ANATOMY / CHAPTER 01
The Hair Follicle
A dynamic mini-organ that cycles through growth, regression, rest, and shedding.
Overview
The follicle is an epidermal appendage extending into the dermis and, depending on body site and growth phase, the subcutis. Its bulb contains matrix keratinocytes surrounding the dermal papilla, which provides key inductive signals.
Key structures
- Infundibulum: from follicular opening to sebaceous duct.
- Isthmus: from sebaceous duct to arrector pili insertion.
- Bulge: epithelial stem-cell niche near the arrector pili attachment.
- Bulb and matrix: proliferative compartment producing the hair shaft.
- Dermal papilla: mesenchymal signaling center influencing growth.
Hair cycle
Anagen is active growth; catagen is programmed regression; telogen is relative rest; exogen describes shedding. Cycle duration varies substantially by site and individual.