How Does Hair Grow? The Science of the Hair Cycle
Hair growth is driven by a complex, cyclical process occurring within the hair follicle, a specialized pocket of skin containing stem cells, blood vessels, and sebaceous glands. Each hair shaft is made primarily of keratin, a structural protein also found in nails and skin, layered into three distinct tissue structures: the medulla, cortex, and cuticle. Rather than growing continuously, hair moves through four distinct phases—anagen (growth), catagen (transition), telogen (resting), and exogen (shedding)—which determine whether a hair is actively lengthening, preparing to detach, or falling out. Hair color stems from melanin production in the follicle's base, while hair graying occurs when melanin-producing cells exhaust their stem cell reserves.