Hair is one of the defining characteristics of mammals. The human body, apart from areas of glabrous skin, is covered in follicles that produce thick terminal and fine vellus hair.

The most common interest in hair is focused on hair growth, hair types, and hair care, but hair is also an important biomaterial primarily composed of protein that has many uses.

Hair is made of a tough protein called keratin. A cross-section of a hair shaft reveals three layers. The innermost layer is the medulla; the middle layer is the cortex, and the outermost layer is the cuticle.

The cortex makes up the bulk of the hair shaft. The cuticle is a thin, tough outer layer composed of overlapping scales that protect the cortex and medulla from damage.

Hair growth occurs in cycles. The growing phase, or anagen phase, can last anywhere from two to eight years. This is followed by a brief transitional phase, or catagen phase, which lasts about two weeks.

The final phase is the resting phase, or telogen phase, which lasts around three months. At the end of the telogen phase, the hair follicle begins the anagen phase again and a new hair shaft is produced.

Hair regeneration is the ability of hair to grow back after it has been lost. This can occur naturally as part of the hair growth cycle or it can be induced through a variety of methods including surgery, medication, and lasers.

Hair loss, also known as alopecia, can be a result of a number of factors including aging, genetics, hormones, and illness.

Grey hair is caused by the decreased production of melanin, which is the pigment that gives hair its color. There are a number of ways to prevent or treat grey hair, including dyeing or bleaching, but the most effective method is to use a hair product that contains melanin.

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