Human Hair

HUMAN HAIR

PKGhatak,MD

The hair is an appendage of the skin. The entire skin surface is covered with hair except on the palms of hands and soles of feet. Human hair is subdivided into two categories.

1. Terminal Hair. The coarser hairs that are present on the scalp, eyebrows, eyelashes, nostrils, under the armpits, pubic area; and in men mustaches and beards.

2. Body Hair. The finer hair covers the rest of the body.

Each hair has a bulbous end called a hair Follicle located in the deep layers of skin. It is supplied with nerve fibers and blood vessels. The part of the hair that remains underneath the superficial layer of skin (epidermis) is called the Root of the hair. The part of the hair on the top of the skin is called the Shaft. The cells of the hair follicles are responsible for the growth and maintenance of hair.

Under the microscope, the hair has loosely packed cells in the center called the Core. The core is surrounded by several layers of densely packed cells containing pigments, and a protein called Keratin; the outer layer of hair is made up of a single layer of cells called Cuticle.

The hair is surprisingly strong for its size. 100 strands of hair can support 20 lbs. of weight without breaking. Hair is wettable, soaks up water about a third of its weight. With air sacs in the core and water retaining property, the hair protects the head against strong sun and chill winds.

There is a small gland underneath the skin at the point where the hair emerges from the skin. It secretes a tiny amount of oil and the oil keeps hair shiny and silky looking. This oil coats all surfaces of straight hair evenly; in persons with wavy or kinky hairs the oil cannot get to the entire shaft uniformly and these types of hair look dry and disorderly.

The body hairs grow slowly, whereas scalp hairs grow about 1 inch in 3 months. The rate of growth is slower in older people and fastest in adolescence. Growth takes place at the root of the hair, and as hair grows it sprouts out of the skin surface. Each individual hair has its own life cycle; independent of its neighbors. It decides when to grow, when to rest and when to die and fall off. And then it begins to grow again and the cycle continues. Because we lose so few hairs at a time that we do not notice the hair loss. Hair on the scalp grows to shoulder length in 3 years and reaches waist length in 5 years. Generally, after 6 years growth stops and hair rests for a period of time and at the end of 6 to 7 years it dies and falls off. During its lifetime, a hair may grow 5 feet in length if not trimmed. In the Guinness Book of Records, you may find evidence of longer hair. Body hair has a shorter life and growth remains limited.

The keratin of hair gives hair its flexibility and strength. It is made of amino acids of which Cystine has a disulfide bond. This disulfide bond gives keratin its special properties. When hairs are treated with alkaline agents this disulfide bond is weakened and curly hairs could be strengthened or straight hair may be given a curly shape. Then an acidic agent is applied to neutralize alkali and to make the changes last longer. The color of hair is due to the presence of a pigment called Melanin produced at the root of the hair by cells called Melanocytes. Two types of melanin are produced by these cells. Long arms of these cells deposit the pigments in rows, like a string of pearls, in the cortex cells of the hair. One color is black and the other is red. By the various combination of these two pigments, hairs get various shades of black, brown, yellow and red. The gray color of hair is due to the presence of only a few melanin pigments, non-pigmented hair looks yellowish due to the color of keratin.

The terminal hair depends on Androgen for its regulation of longitudinal growth, thickness, and vigor, whereas Estrogen slows the growth rate but prolongs the growth period. The thyroid hormone maintains its health. Body hair depends mostly on thyroid hormone for its health and growth. Zinc and B vitamins, particularly pantothenic acid, is required for the nutrition of hair. Steroid hormone may slow or weaken roots of hairs when used for a prolonged period. Certain chemicals like arsenic are taken up by the hair and remain unchanged in hairs for years even after burials. Drugs used in cancer treatment may arrest the growth of all fast growing cells of the roots of hair all at the same time, making hairs fall off in 2 to 3 weeks. But after completion of treatment hair will grow back. Thallium used in cardiac stress laboratories may cause hair to fall for the same reason. Many other drugs may influence hair one way or other.

The roots of the hair are the living part of the hair. The shaft of hair is a dead tissue. The DNA of hair cells can help identify an individual.

In hair, we admire the texture, wave, color, and silkiness - all about a dead tissue and the living part is unseen and remains buried under the skin.

 

 

revised 2020

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