How does a hair get its shape?

Curly? Wavy? Straight?

Why do some people have straight hair and others have curly? Why do some people have very thick hair and other people have very thin? A part of the hair follicle deep under the scalp known as the inner root sheath (IRS) is responsible for the shape of a person's hair.

Underneath the scalp, the hair fiber is surrounded by the IRS. The IRS in turn is completely surrounded by the outer root sheath. The IRS is a very rigid structure and one can imagine the IRS as being similar to the circular opening of a tube of toothpaste (see picture). The opening is rigid and as one squeezes the tube of toothpaste a cylinder of gooey toothpaste comes out. If the opening of the toothpaste container was a different shape rather than a circle, it's easy to imagine that a different shape would be created. Some people have an IRS that is very circular (...and they tend to produce straight hair!) whereas other people have an IRS that is more oval-shaped and even crescent-shaped (...and they tend to produce wavy and curly hair!). When the hair follicle is first manufactured deep under the scalp it is very soft - and the IRS helps guide the new shape of the developing follicle. Only after a few days time does the new hair follicle harden up (keratinize) and take on the qualities that we all know when it finally emerges from the scalp.