RT Book, Section A1 Alessandrini, Aurora A1 Starace, Michela A1 Piraccini, Bianca Maria A2 Tosti, Antonella A2 Piraccini, Bianca Maria SR Print(0) ID 1201699276 T1 Hair Anatomy and Physiology: What You Need to Know T2 Disorders of the Hair and Nail: Diagnosis and Treatment YR 2023 FD 2023 PB McGraw Hill PP New York, NY SN 9781260462470 LK dermatology.mhmedical.com/content.aspx?aid=1201699276 RD 2023/11/30 AB SUMMARYThe hair follicle is composed by two segments: upper, or permanent, and lower or dynamic, subject to changes that occur during the follicular cycle.The hair shaft consists of three structures: cuticle, cortex, and medulla.The follicular cycle is composed by three phases: anagen or growth phase; catagen or involution phase; and telogen or resting phase.Hair follicles appear around on the scalp in the fourth month of intrauterine life.