Hair

Hair

by Jasmine Chan 

“Oily Hair.” Sesderma, www.sesderma.com/ca_en/hair-body/hair-care/oily-hair/. 

INTRODUCTION

Humans have hair all around our body. We have it on our arms, our legs and of course, our head as well. In this article, I will look into hair from the scalp of our heads, and look into details of their function, composition, and how hair is unique to each person.

Humans have 80,000-150,000 hairs located on the scalp of their heads. Compared to other organisms in the world, humans are one of the only species that have a large concentration of hair in one designated spot - their heads. Humans have hair on top of their heads to prevent direct ultraviolet light and heat to their heads, to provide insulation and warmth to the head as well to provide cooling when sweat evaporates. [1] The hairs on our heads are classified as ‘terminal hairs’ where they are thicker, longer and more pigmented compared to vellus hairs (eg. hair on the arms). [2]

STRUCTURE OF HAIR

Fig.1 Structure of Hair.

“Structure and Composition of the Hair.” Activilong, activilong.com/en/content/95-structure-composition-of-the-hair. 

Hair is a derivative of the epidermis which is made of a protein known as keratin. Keratin is a protective protein, which is less prone to tearing and scratching. [3] Hair consists of two parts: the follicle and the hair shaft. 

The terminal hair follicle starts from the surface of the epidermis and extends into the dermis. Each hair is attached to a sebaceous gland and an arrector pili muscle. The terminal hair can be separated into three sections: the infundibulum, the isthmus and the inferior segment. The infundibulum is the upper section of the hair follicle. It starts at the surface of the epidermis and extends to the opening of the sebaceous duct. The isthmus is the area between the sebaceous duct opening and the bulge, which is the area in the hair follicle where the arrector pili muscle can be found. The inferior segment of the hair follicle is from the bulge to the base of the follicle and includes the bulb. The bulb contains the dermal papilla which is surrounded by the follicular matrix. The dermal papilla contains capillaries and the matrix contains keratinocytes (the primary type of cell found in the epidermis) which forms the hair shaft of growing hair. Melanocytes are found within the matrix to provide the hair shaft with colour. [4]

Fig.2 Example of Hair Growth Cycle.

“Hair Growth Cycle.” VectorStock, www.vectorstock.com/royalty-free-vector/hair-growth-cycle-vector-8282990.

The follicle has a continuous growth and rest cycle, known as the hair cycle which allows our hairs to grow. [2] The hair cycle consists of three steps: anagen, catagen and telogen. The anagen phase is the growth phase where most hair is growing at a given time. Each hair on the scalp spends a few years in this phase. The rate of growth varies from person to person. On average, hair grows about half an inch each month. Catagen phase causes the hair growth to slow down for a few weeks which causes the hair follicle to shrink. The telogen phase is the resting phase which means that the hair growth stops and detaches from the hair follicle as a new hair grows and pushes the old hair out. The cycle repeats over and over again. [5] 

The hair shaft has a medulla which is surrounded by the cortex, which makes up the bulk of the hair. Around the hair shaft, there is a layer of cells known as the shaft cuticle which forms the inner root sheath. The inner sheath plays a major role in shaping the hair shaft. [4]

The sebaceous glands open onto the hair follicles. Sebaceous glands secrete lipid-rich sebum that protects the hair and provides the skin with a hydrophobic barrier for protection when stimulated by a hormone called androgens.

NATURAL HAIR COLOURS

The natural hair colour is determined by the concentration of melanin present in the hair. Melanin is a term of for any pigment in the hair or skin which is derived from an amino acid called tyrosine. [6] The MC1R (Melanocortin 1 Receptor) is the gene responsible for determining hair colour which is primarily found on the surface of melanocytes. [7] Hair colour is determined by two types of melanin: eumelanin and pheomelanin which are produced by melanocytes. Eumelanins are produced when the MC1R is active, which results in dark hair. Eumelanins help to protect the skin (in this case, the scalp) from ultraviolet radiation from the sun. Pheomelanins are produced when the MC1R is inactive which means that this pigment is unable to protect the skin from ultraviolet radiation.

There are five main hair colour types: black, brown, blonde, red and grey.

Fig.3 Example of Natural Hair Colours.

Hoffman, Matthew. “Hair (Human Anatomy): Image, Parts, Follicle, Growth, Problems, and More.” WebMD, 18 May 2019, www.webmd.com/skin-problems-and-treatments/picture-of-the-hair.

Black hair colours are seen commonly from Asian and African countries which contain black eumelanin. This is because these people tend to have low concentrations of tyrosinase in their bodies. Tyrosinase is an enzyme responsible to convert tyrosine into a compound known as dopaquinone. [8] 

Brown and blonde hair are created due to ranging concentrations of brown eumelanin. As the brown eumelanin concentration decreases, the lighter the hair appears. There is a range of natural browns and blondes due to the presence of an allele at a specific position on the chromosome. For example, some blonde hairs are caused due to the presence of an allele which suppresses the enzyme production of brown eumelanins. [8] In addition, people with brown or blonde hair can have a range of undertones due to the production of pheomelanin in addition to the brown eumelanins present. Yellow-orange pheomelanin causes the hair to have a warm, yellow undertone, whereas red-yellow pheomelanin causes a warm, red undertone. [9]

Red hair colours are due to high concentrations of pheomelanins. The gene for red hair is recessive, making people with red hair colours rare around the world. Pheomelanins are produced in high concentrations due to a series of mutations in the MC1R gene.

Grey hairs are due to the lack of melanin pigments. The lighter the grey hairs appear, the lower the concentration of melanin present. The gene that is responsible for grey hairs is known as the IRF4 gene which plays a part in the production and storage of melanin. When coloured hair changes colour to grey, it means that the melanocyte activity has stopped. [8]

NATURAL HAIR SHAPES

The texture and shape of the hair on our heads are mostly due to the shape of the hair follicles. For people with straight hair, the follicle is perfectly round. For people with curly hair, the follicle is more oval-like. As the oval becomes flatter, the curlier the natural hair becomes. In addition, the shape is determined by how the follicle is placed in the scalp. People with straight hair have hair follicles which are positioned vertically down the surface of the skin into the dermis whereas people with curly hair have follicles positioned at an angle. The more angled the position of the follicles are, the more curly the hair becomes. [10] 

Fig.4 Example of how the shape of the follicle affects hair shape.

Chong, Trisha. “Hair Texture Can Change.” The Tech Interactive, 22 May 2018, genetics.thetech.org/ask-a-geneticist/hair-texture-can-change. 

CONCLUSION

After looking into the anatomy of hair, I am fascinated by how complex a piece of hair is. Hair on our heads is something that we value so much, but barely know much about. Is it possible to have new natural hair colours in the future, perhaps from further mutations of the MC1R gene? Only time will tell.

BIBLIOGRAPHY

[1] Wagner, Heather Elise. “SC200.” Why Do Humans Have Hair on Top of Our Heads?, 18 Sept. 2014, sites.psu.edu/siowfa14/2014/09/18/why-do-humans-have-hair-on-top-of-our-heads/. 

[2] Erdoğan, Bilgen. “Anatomy and Physiology of Hair.” IntechOpen, 3 May 2017, www.intechopen.com/books/hair-and-scalp-disorders/anatomy-and-physiology-of-hair. 

[3] Watson, Kathryn. “Keratin: Benefits and Uses for Hair.” Healthline, 17 Sept. 2018, www.healthline.com/health/keratin. 

[4] Martel, Julianna L. “Anatomy, Hair Follicle.” StatPearls, U.S. National Library of Medicine, 15 Aug. 2020, www.ncbi.nlm.nih.gov/books/NBK470321/. 

[5] Hoffman, Matthew. “Hair (Human Anatomy): Image, Parts, Follicle, Growth, Problems, and More.” WebMD, 18 May 2019, www.webmd.com/skin-problems-and-treatments/picture-of-the-hair.

[6] Thomas, Dr. Liji. “Genetics of Hair Color.” News Medical, 27 Feb. 2019, www.news-medical.net/health/Genetics-of-Hair-Color.aspx. 

[7] Veloz, Liz. “Why Do People Have Different Hair Color?” Sciencing, 2 Mar. 2019, sciencing.com/do-people-different-hair-color-5438253.html. 

[8] “TYR Gene: MedlinePlus Genetics.” MedlinePlus, U.S. National Library of Medicine, 18 Aug. 2020, medlineplus.gov/genetics/gene/tyr/. 

[9] “The Color of Our Eyes, Hair and Skin.” Causes of Color, www.webexhibits.org/causesofcolor/7F.html. 

[10] “The Science of Curls.” Helix Magazine, helix.northwestern.edu/blog/2014/05/science-curls.