StructureHair consists 90% of a biological polymer, a-keratin, and about 10% water, which modifies its mechanical properties. This a-helically coiled protein is further wound into supermolecular coiled-coil microfibrils, many of which are held together with a protein glue to form long macrofibrils, which are packed inside dead hair cells about 100 Ám long by 3 Ám across. Several of these associate to form one strand of hair, which is covered with tiny surface scales. The ends of individual keratin chains are high in the amino acids proline (an a-helix breaker) and cysteine. Adjacent keratin chains are held together by many disulfide bonds bridging their cysteines. These links are very robust; virtually intact hair has been recovered from ancient Egyptian tombs. Different parts of the hair have different cysteine levels, leading to harder or softer material.
Hair consists of an inner cortex, comprising spindle-shaped cells, and an outer sheath, called the cuticle. Within each cortical cell are the many fibrils, running parallel to the fibre axis, and between the fibrils is a softer material called the matrix. It grows from a hair follicle.
The cuticle is responsible for much of the mechanical strength of the hair fibre. It consists of scale-shaped layers. Human hair typically has 6-8 layers of cuticle. Wool has only one, and other animal hair may have many more layers. Hair responds to its environment, and to its mechanical and chemical history. For example, hair which is wetted, styled and then dried, acquires a temporary "set", which can hold it in style. This style is lost when the hair gets wet again. For more permanent styling, chemical treatments (perms) break and re-form the disulphide links within the hair structure.
The diameter of a human hair ranges from about 18 Ám to 180 Ám. In people of European descent, blond hair and black hair are at the finer end of the scale, while red hair is the coarsest. The hair of people of Asian descent is typically coarser than the hair of other groups.
The cross-sectional shape of human hair is typically round in people of Asian descent, round to oval in European descent, and nearly flat in African peoples; it is that flatness which allows African hair to attain its frizzly form. In contrast, hair that has a round cross section will be straight. A strand of straight round cross-section hair that has been flattened, for example, with an edge of a coin, will curl up into a micro-afro.
The speed of growth is roughly 11 cm/yr = 0.3 mm/day = 3 nm/s. Cells at the base of the hair follicle divide and grow extremely rapidly. Drugs used in cancer chemotherapy frequently cause a temporary loss of hair, noticeable on the head and eyebrows, because they kill all rapidly dividing cells, not just the cancerous ones. Other diseases and traumas can cause temporary or permanent loss of hair, generally or in patches.
Hair is strong. A single strand can hold 100g (3.5oz) of weight. A head of hair could support 12 tonnes. It is equivalent in strength to aluminium or Kevlar. Wet hair, however, is very fragile.
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Types of hair
Hair change with aging