Hair is made up of different layers and structures.
It is the tough protein keratin that forms the strength of your hair, nails and the outer layer of skin. Keratin is a large molecule made up of smaller units called amino acids, which join together to form a chain.
The Hair Root
The epidermis, as shown in the diagram below, is the outermost layer of the skin. Each hair arises from an indentation on the epidermis. Hair is made up of two parts, the hair follicle and the hair shaft.
The hair follicle is the point from which the hair grows.
- The terminal part of the hair follicle within the skin is called a hair bulb.
- The hair bulb is the structure formed by actively growing cells.
- These cells produce the long, fine and cylindrically-shaped hair fibres.
- The hair bulb has special cells which produce the pigment that gives hair its colour.
- The pigment is called melanin and the cells producing it are known as melanocytes.
- Androgens, which are receptors for the male hormones, are located on the cells of this structure.
- The dermal papilla that feeds from the bloodstream provides nourishment to form new hair and is found at the base of the hair bulb.
- Within the skin, internal and external root sheaths cover the hair follicles.
- The outer rooting of a hair follicle has a continuous growth cycle along with the epidermis.
- Adjacent to the hair follicles are glands.
- The most important one of these glands is the sebaceous gland, as it produces and secretes the natural oils which lubricate the hair.
The part of the hair seen above the skin is called the hair shaft.
- The hair shaft is made up of dead cells that have turned into keratin and binding material, together with small amounts of water.
- This structure explains why we do not feel any pain whilst our hair is being cut.
- The hair shaft is formed of three layers:
- The medulla – the deepest layer of the hair shaft, only seen in large and thick hairs.
- The cortex – the middle layer of the hair shaft which provides the strength, colour and texture of a hair fibre.
- The cuticle – the outer layer of the hair shaft is thin and colourless. It serves as protection to the cortex.
Hair Growth Cycle
Each hair follicle lives in a cycle of a long period of growth, followed by a relatively short period of rest. During the rest period the hair is still attached to the hair follicle, but it is not growing. After the resting phase the hair is shed and a newer hair begins to grow, starting a new hair growth cycle.
Genetics determine hair follicles’ pattern of growth and rest. As we age, hair follicles are programmed to stop producing hair and spend more time in the resting stage.
The growth cycle of a hair consists of 3 stages:
- ANAGEN (GROWING) STAGE
- The anagen stage is the growing period of a hair follicle.
- This stage typically lasts about 3 to 5 years.
- CATAGEN (INTERMEDIATE) STAGE
- The catagen stage is the intermediate period of hair growth.
- Hair follicles prepare themselves for the resting phase.
- It lasts around 1-2 weeks.
- During this phase, the deeper portions of the hair follicles start to collapse.
- TELOGEN (RESTING OR SHEDDING) STAGE
- The telogen stage is the resting and shedding period of the hair cycle.
- It usually lasts 3 to 4 months.
- At the end of this period, older hairs that have finished their life will fall out and newer hairs will begin to grow.
- The anagen phase constitutes about 90% (1000 days or more) of the growth cycle.
- The catagen phase (10 days) and telogen phase (100 days) constitute only 10% of the hair growth cycle.
- During the catagen and telogen phase of the hair growth cycle, as hairs are at the shedding and rest-from-growth period, no bald spots are shown as hairs are randomly distributed over the scalp.