What is the structure of hair and how does it grow?

Aside from a few places, like the palms of our hands or the soles of our feet, the entire surface of our body has hair on it. The two main types of hair are the shorter and thinner "vellus" hairs (peach fuzz) found on the body and the longer and thicker "terminal" hairs. Examples of terminal hairs include the hair on your head, facial hair, eyelashes, eyebrows, pubic hair, chest hair and belly hair.

How much of each hair type you have varies from person to person and also depends on your age and sex. Children’s bodies mostly have vellus hair, for instance. About 30 percent of the body’s surface is covered with terminal hair in women, compared to about 90 percent in men.

Each hair has a hair shaft and a hair root. The shaft is the visible part of the hair that sticks out of the skin. The hair root is in the skin and extends down to the deeper layers of the skin. It is surrounded by the hair follicle (a sheath of skin and connective tissue), which is also connected to a sebaceous gland.

Each hair follicle is attached to a tiny muscle (arrector pili) that can make the hair stand up. Many nerves end at the hair follicle too. These nerves sense hair movement and are sensitive to even the slightest draft.

At the base of the hair, the hair root widens to a round hair bulb. The hair papilla, which supplies the hair root with blood, is found inside the bottom of the hair bulb. New hair cells are constantly being made in the hair bulb, close to the papilla.

New cells are constantly forming in the hair bulb. These cells stick together and harden. The full strand of hair develops from this group of hardened hair cells. Because new hardened cells keep on attaching to the hair from below, it is gradually pushed up out of the skin. In this way, a single hair on your head grows at a rate of about 1 cm per month. Facial hair, and especially eyelashes, eyebrows and body hair grows at a slower pace.

Whether it is straight or curly will depend on the cross-sectional shape of hair. Round hair grows straight out of the skin. The more oval-shaped the cross-section is, the curlier the hair will be.

The color of the hair is determined by the amount of melanin in the hardened cells. This can vary a lot from person to person, and it changes over the course of a lifetime. The amount of melanin typically decreases as people get older, and more air gets trapped inside the hair – it then loses its color and turns white. Depending on someone’s original hair color and the number of white hairs that grow, the hair on their head then turns gray or white.

As long as new hair cells continue to grow in the hair bulb, the hair continues to grow longer. This growth phase is also called the anagen phase. At any point in time, about 90 percent of a person’s total amount of hair is in this growth phase.

Depending on where on the body a hair grows, the growth phase will last longer or shorter: For instance, the growth phase of hair on your head can last several years,, so it can grow to over a meter in length if you don't have it cut. The growth phase is especially short for eyelashes, eyebrows, nasal hair and ear hair. Those hairs only grow for about 100 to 150 days, so they can’t get that long.

At the end of the growth phase, the hair root separates from the papilla. Then a transitional phase called the catagen phase starts, lasting about two to four weeks. When the hair has separated completely from the papilla, the supply of blood is cut off in the final resting phase, which is also called the telogen phase. The hair is gradually pushed out of the skin and eventually falls out. The resting phase can last several months.

New hair cells then start to multiply at the base of the “empty” hair follicle to form a new hair, and the growth phase of the hair growth cycle starts all over again.

Because hairs continue to enter the resting phase and then fall out, we are constantly losing hair. A healthy adult may lose about 70 to 100 hairs on their head per day. But because new hairs are always growing and replacing them, this natural hair loss isn't noticeable.

The rate of hair loss may increase noticeably if the hair roots are damaged during the growth phase or if a lot of hairs go into the resting phase at the same time. If no new hair grows and replaces the hair, that part of the skin becomes bald. This type of hair loss is referred to as alopecia – regardless of how large the bald spot is or whether it affects the scalp or body hair. In some types of alopecia, the hair may grow back. But baldness can also be permanent – one typical example is gradual hair loss in men (male pattern hair loss).