What determines the border between hair shaft and hair root?

The border between the hair shaft and hair root is determined by the transition from the visible part of the hair, which extends above the skin's surface (the hair shaft), to the part that is embedded in the follicle beneath the skin (the hair root). This junction is marked by the opening of the hair follicle, where the hair shaft emerges, while the root includes the bulb and the lower portions of the follicle that anchor the hair in place. Additionally, the root is associated with structures such as the dermal papilla and sebaceous glands, which provide nourishment and lubrication to the hair.