How does your hair know how long to grow?

As mammals we have hair on our bodies. Our hair grows to different lengths. Our eyebrows are short and the hair on the scalp can be a meter long. We will explore the mechanisms that determine how long your hairs grow.

First we need to discuss some of hair's characteristics. The only hair cells that are growing are in the hair root. The cells of the hair shaft are dead. The hair shaft is the hair that you see and cut. The outer cells are made of tough proteins. This protects the hair from wearing out. The cells on the inside are stretchy. This protects them from breaking off when you comb them. Some of these proteins are high in sulfur. You can smell this burning sulfur when hair burns. A single hair has a thickness of 0.02-0.04mm.

How many hairs next to each other would you need to equal one millimeter?

There are no nerve cells in the hair. The hair root has no way of knowing how long the hair is. Your eyebrows are always the same length. If they are shaved they grow back to the original length and then stay that length. Your eye lashes do the same thing but they are about twice as long. The hairs on your arm or leg "know" how to grow to their proper length. To understand how this works we need to learn more about how hairs grow.

When you brush your hair a few hairs come out. We lose between 50 and 100 hairs each day from our scalp. After the hair falls out the root rests a few days or weeks and then starts to grow again.

The growing phase is called anogen. The anogen phase is different lengths of time for different types of hair. Short hairs like eye brows only grow a few weeks. Your scalp hair can grow for years. 85% of your hairs are in anogen.

After the anogen phase the hair enters Catagen. Catagen is only lasts a week or 2. This is a transition period. The living cells of the hairs root slow down and change their structure. Only 1% of your hairs are in catagen.

Telogen is the final phase. Telogen is a resting phase. During telogen the hair doesn't grow. Telogen lasts a few months. 14% of hairs are in telogen. Some hairs fall out during Telogen. A few hairs don't fall out until the new hair starts to grow in the next anogen phase.

Human beings have about one million four hundred thousand hairs on their body. The average number of hairs on the scalp of an adult is 100,000. Blondes have more hairs, about 140,000. Redheads have the least with about 90,000.

Scientists have observed hairs on different parts of the body. They find these ranges of growth for different kinds of hair.

Scalp hair

• Anagen , grows 4 to 8 years
• Telogen, rests 2 to 4 months

Average growth rate (mm/day)

• Scalp 0.44(mm/day)
• Temple 0.39(mm/day)

Eyelashes. trunk, and extremities

• Anagen, grows 1 to 6 months
• Telogen, rests 2 to 4 months

• Average growth rate
• Coarse body hairs 0.27(mm/day)
• Eyebrows 0.15(mm/day)
• Mustaches (beards or whiskers) 0.4(mm/day)
• Armpit hairs 0.3(mm/day)
• Pubic hairs 0.2(mm/day

Hairs don't grow at a constant rate. Hairs grow at a faster rate in the spring and summer. They grow at a slower rate in autumn and winter. Can we use these numbers to explain that our hairs "know" how long to grow? Yes, we can use a model to help us explore the process.

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