Hairs of head

Hair of the head (scalp hair) refers to the keratinized filamentous structures produced by hair follicles located on the scalp.

Definition and General Characteristics

Scalp hair is a defining characteristic of mammals and, in humans, consists primarily of terminally differentiated keratinocytes produced by hair follicles — small, complex epithelial mini-organs embedded in the skin. The healthy adult scalp covers an area of approximately 700 cm² and contains roughly 100,000 to 150,000 hair follicles, all of which are formed during embryogenesis and are not generated de novo after birth.

Types of Scalp Hair

Approximately 90% of scalp hairs are terminal hairs — large, pigmented fibers with a shaft diameter of 30–150 µm. The remaining 10% are vellus hairs, which are short, fine, barely visible, and depigmented. An intermediate category, termed indeterminate hairs (shaft diameter 0.03–0.06 mm), also exists. A normal terminal-to-vellus hair ratio on the scalp is greater than 3:1 (or ≥2:1 in individuals over 50 years of age).

Hair Fiber Structure

Individual hair fibers are composite, multilayered structures organized into three concentric layers: the cuticle (outermost protective layer), the cortex (the main structural body containing melanosomes responsible for pigmentation), and the medulla (the innermost core, which may be absent in fine hairs). These layers are bound together by the cell membrane complex (CMC). The concentration and ratio of eumelanin and pheomelanin within the melanosomes account for the wide variation in hair color.

Hair Growth Cycle

Hair growth is cyclic and consists of three phases:

• Anagen (growth phase): The active growth phase during which the hair matrix surrounding the dermal papilla produces the hair shaft. On a normal scalp, 90–95% of follicles are in anagen at any given time. The duration of anagen determines hair length.

• Catagen (involution/regression phase): A brief transitional phase driven by apoptosis, affecting less than 1% of follicles at any time.

• Telogen (resting phase): The quiescent phase during which the fully keratinized "club hair" is retained and eventually shed. Approximately 5–10% of scalp follicles are in telogen. Up to 100 telogen hairs are shed daily from the scalp under normal conditions.

Follicular Anatomy

Each hair follicle can be divided into distinct anatomical regions: the bulb (containing the dermal papilla and hair matrix), the suprabulbar zone, the isthmus, and the infundibulum (the upper portion opening to the skin surface). The follicle also houses the sebaceous gland (forming the pilosebaceous unit) and the bulge region, which contains epithelial stem cells critical for follicle regeneration.

Functions

In humans, scalp hair serves functions beyond thermoregulation and physical protection of the cranium. It plays a significant role in social and sexual signaling, is a key perceptual indicator of age, health, and beauty, and is often reflective of cultural identity. The loss of scalp hair can have substantial adverse psychological effects, including anxiety and diminished self-esteem.

Regulation

Hair growth is regulated by complex epithelial-mesenchymal signaling, as well as systemic influences including androgens, thyroid hormones, nutritional status (iron, zinc, selenium, vitamin D, niacin), and immune mechanisms. Androgens, in particular, modulate follicle size — enlarging follicles in the beard and body at puberty while paradoxically miniaturizing follicles in the bitemporal and vertex scalp regions, leading to androgenetic alopecia