Article Sections

Introduction

Alopecia (hair loss) can be classified as scarring or nonscarring based on the presence or absence of permanent hair follicle damage.  This article provides an overview of hair follicle cycling and discusses several common types of alopecia based on this classification.

Hair follicle cycling

Hair growth occurs in a cyclical pattern with 3 primary phases ( Figure 1):

  • The anagen phase is characterized by linear growth of the hair shaft associated with proliferation of hair follicle cells.  At any given time, 80%-90% of follicles are in the anagen phase, which lasts up to 5 years.
  • The catagen phase is the transition phase characterized by regression and apoptosis of follicular cells.  The hair shaft may continue to be extruded from the follicle but is no longer growing.  This phase lasts up to 2 weeks and typically accounts for <1% of follicles.

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