What is the outer layer of hair that consists of heavily keratinized cells?

The outer layer of hair is known as the cuticle, which is made up of heavily keratinized cells. This structure serves as a protective layer for the hair shaft underneath. The cuticle consists of overlapping, scale-like cells that help to protect the inner layers of the hair and maintain its strength and resilience.

The cortex, on the other hand, is the thickest part of the hair and contains the bulk of the hair's mass, providing strength and pigmentation, but it is not the outer layer. The hair shaft refers to the part of the hair that is visible above the skin's surface, comprising both the cuticle and the cortex. The root is the part of the hair that is embedded in the follicle beneath the skin, and while it is integral to hair growth, it doesn't refer specifically to the outer layer. Thus, the cuticle is correctly identified as the outer layer of hair made up of protective keratinized cells.