Hair

Hair is a protein filament that grows from follicles located in the dermis, a layer of skin. It is one of the defining biological characteristics of mammals. Most human skin is covered in follicles that produce different types of hair, such as thick terminal hair or fine vellus hair. While many people focus on hair care or styles, hair is actually a vital biomaterial. It is primarily composed of a protein known as alpha-keratin.

To understand hair, we must distinguish between its two main structures. The hair follicle is the part located beneath the skin. It contains stem cells that can regrow hair after it falls out. These stem cells can also help regrow skin after a wound. The hair shaft is the hard, visible part that extends above the skin surface. The shaft is made of multi-layered, keratinized dead cells. These cells form rope-like filaments that provide strength and structure.

The hair shaft is organized into three distinct zones. The outermost layer is the cuticle, which consists of thin, flat cells that overlap like roof shingles. This structure slides as the hair swells. A single molecular layer of lipid covers the cuticle to help the hair repel water. The middle layer is the cortex, which is the primary source of mechanical strength and water uptake. The cortex also contains melanin, the pigment that colors the hair. The innermost region is the medulla, which is an open and disorganized area. The medulla is not always present in every hair fiber.

Hair color is determined by the distribution of melanin granules within the cortex. There are two main types of these pigments. Eumelanin is the dominant pigment found in black and brown hair. Pheomelanin is the pigment that makes red hair appear. Blond hair results from having very little pigmentation in the strand. As people age, melanin production may decrease or stop, resulting in gray hair. A condition called poliosis can cause white hair to appear in specific spots.

Growth is a continuous cycle involving three specific phases: anagen, catagen, and telogen. Each individual strand of hair is at a different stage of this cycle at any given time. Growth begins inside the follicle, which is the only living portion of the hair. The base of the root is called the bulb, and it contains the cells that produce the shaft. On average, hair grows about 1 centimeter per month. However, scalp hair growth can vary between 0.6 cm and 3.36 cm per month.

Texture and curl patterns are closely linked to the shape of the hair follicle. A round follicle produces a round hair fiber, which results in straight hair. Oval or other shaped fibers result in wavy or curly hair. This happens because the shape affects how disulfide bonds are arranged. In straight hair, these bonds are in a direct line. In curlier hair, the flatter shape allows more cysteines to compact together, creating bends.

Many scientists use the Andre Walker Hair Typing System to classify hair into four categories. Type 1 is straight hair, which is often shiny and oily because sebum spreads easily along the shaft. Type 2 is wavy hair, which sits between straight and curly textures. Type 3 is curly hair that often forms an S-shape. Type 4 is kinky hair, which features a very tight coil and high density. This type is often fragile because it has fewer cuticle layers.

Human hair characteristics also vary significantly by ancestry and diameter. The diameter of human hair ranges from 20 to 120 micrometers. People of African ancestry often have hair with a diameter of 60–90 μm and a flat cross-section. People of European or Middle Eastern ancestry often have a diameter of 70–100 μm and an oval cross-section. People of Asian or Native American ancestry often have a diameter of 90–120 μm and a round cross-section.