What is the structure and composition of hair?

Hair has a complex structure. It is made up of 90-95% keratin, a protein that is the main component of the dander (hair, nails, horns, feathers, beaks). The colour of hair is produced by melatonin, a natural dye. 

A hair grows in a three-stage cycle: 

  1. Anagen phase or growth phase. It lasts from 2 to 5 years. 
  2. Catagen phase or resting phase. The hair stops growing. It lasts a few weeks. 
  3. Telogen phase or expulsion phase. The hair falls out. It lasts several months. 

Most of the hair is in the anagen phase, i.e. in growth. 

A hair consists of three parts: the bulb, the root and the shaft. 

  • The bulb is the "living" part of the hair. It consists of a small cavity including the papilla. The papilla is the place where new hair cells are formed. Hair synthesis takes place here. These cells will push the old ones along the pilosebaceous follicle, at an average rate of 1 cm/month, to form the hair shaft. The blood supply to the papilla is via arterioles and venules. 
  • The root is the internal part of the hair, from the bulb to the surface of the epidermis. It is surrounded by the pilosebaceous follicle. This part is not accessible during hair removal. 
  • The last part is the stem. This consists of the old cells that are keratinised. This part is considered "dead", i.e. biologically inactive. The shaft consists of three layers of cells: the cuticle, the cortex and the medullary region.

The cuticle, also called the epidermis, is the outer envelope of the stem. It is composed of 3 to 10 rows of cells called "scales" which are connected by a lipid-rich intercellular cement. The role of the epidermis is to protect the cortex.

The cortex, also called the bark, makes up 90% of the total weight of a hair. It is located between the cuticle and the medullary region. It is made up of cells filled with keratin, linked by a flexible, lipid-rich intercellular cement. It is in the cortex that melanin is found, which is responsible for the colour of the hair.

The medullary region constitutes the centre of the hair. It is not homogeneous in the shaft. In fact, it is present in a discontinuous manner in the hair and may be absent in certain areas.