Principles of hair cycle control

Area of Science:

• Dermatology
• Cell Biology
• Chronobiology

Background:

• Hair follicles (HFs) undergo continuous cyclic transformations: anagen (growth), catagen (regression), and telogen (resting).
• Contrary to prior belief, the telogen phase is metabolically active and involves hair shaft extrusion (exogen).
• HF cycling is a complex process, with some follicles undergoing programmed deletion by immune cells.

Purpose of the Study:

• To elucidate the intrinsic mechanisms controlling the hair follicle cycle.
• To investigate the role of an internal 'hair cycle clock' versus external factors.
• To identify therapeutic targets for hair growth disorders by understanding catagen control.

Main Methods:

• Review of existing literature on hair follicle biology and cycling.
• Analysis of signaling pathways and cellular activities within the HF.
• Discussion of clinical implications for alopecia and hirsutism.

Main Results:

• The hair follicle cycle is driven by an intrinsic, intrafollicular 'hair cycle clock,' not external vascular, neural, or extrafollicular cell stimuli.
• Telogen is an active phase with metabolic and proliferative activity, including potential hair extrusion (exogen).
• While various factors influence hair growth, they are secondary to the primary internal oscillator.

Conclusions:

• The precise nature and location of the 'hair cycle clock' oscillator remain unknown.
• Control of catagen, particularly keratinocyte apoptosis and dermal papilla secretion, are key targets for therapeutic intervention in hair disorders.
• Understanding these internal controls is crucial for managing conditions like alopecia and hirsutism.