- The cuticle, which is the outermost layer consisting of scale-like overlapping cells five to twelve deep. It is formed from dead cells, and gives the hair shaft strength and provides protection for the softer inner structures. The cuticle can be damaged by excessive use of heat (such as straightening irons) and by chemical damage (such as colouring, etc). Everyday elements such as the sun or wind, brushing, and harsh handling will also damage the cuticle.
- The cortex, which is the fibrous middle layer of hair consisting of keratin-filled, elongated cells that are cemented together with lipid-rich, flexible keratin. This gives the cortex the properties of elasticity and a high tensile strength. The cortex contains the pigment (melanin) that gives hair its colour.
- The medulla, which is the innermost layer of the hair structure consisting of sparse cells and air bubbles. Its role in humans is unknown, however in animals it helps to control body temperature.
The Hair Growth Cycles
There are three stages in the hair growth cycle. The entire cycle can be as short as one year or as long as seven years, with the average growth cycle being four years. Many factors can interrupt the cycle and force the hair into the Telogen stage (hair shedding); this differs for everyone. It is understood that the protein fgf-5 triggers the hair to change from Anagen to Catagen and then to Telogen.