“Why our eyebrow does not grow as long as the hair on our head ?”

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The purpose why there a lot of tiny hair over our is to protect our skin, the tiny hair will regulate the heat when we're cold. It also will help to cool down our body temperature when we're facing overheating.

All of our skin including our head is covered by hair, but what does the two hair strip above our eyes do. Is it cold over there and the two strip hair roles to heat the part of skin over there? For your information, the presence of our eyebrow above our eyes is to make sure sweat from our head not fall into our eyes. Now, you know why you have eyebrow right, but we still don't know why our eyebrow not growing longer as our hair on the head. There are many kinds and shapes of the eyebrow, yours is different from others eyebrows. But, it is very rare if the eyebrow grows longer than a few millimeters. There is a man from China, Zheng Shusen the Guinness World Record holder. He had a single 19.1cm eyebrow. Very long right? There is the picture of the long eyebrow man above.

There are three different phase will go through by our hair , Anagen(growing), Catagen(Regressing) and Telogen(Resting) phase. There are millions of little hair pocket on the scalp, it is called as follicles. The follicles function is where the proteins and cells build the root of the hair.

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Anagen Phase

Anagen is growing phase for the hair, the hair cells in the root divide rapidly in this phase, so hair will grow. In 28 days, our hair will grow 1cm or 0.5cm or shorter during the Anagen phase, the hair growth depends on your gene also. For your information, the scalp will stay in this active phase for a few years. About 85% of our hair on the head is undergoing this process.Those are who have long hair, these people actually will have longer Anagen phase, that's why their hair can grow very long. There will be an unknown signal, this signal will cause the change of the phase from Anagen to Catagen Phase, and this happens at the end of the Anagen phase. But, this Anagen phase can be shortened for people especially who have calorie-restricted diet, stress, childbirth or any traumatic event. There will be a process called Telogen Effluvium, this process occurs when the hair follicle starts to go into Telogen phase at the same time, this will cause the diffuse hair loss. There also a syndrome in Anagen phase, called Loose Anagen Syndrome. This syndrome often happens in some children. Their hair can be easy to pull out and their hair also sparse. But, this syndrome may be caused by an inherited condition and this syndrome can be recovered itself as the children grow.

Catagen Phase

Of course, this is stage is happening next Anagen phase and this stage occurs at the end of Anagen phase. This phase is a signal to the hair for stop the active growing. This phase lasts for two to three weeks, in this phase, our hair will be converted into Club Hair. The club hair form when the hair follicle and the lower portion of the hair contact each other, this will cause the lower part of the hair to attached to the hair shaft. Blood supply and the cells that produce a new will be cut off and this process will occur for 2 weeks. After that, the hair follicles will be undergoing the final phase, Telogen. During this second phase, the hair follicles will shrink quickly and this is happening because to encourage the upward push and this will help the hair to grow.

Telogen Phase

This the last process for the hair cycles and this is the third phase of the hair's cycles. This process will last for a few month(3-5 month). During this phase, our hair will stop growing and our hair is fully keratinized. Our hair doesn't need nutrient because it is no longer needs to sustance and it is fully grown. During this stage, the hair follicles restructure and relocates its cells. The new hair follicles start to expand and grows. This condition will weaken the base of the hair, and this weakness makes the hair can break away and leave the scalp easily. So, hair can pass the follicle easily because of the enlargement of the follicles. This process is also known as club hair loss or shedding.

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There will be a major difference between eyebrows and our hair on the head. The eyebrows Anagen will only growth to a certain length and this growing stage will last less than a month, then it will moves to the next phase, Catagen phase. The eyebrows growing cycles takes only 4 months to complete their cycles. Compare to the scalp it needs 3-4 years to finish the hair growth cycles. So, that's why our hair on the head is longer than eyebrows. The epidermal growth factor is a chemical signal, and this chemical signal will control the growth cycles. There also will be a regulator of the cycles and the hair follicles, it called DLX3.

To protect our eyes from the sweat it is important to have the two strip. The short Anagen phase for an eyebrow is to make sure the eyebrow does not make a contact with our eyes, it will make us pain if we wake up from sleep and the eyebrows poke our eyes, that's why it does not grow long as the hair on the head and you no need to cut it regularly and comb it.Thanks for reading and keep following My Biologist's Adventure.

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Reference and Further Reading

TOPIC	WEBSITES
Wikipedia Hair Follicles	https://en.wikipedia.org/wiki/Ageing
What cause Anagen ?	https://www.verywell.com/what-is-the-anagen-phase-of-hair-growth-1069411
DLX3	https://en.wikipedia.org/wiki/DLX3
Why do eyebrows stop growing ?	http://www.sciencemadesimple.co.uk/curriculum-blogs/biology-blogs/why-do-eyebrow-hairs-stop-growing
What is Telogen ?	https://www.hairtransplantmentor.com/telogen-phase/
What is Catagen ?	https://www.hairtransplantmentor.com/catagen-phase/
Longest Eyebrows Guiness World Record	http://www.guinnessworldrecords.com/news/2016/2/eighty-one-year-old-chinese-man-has-longest-eyebrow-hair-in-the-world-417851
Video Source	https://www.youtube.com/channel/UCpyGoMXd9txkMG9dU0jlj4g
Image Source	https://www.hairtransplantmentor.com/catagen-phase/
Image Source	https://en.wikipedia.org/wiki/DLX3

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