Which part of the hair is found deep to the shaft and penetrates the dermis?

The root of the hair is the part that is found deep to the hair shaft and extends into the dermis. This is where hair growth occurs, as the root is embedded within the hair follicle, which is surrounded by living cells that are actively growing and producing keratin. The dermis provides the necessary nutrients and blood supply to support the living cells in the hair bulb located at the base of the root.

In contrast, the cortex, cuticle, and medulla are components of the hair shaft itself. The cortex is the thick, protective middle layer of the hair that contains fibrous proteins and is responsible for the strength and color of the hair. The cuticle is the outer layer of overlapping cells that protects the inner layers, and the medulla is the central core that is typically present in thick hair but may be absent in finer or thinner hair types. These components contribute to the hair's overall structure but do not penetrate the dermis.