We view our hair very differently from most of our fellow mammals. We pluck it and shave it, bleach it and color it, and curl it and straighten it as an expression of individual style. But for all the consideration and attention our hair seems to get, most of us don’t know very much about its origins and primary functions. To see hair in its entirety, you have to first go to the roots. This article covers hair structure and function, hair types, growth of hair and the hair cycle.

 

1. Hair Structure and Function

In mammals, hair is the collective name for all the fine, thread-like filaments that grow outward from the epidermis. When hair is very dense, as it is in many animals, it is usually called fur or wool. Each tiny hair filament grows outward from a deep, pouch-like depression within the epidermis called a follicle, and extending into the follicles is a group of capillaries and nerves called the papilla. The papilla is the center of hair growth; it is the blood supply of the hair and contains the cells that form individual hair shafts. As cells at the base of the hair divide, they force cells above them to move upward. Gradually, the cells moving upward die and harden into the hair shaft, which is composed of two layers, the cuticle and the cortex. The outer cuticle layer is made up of flat cells that overlap one another kind of like fish scales. Below this is the cortex. Cortex contains all the hair’s pigment and a tough, fibrous protein called keratin.

Hair is characteristic of all mammals, and all mammals are built with the structural components needed to produce hair. The increased density of hair we see in mammals living in colder regions indicates that the primary outward function of hair is in regulating body temperature by insulating the body. The exceptions to this general rule are animals that have been bred to reduce or eliminate this visible hair. Some hairs, like the whiskers on cats, have a specialized sensory function and are extremely sensitive like our fingers. Other functions of hair include camouflage and protection against harmful particles and pathogens. Keeps harmful particles suspended in the air like dust and sand from entering the body, and plays a part in the removal of pathogens within the body as seen in the very rapid growth of hair from moles. Plants and insects also have outward-growing filaments that serve similar functions as hair in mammals. In plants, these “hairs” are called “trichomes”, and in insects and spiders they are called “insect bristles”.

In mammals, hair is the collective name for all the fine, thread-like filaments that grow outward from the epidermis. When hair is very dense, as it is in many animals, it is usually called fur or wool. Each tiny hair filament grows outward from a deep, pouch-like depression within the epidermis called a follicle, and extending into the follicles are groups of capillaries and nerves called the papilla. The papilla serve as the hair’s blood supply and provide the cells that form individual hair shafts. Hair is lubricated by sebaceous glands within the hair follicles.

 

2. Hair Function in Mammals

All mammals are built with the structural components needed to produce hair. The increased density of hair we see in mammals living in colder regions indicates that the primary outward function of hair is in regulating body temperature by insulating the body. The exceptions to this general rule are animals that have been bred to reduce or eliminate this visible hair. Some hairs, like the whiskers on cats, have a specialized sensory function and are extremely sensitive like our fingers. Hair prevents particles suspended in the air -- like dust and sand -- from entering the body, and can serve also in the removal of pathogens from within the body, as seen in the very rapid growth of hair from moles. Other functions of hair include camouflage and protection against the harmful ultraviolet component of sunlight. Plants and insects also have outward-growing filaments that serve similar functions as hair in mammals. In plants, these “hairs” are called “trichomes”, and in insects and spiders they are called “insect bristles”. 

 

3. Human Hair Types

The most noticeable hair on humans is the hair on the head, which shows far more variations in texture and color and can grow much longer than the hair on other mammals. It is commonly theorized that the long head hair characteristic of humans has evolved as a byproduct of secondary natural selection, serving as a kind of adornment after other types of somatic hair in humans disappeared. Another theory is that this characteristic is the result of runaway sexual selection, in which longer hair would serve as an indication of an individual’s health. The absence of head hair in humans is called alopecia, commonly known as baldness. 

Other types of hair on the human body can vary greatly in their occurrence and appearance according to the age, sex, health, or race of the individual. Beginning in childhood and regardless of sex or race, fine vellus hair covers much of the body. The only places on the body not covered by vellus hair are the lips, nipples, palms, soles of the feet, naval (along with any other scar tissue), and some external genital areas. The thickness, color, and density of these hairs vary from one person to another. 

During puberty, increasing levels of male hormones called androgens (primarily testosterone and its derivatives) trigger hair follicles to transform the fine vellus hair into terminal hair on many parts of the body for both sexes. The terminal hair in these areas is generally characterized by an increase in both thickness and the rate of growth and is often called androgenic hair. As testosterone levels in the body increase, the appearance of androgenic hair changes in response to androgen sensitivity. Most sensitive are the pubic areas, which are generally the first to respond to changing androgen levels through an increase in hair thickness. 

Both men and women exhibit terminal hair growth in the underarm and pubic areas as androgen levels increase during puberty. However, in general only men grow androgenic hair in other areas, in particular on the face, chest, abdomen, legs, and arms. Females usually maintain the finer vellus hair in these areas, both during puberty and throughout adulthood. Increased androgenic hair on women in these areas is referred to as hirsutism, while excessive and abnormal hair growth on these and other areas (for both sexes) is called hypertrichosis. 

Hair color and texture are inherited characteristics. Hair color, or pigmentation, can lessen with illness or stress and eventually cause the hair shaft to whiten. The normal whitening of the hair that occurs with increasing age, however, is largely determined by heredity. Individual hair shape and texture varies greatly with race. Native Americans, Eskimos, and Mongoloids exhibit very straight hair that is almost perfectly round in cross section. Thick, kinky hair that is very flat in cross section is characteristic of Negro populations, and wavy or curly hair that is oval in cross section is commonly exhibited among Caucasians. 

 

4. Human Hair Growth Cycles

Individual hairs exhibit varying cycles of growth and dormancy. During growth phases within these cycles, the hair follicles elongate and their shape becomes more bulbous. Growth phases generally last three to six months, with hair growing about one third of a millimeter per day throughout. At the end of each growth phase, the follicles shrink and the hair root grows rigid, signaling the beginning of a dormancy phase. When another growth cycle begins, new hairs push the old ones out of the follicles as they grow. Head hair is again different from other types of hair in that it grows for long durations and to very long lengths before it is shed. The rate of growth for head hair is about 1.25 centimeters (about one-half inch) per month, and head hairs are usually shed every two to four years.

The hair that gives the beauty and the looks to the people is nothing but fibers made of keratinized protein emerging from the hair follicle and are called the hair shaft. In the process of the growth of the hair from the follicles it loses its nucleus and in fact is no longer alive. This is the actual hair that is visible outside. The outer portion of the hair emerged out of the follicle known as the shaft has unique construction. It has three layers, the outer one covering the keratin scales is known as the cuticle, the middle layer that consist of keratinized protein fibers is called the cortex and the third layer of round cells is called the medulla. This is the constitution of the outer hair or the hair shaft. The third layer described above that is the medulla is usually found only in coarse hair and not observed in blonde and fine hairs.

As far as the chemical constitution of the hair is concerned, it consists of around ninety one percent of protein and is made up of amino acids. These chains of amino acids are included within the fibers of the cortex. The Amino acids on the other hand consists of elements like carbon, hydrogen, oxygen, nitrogen and sulfur, the sulfur being the building blocks for the nails as wells the skin. These elements are further joined by the peptide bonds the chains of which are also known as polypeptide chains.

The polypeptide chains mentioned above are further joined by side bonds. These side bonds are usually of three types’ viz. salt bonds and hydrogen bonds and disulfide bonds. The first two are comparatively weak and the most common. It is these bonds that allow the change in the amount of curl in hair by methods like flat iron, and wet rollers etc. The third one that is the disulfide bonds are much stronger and can be broken by relaxed or permanent wave service to make change in the curl. These sides contribute in equal proportion to the strength of the hair.

 

5. How the hair is formed?

The hair is grown from the follicles with the skin and the part that is inside the follicle is called the hair root. The hair bulb is situated at the hair root and is responsible for receiving the nutrients and formation of new cells. The hair that we see protruding from the skin is the hair shaft. It would be better to understand the function of dermal papilla that is found within the hair follicle that is cone shaped. The function of dermal papilla feeds the blood and the nutrients to the hair bulb. The sebaceous gland keeps the hair oily and shiny. The arrector pili are the tiny muscle at the base of the follicle that is sensitive to stimulus like scare, excitement or cold and make the hair stand upright.

The process of hair growth can be explained as follows. Using the nutrients from the dermal papilla the hair bulb is responsible in generating the new hair cells, and moves through the root of the hair and the process of keratinization happens in which the hair is filled with fibrous protein and loses its nucleus. The cell is no longer is alive the moment it loses its nucleus and the hair that protrudes from the skin is merely fiber of keratinized protein. 

 

6. Hair Growth Cycle

It is better to have a first hand knowledge about the hair growth cycle to appreciate better the reasons for the problems concerned with the hair such as hairless patches and total baldness in both men and in some unfortunate women.

The hair is formed in the hair bulb situated at the root of the hair deep in the hair follicle. It is the hair follicle that is responsible for the production of the hair. The follicle continues to produce hair from the birth of a person until his death. The hair lasts and grows for very longer periods and the same is tortured by shampooing, cut conditioned and exposed to cold and heat and affects only the hair shaft and the hair bulb and the root remain strong. The hair final falls out, leaving the follicle to produce another one after a short break. The process is called the hair cycle.

7. The various stages in the Hair cycle

The hair passes through three different stages namely the growing; the intermediary and the shedding phase each known by distinct names called anagen, Catagen, and telogen. Let us look into these stages.

Anagen (the growing phase)

The growing phase of the hair is known as the Anagen at this stage there is active growth in hair bulb. The stage last for a few years says between three and seven years. The hair grows at this stage without interruption and the normal growth is around I centimeter per month. The growth of the hair also varies according to the season. While in winter the growth is more the same is summer is comparatively less. It is at this stage the hair gets the pigment. But usually the pigment is less in the older people and the hair turns gray but the hair may still be strong in them. In some cases the hair cycle gets shortened in older people and the hair may become thinner and may eventually lead to baldness also.

Catagen and Telogen

Catagen is the second stage in the hair cycle growth that lasts between two and four weeks. The follicle stops producing the hair at this stage, the base of the hair follicle moves upwards to the surface of the skin. The catagen is followed by the telogen that lasts for three to four months. This is followed after the resting period of the follicles when it does not produce any hair and thereafter a new hair begins to grow from the hair follicles. As the new hair grows the old one is naturally shed. The hair at this stage is very weak and may fall out while shampooing or combing. This is a natural process and there is nothing to worry on this phase as usually one in ten hair is shed in this process and new one are grown. The new hair also emerges from the same openings and the cycle repeats.

The hair growth is a natural phenomenon and the nutrients and the health of the hair follicle play an important role in the growth and life of the hair.