Normal head-hair length is correlated with its diameter

Area of Science:

• Dermatology
• Trichology
• Human Hair Biology

Background:

• Shed head-hair fibers are crucial for understanding hair growth cycles.
• Previous research has not fully characterized all types of shed hair fibers.
• The morphology and growth dynamics of specific hair fiber types require further investigation.

Purpose of the Study:

• To identify and characterize a novel class of shed head-hair fibers exhibiting both exogen clubs and anagen tips (EA).
• To determine the prevalence and length distribution of EA fibers in nonalopecic women.
• To analyze the diameter changes of EA fibers throughout their growth and shedding phases.

Main Methods:

• Collection and analysis of shed head-hair fibers from young, nonalopecic women (n=25).
• Measurement of fiber lengths and major-axis diameters.
• Statistical analysis to determine distribution, correlations, and growth rates.

Main Results:

• EA fibers comprise approximately 44% of shed hair.
• EA fibers exhibit a uni-modal, positively skewed length distribution (mean 16.7 cm).
• Fiber diameter increases to a maximum early in growth and then decreases towards the exogen club, with specific rates for maximal and proximal diameters.

Conclusions:

• EA fibers represent a significant, previously uncharacterized component of normal hair shedding.
• The study suggests a revision of the concept of hair miniaturization in androgenetic alopecia (AGA).
• Normal hair diameter variability may be overestimated, opening avenues for its use as a biomarker.