Hair structure anatomy and comparative anatomy1 1Dedicated to the memory of the late Dr. R. E. Rivett whose untimely death prevented his role in co-authorship.

Human hair, often described as a "mini-organ," is highly complex, consisting of the hair shaft and the root, which includes the hair follicle and its associated structures. The hair follicle undergoes a continuous cycle of growth and regression, regulated by a sophisticated interaction between stem cell niches and signaling pathways. Hair development begins around the 8th week of embryonic life and involves three phases: induction, organogenesis, and cytodifferentiation, requiring close interaction between the ectoderm and mesoderm through various biochemical signals. Initial hair growth occurs in waves and differs from terminal hair, which appears between 12 and 18 months. Understanding these processes is crucial for interpreting drug incorporation into hair, especially in early childhood.