Hair - General Information

The skin is the largest organ of the body. It is made up of the epidermis which is the outermost protective layer, the dermis which has oil glands, sweat glands and hair arising from it. The lowest layer is below the dermis and is made of fat and connective tissue.

Hair follicles are the factories in which hairs are produced. The hair roots are deep in the dermis. The size depth and angles of the hair growth varies as does the colour.These differences depend on location, race, genetics as well as some hormonal differences

The hair follicle is the only organ in the body that can permanently regenerate. The hair is made up of dead keratinocytes that produce keratin. Most hairs grow in cycles. The hair cycles are called anagen (growth), catagen (regression), telogen (resting) and, lastly, hair shedding. The cycle is controlled by an internal "hair cycle clock". The nature of this is poorly understood. The hair cycle is influenced by endocrine, vascular and genetic control. The immune system also has an influence on the hair cycle.

The total number of hairs on the body would be approximately 5 million. About 100,000 hairs are found on the scalp. Blondes have 20% more hair while redheads tend to have 20% less hair than average. The African American and Asian hair density is lower. The hair cycle usually is made up of anagen hairs in 85-90% of hairs, telogen in 10-15% and less than 1% is in the catagen stage. The anagen phase usually lasts for two to six years; the catagen stage lasts for two to three weeks and the hairs in the telogen phase for about three months. There is significant variation in the length of anagen growth, depending on the location. The moustache anagen cycle lasts between 1-3 months whereas the anagen phase on arm hair is usually between 6-12 weeks and between 20-26 weeks on the legs. It is known that plucking of the hair will induce new growth of hair. Estrogen prolongs the anagen growth phase. Thyroxin also increases growth whereas corticosteroids slow down the onset of anagen.

The number of hair cycles in a lifetime varies between 10-20.

The shedding of hair from the scalp varies between 100-200 daily. There is some individual variation as well as seasonal variation.

The hair production is not influenced by shaving or cutting. The rate of growth is reduced by estrogens whereas androgens such as testosterone increase the hair growth rate as well as the diameter of the hair, particularly in the beard area.

Types of Hair

The shape of the hair shaft is different according to races. Asian hair tends to be a round circle whereas Caucasian hair is an elliptical shape. The oval shaped hair tends to become more curly.

Graying of hair usually starts in the third decade, starting at the temples and later spreading to the crown and to the back of the head. By the age of fifty, 50% of the population has gray hair.