What portion of the hair penetrates the skin?

The part of the hair that penetrates the skin is the hair root. The hair root is the portion of the hair that is embedded within the hair follicle, which is a structure that extends into the dermis (the deeper layer of skin). This section not only anchors the hair but is also the area where hair growth begins, as it is surrounded by the hair bulb that contains living cells dividing and growing to form the hair strand.

The hair shaft, by contrast, is the visible part of the hair that extends above the skin's surface; it does not penetrate the skin. The hair follicle encompasses the hair root but is not solely the portion that penetrates the skin; it is the housing for the root. The hair bulb is the base of the hair follicle structure, where cells rapidly divide, but it is also not the part that directly penetrates the skin; rather, it is located at the base of the hair root. Hence, the hair root is the correct answer as it is the segment of hair that is actually embedded in the skin, facilitating the growth and development of hair.