The hair that you see on the skin is actually dead. The living portion of hair is the follicle which lies embedded in the dermis (Fig. 1). Let us examine the hair and its follicle to understand what hair is made of and how it grows.

Hair is comprised of three layers-an inner core called the medulla, an outer layer called the cuticle and a layer in between called the cortex (Fig. 2).

The medulla is composed of round cells and was at one time wrongly believed to be the lifeline of hair through which nutrients from the body reached all parts of the hair. This misconception even led to the practice of singeing the ends of hair to prevent the escape of "sap" from the medulla. The outer layer, the cuticle, is composed of transparent overlapping cells and serves as a protective layer for hair. If the "imbrications" or the spaces between the overlapping layers of the cuticle are open, then hairs tend to get entangled. Alkaline substances such as soap tend to open the imbrications whereas acidic substances close them down. Closed imbrications make hairs smooth and more manageable.

Indeed, the key to making hair look good is ensuring that the cuticle is in good condition. Most hair care products affect the state of the cuticle and this is what hair care is really about.

The cortex or middle layer forms the greater part of hair. It is composed of tightly cemented cells and most of the hair's pigment, melanin. It also provides strength and elasticity to the hair.

Hair protrudes through small pores in the skin and is produced by the hair follicles embedded in the dermis (see Fig. 1). The base of the follicle comprises an inner portion known as the hair bulb, which contains actively dividing cells. This surrounds another structure known as the papilla (see Fig. 2)

The papilla contains a rich network of blood vessels which supplies the actively dividing cells of the hair bulb with essential nutrients. New cells produced by the hair bulb are pushed towards the surface by still newer cells coming from below.

As these cells move towards the surface, they accumulate into a hard substance called keratin and die. These dead keratin laden cells form hair. Hair is, therefore, dead. It won't bleed, leak body fluids or hurt when cut. Keratin is also the same hard substance found in nails.

Halfway up the follicle is the arrector pili muscle.

Contraction of this muscle causes the hair to stand on end and form goose pimples. A sebaceous (oil) gland opens into the follicle just above the arrector pili muscle and produces an oily secretion called sebum. Sebum smoothens the overlapping cuticle cells, making hair shiny, smooth and less likely to get entangled. Too much sebum makes the scalp greasy.

The follicle goes through two main stages: anagen and telogen. There is also an intermediate stage called catagen, which lasts only a few weeks.

Anagen is the growth stage. During anagen, the hair bulb produces new cells and causes the hair to increase in length at a rate of half an inch per month.

The anagen stage lasts about five years (or a range of two to five years). Using these figures, we can calculate the maximum length hair will attain if it is not cut. Since hair grows half an inch a month for five years, the total length achieved will be 30 inches. However, some people have longer and some shorter anagen stages. People with longer anagen stages are able to grow longer hairs than others. The length of anagen also varies according to the area of the skin. Eyebrow hair, for example, has a short anagen stage of three months. This is why eyebrow hair does not grow very long.

The other stage is the resting stage or telogen. This lasts about five months (or a range of two to five months). Hair stops growing and is shed at the end of it.

An average adult between the ages of 20 to 30 years has 100,000 hairs. Of these, about 15% are in telogen and the remaining 85% are in anagen. In other words, 15,000 hairs are in telogen. If we assume that telogen is five months or 150 days long, then we can expect to lose 100 hairs per day.

(15,000 hairs divided by 150 days). This is why it is often said that a normal person can lose up to 100 hairs per day. However, this figure is merely a guide because the proportion of hairs in telogen and the duration of telogen varies from person to person. The normal range of hair loss varies from 50 to 100 per day.

In humans, the follicles develop out of phase with one another; meaning some hairs are falling while others are growing. This ensures that there is always hair on the scalp.

In some animals, the follicle develops in phase with each other in certain regions of the skin. At the end of telogen, the entire area becomes devoid of hair. This result_ in seasonal shedding of the hair. A similar situation may occur in humans when the follicles are prematurely pushed into telogen. Such a situation may occur after extreme stress, childbirth or surgery. This condition is known as telogen effluvium.

The sheep is an interesting animal. Its hair follicles are always in anagen and hair keeps on growing longer and longer, until cut. Unfortunately, our scalp hairs do not behave in the same way. Otherwise, we would not have to contend with some forms of baldness. Fig. 3 shows the follicle in different stages of development. The follicle is longest during anagen and shortens gradually until it reaches telogen. As the follicle shortens, the papilla becomes separated from the hair bulb. Below the hair bulb, a secondary germ forms. This divides to form a new bulb which grows downwards and encloses the papilla, forming a new follicle. The new follicle enters anagen and the cells of the hair bulb divide to form a new hair. The growth of the new hair dislodges the old telogen hair.

You should now understand the structure of hair and its growth cycle. Because the protruding hair is dead, no amount of nourishment poured onto it will make any difference. The growing portion of hair is the follicle which receives its nourishment from the blood vessels in the papilla meaning, from the inside rather than from the outside. This is why rubbing nutrients on the scalp does not make sense. However, the dead cells of the hair allows it to be cut, remodelled and coloured to whatever colour you desire without causing any pain or discomfort.