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.