13/03/2026
What is hair?
Hair is a complex, keratin-based filament growing from follicles, consisting of a protective cuticle, fibrous cortex (color/strength), and sometimes a medulla core
It grows in three cycles—anagen (growth), catagen (transition), and telogen (rest)—with a shaft above the skin and a root anchored below.
Key Hair Structure and Composition
Cuticle:
The outermost, transparent layer composed of overlapping cells (like shingles) that protects the inner structure.
Cortex: The middle, thickest layer containing keratin proteins and melanin, which determines hair strength, elasticity, and color.
Medulla: The innermost, often hollow core, usually found in thicker or coarser hair.
Follicle: The living organ below the skin that anchors the hair root and produces new cells.
Sebum: Natural oil produced by sebaceous glands that moisturizes the hair and scalp.
Hair Growth Cycle:
Anagen (Growth Phase): Lasts 3-5 years (or 2-6 years), where 90% of hair is actively growing.
Catagen (Transition Phase): A 10-day to 2-week period where the hair follicle shrinks and detaches.
Telogen (Resting Phase): Lasts about 3 months, during which the hair is inactive and eventually sheds.
Hair Characteristics and Types:
Types: Classified as straight, wavy, curly, or coily based on follicle shape.
Texture: Ranging from fine (flat, oily) to medium to coarse (thick, dry, frizzy).
Density: The average scalp contains about 100,000 to 120,000 hair follicles.
Key Facts
Hair grows roughly 0.5 inches per month.
The only living part of hair is in the follicle.
Hair color is determined by melanin, specifically eumelanin (brown/black) and pheomelanin (red/yellow).
