Hair Follicles: Understanding Hair Growth and Structure

Hair grows from hair follicles—tubular invaginations of the epidermis into the dermis/subcutis.

Follicle Anatomy

• Hair bulb (base): Contains dermal papilla (blood supply) and matrix cells (proliferate → hair shaft).
• Inner/outer root sheaths: Guide and shape the shaft.
• Sebaceous gland: Secretes sebum (lubricates hair/skin).
• Arrector pili muscle: Contracts → goosebumps (vestigial).

Hair Shaft

• Composed of keratin (dead, cornified protein).
• Three layers: medulla (core), cortex (strength/color), cuticle (protective scales).

Growth Cycle

1. Anagen (growth): 2–7 years (scalp); determines length.
2. Catagen (transition): 2–3 weeks; follicle regresses.
3. Telogen (rest): 3 months; old hair shed, new anagen begins.

~85–90% of scalp hairs in anagen. Disruption → hair loss (telogen effluvium, androgenetic alopecia).