Anatomy and Biology of Hair at Different Ages

December 2023

Anne‐Laure Pélissier‐Alicot

hair shaft hair root pilosebaceous unit hair follicles stem cell niches signaling pathways hair development fetal development induction phase organogenesis cytodifferentiation ectoderm mesoderm terminal hair

TLDR Hair grows in cycles and changes with age, starting from fetal development.

The document provides a comprehensive examination of the anatomy and biology of human hair, emphasizing its complex structure and functions across different ages. It describes hair as a "mini-organ" with a continuous growth cycle involving stem cells and signaling pathways. Hair development begins in the fetal stage, with intricate interactions between the ectoderm and mesoderm, and changes significantly from fetal life to adulthood. The document highlights the classification of hair types, the role of various compartments in hair growth and pigmentation, and the phases of the hair cycle, including newly identified exogen and kenogen phases. It underscores the importance of understanding these biological aspects for interpreting drug incorporation into hair and analyzing hair characteristics.

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