The portion of the hair that we can see is called the shaft. Each shaft of hair protrudes from its follicle, which is a tube-like pouch just below the surface of the skin. The hair is attached to the base of the follicle by the hair root, which is where the hair actually grows from and where it is nourished by blood capillaries. Like the rest of the body, hairs are made of cells. As new cells form at its root, the hair is gradually pushed further and further out of the follicle. The cells at the base of each hair are close to the blood capillaries, and are living.

It is the follicle, which lies deep in the skin that is essential in growing hair. Also, the thickness of each hair depends on the size of the follicle from which it is growing. Hair growth is not a continuous process: it has several stages.

Hair grows in a cyclical manner. There are three distinct phases referred to as anagen, catagen and telogen.

The hair follicle has portions that are permanent and portions that are transient. During the catagen and telogen phases the transient parts are lost and are later regenerated.

In the lifetime of a hair follicle it is thought that the hair cycles perhaps ten to twenty times. The individual phases and cycles will vary from person to person and from hair to hair.

The anagen or growth phase lasts from about three to ten years. Catagen lasts approximately two to three weeks. Telogen is viewed as the final stage of the hair cycle and lasts three to four months.

At any given time it is estimated that the normal ratio of anagen to telogen is approximately 80-90% anagen to 10-20% telogen.

Hair loss is essentially divided into two types of hair loss; non scarring and scarring alopecias. Alopecia is simply the medical term for hair loss no matter what the cause is.

Non scarring hair loss would include male pattern hair loss, female pattern hair loss, alopecia areata, telogen effluvium and anagen effluvium

Scarring alopecias would include hair loss as a result of  skin diseases such as lupus erythematosus, lichen planopilaris. Scarring can also occur due to direct injury such as physical trauma caused by chemicals, radiation, thermal burns or traction on the hair. Various skin cancers and growths can also cause hair loss.

Infections with bacteria, fungus and viruses can also result in hair loss that can be scarring.

Recently, scientists at Columbia University in New York announced the discovery of a gene that appears to be the master switch for hair growth. They found the gene after comparing the genes of hairless mice belonging to a mutant breed, and comparing the genes of 11 members of a family who had lost all their hair. This discovery is a step towards understanding how the hair follicle works and how baldness happens, and may lead to effective treatments becoming available in the future.