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 discusses the complex anatomy and biology of human hair, highlighting its division into the hair shaft and root, which together form the pilosebaceous unit. Hair follicles undergo a continuous cycle of growth and regression, influenced by stem cell niches and signaling pathways. Hair development begins in the eighth week of fetal development and involves three phases: induction, organogenesis, and cytodifferentiation, requiring interactions between the ectoderm and mesoderm. The first hair emerges in waves, differing from terminal hair that appears between 12 and 18 months. Understanding these processes is crucial for interpreting drug incorporation into hair and concentration analysis, especially in early childhood.

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