During which zone does hair undergo keratinization as it ascends up the follicle?

The keratogenous zone is the specific area within the hair follicle where hair undergoes keratinization. This is the process by which cells that are in the hair's growth phase become filled with keratin protein, ultimately forming the structure of the hair shaft. As hair cells move up through this zone, they accumulate keratin and eventually die off, leading to the formation of mature hair that emerges from the scalp or skin surface. Understanding this process is critical in waxing and hair removal practices, as it impacts the effectiveness of hair removal methods and the timing for subsequent treatments.

In contrast, the other zones mentioned – undifferentiated, telogen, and anagen – have different roles in the hair growth cycle. The undifferentiated zone refers to areas where cells are still in a state of development and haven't yet began to keratinize. The telogen zone signifies the resting phase when hair is not actively growing. The anagen zone is when hair is actively growing but is not specifically focused on the keratinization process that occurs in the keratogenous zone.