The Science of Hair: Hair Growth Cycle & How Hair Works

The Science of Hair: Hair Growth Cycle & How Hair Works

The process of hair growth and loss is a complicated one. For people suffering from unusual hair thinning or loss, it is important to understand this process before taking any remedial action. In this article, we will talk about the science of hair and the different stages of the hair growth cycle.

The different parts of the hair

Hair is made up of two major parts; the Follicle and the Shaft.

Hair Follicle

The follicle is a tunnel-like structure that runs through the epidermis down to the dermis. This is the part of the hair under the skin and is comprised of different layers. At the base is the papilla, which contains tiny blood vessels for nourishing the cells. Surrounding the papilla is the bulb, which is the living part of the hair and the cells inside it divide every 24 to 72 hours.

The follicle also consists of two sheaths surrounding it. These sheaths protect and form the growing hair shaft.

At the end of the follicle is a muscle called erector pili, which causes the hair to stand up when it contracts. It also causes the sebaceous gland to secrete oil (sebum). This oil serves as a natural moisturizer and conditioner and is responsible for the sticky feeling you get when you haven’t washed your hair in days.

Hair shaft

This is the part of hair you can visibly see. It is made of keratin, a hard protein that is dead. The shaft is made up on three layers; medulla, cortex and cuticle. The medulla and cortex, i.e. the inner-most layers, are responsible for giving your hair its distinctive color.

It is interesting to note that a person does not form new hair follicles during their lifetime. All of them (around 5 million) are formed by the time a fetus is 22 weeks old. About one million follicles are formed on the head, with about 100,000 residing on the scalp. Also note that one follicle may grow more than one hair shaft.

The hair growth cycle

Most mammals have a cyclic or seasonal hair growth and shedding cycle. Humans don’t – they experience growth and shedding randomly. Hair goes through four major stages of growth and shedding. Each of these stages lasts for a different duration of time, and the hair on your body will be in all the four different stages at any given time.

1. Anagen (growth) stage

This is the active hair-growth stage. Depending upon a range of factors (environmental, genetic etc.) this stage can last anywhere between two to six years. The longer the length of this phase, the longer your hair will turn out to be. Remedies that promise to ‘lengthen’ your hair attempt to prolong this stage, with varying degrees of success. But it’s not something that can be altered very easily.

2. Catagen (transition) stage

Once a hair follicle completes its growth, it enters the catagen or transition/regression phase. About 3% of your hair are in this stage at any given time. The outer root sheath shrinks and attaches to the root of the hair. This causes the hair to be detached from the skin, although it may be some months before the hair actually falls. This stage usually lasts for 2-3 weeks.

After each hair follicle completes its anagen phase, it enters the catagen, or regression phase. A hair follicle in the catagen phase will shrink slightly in size and detach from your skin, starting its process of falling out.

3. Telogen (resting) stage

This is the where a shrunken follicle will enter a ‘resting’ stage and will wait to be pushed out of the scalp by the new hair replacing it. About 10% of hair follicles are in this stage at any given time. However, people suffering from depression, stress or other medical conditions have a higher percentage of their follicles entering this stage prematurely, causing a temporary hair loss and shedding.

This stage can last up to 5 months, which is why the balding effect of stress or depression might not manifest itself right away. In many cases, patients report hair loss and balding months after suffering from mental stress or trauma.

4. Exogen (shedding) stage

This is the last stage where the hair has not only detached from the body, it has also been completely pushed out of the scalp by new hair. It is normal for about 50-150 hair to be in this stage each day, and these are the ‘fallen’ hair you see when you use a brush or comb or wash your hair.

Understand how your hair works

Whether you want to save yourself from going bald or get rid of excess hair on your body (such as on the arms, legs and pubic areas), it is important to understand how the hair works. The lifecycle of hair can be interrupted and manipulated, just as with any other process in the human body.

Environmental factors such as illnesses and stress can cause hair to shed more quickly. Lack of protein can cause hair to enter telogen phase prematurely, causing unusual hair fall. Certain medication can help mitigate or advance hair loss, depending on what you want to do. Hair growth creams, for example, attempt to prolong the growth phase whereas hair removal creams cause hair to shrink and enter the telogen phase quicker.

These days, portable laser-based at-home hair removal devices exist that can get rid of unwanted hair from your body. If you are more worried about not losing your (facial or scalp) hair, however, you should consult with your dermatologist to give you a hair growth, hair retention or hair health solution.


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