Hair covers almost every surface of the human body. It provides insulation, traps dust particles and other tiny debris near the eyes and ears, and helps regulate body temperature. It can also be expressively styled, which is why so many people spend time on their hair every day. Hair even provides important clues about our biological sex, age and ethnic ancestry.
The most familiar form of human hair is the one that grows from your head, but there are two types of hair that grow everywhere on the body. Vellus hairs are short, soft, fuzzy hairs that cover most of the body and serve mostly as insulation. Terminal hairs, which are coarser and darker in color, grow on the head, neck and shoulders.
Both kinds of hair are made from the same protein, keratin, that gives strength to nails, hooves and feathers. The difference is that your hair cells are alive and surrounded by an outer layer of living tissue called the cuticle.
Within each hair follicle (which is like a mini-scalp) are stem cells that multiply rapidly, and as they do they make a protein called keratin to harden the structure. As these cells are formed they are pushed upward through the epidermis to form the hair shaft, which sticks up above the skin’s surface. The shape of the follicle determines whether your hair is straight or curly. The curved shape of the follicle creates wavy hair, while the flat shape results in straight hair.
At the base of each hair follicle is a bulb housed in a structure called the dermal papilla, which contains nerves and blood vessels that nourish the living hair cells. The follicle is surrounded by sebaceous glands, which produce oils that protect the growing hair from external substances.
As the hair is pushed upward through the follicle, it passes through a small hole in the skin’s surface. This is the point at which it becomes a separate structure from the rest of the skin, and it is this point that is determined by the shape of the follicle — a circular follicle creates straight hair, while an oval follicle produces wavy or curly hair.
After the hair has grown out of the follicle, it is covered by a thin layer of skin called the epidermis. The epidermis is slightly thicker than the underlying keratinous tissue, and it is this thickness that helps give hair its strength and resilience. The keratin proteins are tightly packed together and fused to each other, which is what makes the hair so resistant to damage.
The outer layers of the hair are arranged in three distinct structures, the cuticle, cortex and (in some cases) the medulla. The cuticle is an outer shell of overlapping cells that look somewhat like fish scales or roof tiles, and it provides both protection and shine to the hair. It is also a very important barrier that keeps moisture in and out of the cortex, maintaining its hydration balance and flexibility.