RT Book, Section A1 Hordinsky, Maria K. A2 Soutor, Carol A2 Hordinsky, Maria K. SR Print(0) ID 1177005978 T1 Hair Disorders T2 Clinical Dermatology YR 2017 FD 2017 PB McGraw-Hill Education PP New York, NY SN 9780071769150 LK dermatology.mhmedical.com/content.aspx?aid=1177005978 RD 2024/03/29 AB The hair follicle is a complex structure which produces the hair fiber consisting of a cortex, medulla, and cuticle (Figures 19-1 and 19-2). Hair follicles demonstrate the unusual ability to completely regenerate themselves. Hair grows, falls out, and then regrows. In the normal human scalp, up to 90% of hair follicles are in the growth phase called anagen, 1% in the transition phase catagen, and up to 10% in telogen or the loss phase. The anagen phase lasts approximately 3 years, catagen 2 to 3 weeks, and telogen 3 months.1