili muscle (m. arrector pili). This bundle of smooth muscle cells extends from the hair sac to the papillary dermis. Its contraction produces "goosebumps" (piloerection).
Hair Growth 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 occurs in the bulb matrix, forming new hair layers.
- Catagen (involution phase). Lasts 2–4 weeks. The hair papilla atrophies, and blood supply decreases. Melanocytes stop producing pigment first, followed by cessation of mitosis. The bulb keratinizes, turning into a hair club. The inner root sheath breaks down, and the club shifts upward along the outer root sheath.
- Telogen (resting phase). Lasts 2–4 months. The old unpigmented hair, shaped like a keratinized club, is held in the follicle until stem cells at the base are reactivated. The new growing hair eventually mechanically pushes out the old one.