Human hair is a protein filament that grows from follicles deep in the dermis. It can be found on most areas of the body, including the face, nose, ears, head, eyebrows, eyelashes, legs and armpits. The strands are usually thin and are composed of non-living cells primarily containing the protein keratin.

Hair is an important biomaterial that provides protection from weather and environmental stress as well as a sense of touch. It also acts as a lubricant and absorbs perspiration (evaporation).

The structure of hair consists of three key parts: the cuticle, the cortex and the medulla. The outer layer, called the cuticle, contains keratin and is about 60 micrometers long and 6 micrometers wide. Within the cuticle, the cortex is a highly keratinized layer, consisting of scale-like cells that overlap one another.

During the growth phase, the fibers form from the follicles in a cylindrical shape. The lower part of the follicle, called the bulb, is where pigmentation from melanocytes is incorporated. The follicle then goes through cycles of activity and rest, each lasting from a few weeks to several years.

Each hair strand is made up of a cluster of small cells that are responsible for the production of keratin and melanin. During the anagen stage, these cells divide into new keratinocytes.

At the end of this stage, the keratinocytes slough off and the medulla is replaced by a club-shaped follicle that is dormant for a time. Fibrous rootlets anchor the club to the surrounding follicular tissue.

A human has up to 100,000 to 150,000 strands of hair on their head and each strand can grow to a length of about 5 inches. The strands grow out in a pattern known as anagen, catagen and telogen phases.

The follicles in each of these stages have specific characteristics and functions that are critical for the hair to be functional. For example, the follicles in the anagen phase are capable of producing a variety of proteins to support growth and repair.

They can help regulate body temperature and act as a barrier to wind and dust. They can also trap and re-release toxins that enter the bloodstream through sweat and other bodily fluids.

Unlike other mammals, humans have an unusual form of hair that is called lanugo. This scaly, soft hair covers the unborn or newborn human. It is formed by division of cells in the region known as the bulb at the base of the follicle.

There are two main types of follicles in the human scalp, which can be distinguished by their shape and size: germ follicles and peg follicles. Germ follicles are smaller and have a globular shape, whereas peg follicles are larger and are cylindrical in shape.

The genes responsible for the development of the follicles and hair color are not well understood. MC1R is the gene that is most associated with hair color and is involved in determining the phenotype of melanocytes, which are the cells that produce melanin.