Human hair: biological significance

The biology of human hair

Beyond being a body part that can be easily transformed to help people achieve their desired look and also self-expression through how it is styled, human hair plays a vital role in the integumentary system which is the largest organ of the body that forms a physical barrier. To understand better the importance of this body part, you might read this post – a summary about the biology of human hair, which provides you a way easier approach to gain profound knowledge on this topic without having to review several complicated scientific articles.

What is hair?

Hair is a slender strand that emerges from a hair follicle situated within the dermis of the skin. Composed primarily of densely arranged cells known as keratinocytes, it is filled with keratin. Throughout the human body, hair follicles are plentiful, though a handful of regions such as mucous membranes, lips, palms of the hands, and soles of the feet are exceptions to this generalization.

Structure of hair

Coming to the structure of hair, the first component to know is hair root which refers to the section of the hair situated inside the follicle, and it is the sole living component of the hair. On the other hand, the visible portion of the hair that protrudes above the skin surface is known as the hair shaft. The hair shaft, devoid of any biochemical processes, is classified as non-living or dead.

Follicle and Root

Image depicts the biology of hair follicle and hair
A hair follicle has a sebaceous gland and an arrector pili muscle

In terms of follicle and root, it is worth knowing that the process of hair growth initiates within a follicle, as depicted in the figure above. Within each hair follicle, there are specialized stem cells that possess the ability to undergo division, enabling the continuous growth of hair. Moreover, these stem cells are capable of regenerating new hair following the shedding of an existing one. Accompanying the hair follicle is a sebaceous gland, responsible for producing an oily substance called sebum. Sebum plays a crucial role in lubricating and waterproofing the hair. Additionally, a small arrector pili muscle is connected to the follicle. Upon contraction, this muscle causes the follicle to move, leading to the erect position of the hair within the follicle.

Shaft

The hair shaft is a durable filament with the potential to grow to considerable lengths. Typically, hair experiences a monthly growth rate of approximately half an inch. When examined in cross-section, the hair shaft can be distinguished into three distinct zones: the cuticle, cortex, and medulla.

  • The cuticle (or also known as outer coat) is the outermost layer of the hair shaft. It is composed of multiple layers of flat, thin keratinocytes that overlap similar to the arrangement of shingles on a roof. This overlapping structure aids the cuticle in effectively repelling water. Moreover, the cuticle is coated with a thin layer of lipids, only one molecule thick, which further enhances its water-repellent properties. It is important to note that the cuticle is the part of the hair shaft visible to the naked eye.
  • The cortex serves as the central region of the hair shaft and is also its broadest segment. It exhibits a high level of organization and structure, composed of bundles of keratin arranged in rod-like structures. These unique formations are responsible for providing hair with its mechanical strength. Moreover, the cortex contains melanin, the pigment responsible for determining the color of the hair.
  • The medulla represents the innermost region of the hair shaft. It is characterized by a small, less structured, and relatively open area located at the center of the hair shaft. Notably, the medulla is not universally present in all hair. However, when it is present, it consists of keratin-filled cells that are highly pigmented.

Characteristics of hair

Hair exhibits two prominent visible traits: color and texture. Additionally, in adult males, the extent of balding is another observable characteristic. These three traits, namely: color, texture, and balding, are all determined by genetic factors.

Hair Colour

Natural hair color can vary widely among individuals. The most common natural hair colors include black, brown (ranging from dark brown to light brown), blonde (ranging from dark blonde to light blonde), and red (ranging from deep red to strawberry blonde). It’s important to remember that there is a wide spectrum of natural hair colors, and individuals can have unique shades and combinations that make their hair color distinct.

Natural hair colors of different ethnicities
Serveral natural hair colors

And the natural color of hair is attributed to the presence of melanin, a pigment produced within hair follicles and stored in granules within the hair. Human hair contains two primary types of melanin: eumelanin and pheomelanin. Eumelanin prevails as the dominant pigment in brown and black hair, while pheomelanin is the dominant pigment in red hair. Blond hair, on the other hand, arises when there is a minimal amount of melanin present. As melanin production gradually decreases and ceases, hair can turn gray or white.

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Hair Texture

Factors define hair textures

Hair displays a diverse range of textures, encompassing various characteristics. The primary components that define hair texture include the curl pattern, thickness, and consistency. These factors collectively contribute to the unique and individualized textures observed in different individuals’ hair.

  • The configuration of the hair follicle plays a pivotal role in shaping the hair shaft itself. Consequently, the shape of the hair shaft influences the curl pattern exhibited by the hair. Straight hair is typically associated with round hair shafts, whereas wavy or curly hair tends to result from hair shafts with oval or other non-round shapes.
  • The size of the hair follicle governs the thickness of the hair. Thicker hair possesses greater volume compared to thinner hair, as it stems from larger hair follicles.
  • The consistency of hair is influenced by a combination of factors, including the volume of the hair follicle and the condition of the hair shaft. Hair consistency is commonly categorized as fine, medium, or coarse. Fine hair exhibits the smallest circumference, while coarse hair boasts the largest circumference. Medium hair falls in between these two extremes. Moreover, coarse hair possesses a more open cuticle compared to thin or medium hair, rendering it more porous.

Example of hair textures

There are several hair texture types, including:

  • Straight hair: Straight hair is characterized by hair strands that are sleek, smooth, and lacking in significant curl or wave. It is typically shiny and reflects light evenly.
  • Wavy hair: Wavy hair has a slight “S” shape or loose curls. It falls between straight and curly hair, with gentle waves that can range from subtle to more pronounced.
  • Curly hair: Curly hair forms well-defined curls or coils. It can vary in texture from loose and soft curls to tighter and more compact coils. Curly hair often has more volume and may be prone to frizz.
  • Kinky hair: Kinky hair, also known as coily or tightly curled hair, has tight, densely packed curls or coils. It has a distinct zig-zag or spring-like pattern and tends to be more prone to dryness and shrinkage.
natural hair types including straight, wavy, curly and Afro
Serveral natural hair textures

Hair texture not only can vary within individuals but also can even vary on different parts of the scalp. Additionally, there is a wide range of diversity within each hair texture category, and individuals may have unique combinations or variations of these textures.

Functions of hair

In humans, head hair serves the purpose of providing insulation and assisting in the retention of heat, thereby helping to regulate body temperature. Additionally, head hair plays a protective role by shielding the scalp from potential damage caused by UV light exposure.

The function of hair in other body locations is a topic of ongoing debate. One hypothesis suggests that body hair aids in preserving warmth during colder weather. When the body experiences low temperatures, the contraction of arrector pili muscles causes the hair to stand up (as depicted in Figure below), creating a layer of trapped warm air above the skin’s surface. However, this mechanism is more effective in mammals with dense hair or fur than in comparatively less-hairy humans.

Image depicts the goose bumps
Arrector pili muscles will make hairs stand erect, more commonly recognized as goose bumps. (1) Epidermis (2) Arrector pili muscle (3) Hair follicle (4) Dermis

Human hair serves a significant sensory role as well. Within the hair follicles, sensory receptors are present, allowing them to perceive hair movement caused by external factors such as a gentle breeze or contact with physical objects. These receptors also contribute to sensory awareness by detecting the presence of parasites on the skin, offering a vital defense mechanism.

Certain types of hair, such as eyelashes, exhibit heightened sensitivity towards potentially harmful substances. Positioned at the edges of the eyelids, eyelashes possess the ability to detect the proximity of dirt, dust, or other potentially dangerous objects near the eye. In response to this sensation, the eye instinctively closes as a protective reflex. Similarly, eyebrows contribute to eye protection by shielding them from dirt, sweat, and rain.

Moreover, eyebrows serve an additional role in nonverbal communication. They actively participate in expressing a range of emotions, including sadness, anger, surprise, and excitement. Through subtle movements and positioning, eyebrows contribute to conveying these emotional cues, as depicted in Figure below.

Two children using their eyebrows to express different feelings
Eyebrows can be used to express emotions

Hair in Human Evolution

One interesting difference that makes humans stand out among mammals is the remarkable reduction in body hair during the course of their evolution. Additionally, humans possess a distinctive feature in their hair texture—curly hair—which is not commonly observed in most other mammalian species. Even among non-human primates (as illustrated in Figure below), straight hair is prevalent. This implies that the development of curly hair occurred at some stage during the evolutionary journey of humans.

Image of tamarin monkey which has straight hair
Like other non-human primates, this tamarin monkey has straight hair.

Loss of Body Hair

One hypothesis explaining the loss of body hair in humans suggests that it would have aided in body cooling through the evaporation of sweat. This hypothesis is supported by the fact that humans have evolved a significantly higher number of sweat glands compared to other mammals. The reduction of body hair allows for faster evaporation of sweat from less hairy skin.

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Another hypothesis for the loss of human body hair is that it would have resulted in fewer parasites residing on the skin. This aspect might have held particular significance when humans began living in larger, more densely populated social groups. By having less body hair, the potential for parasites to find a suitable habitat on the skin would be diminished, reducing the risk of infestations and related health issues.While these hypotheses provide potential explanations for the loss of body hair, they do not fully account for the preservation of hair on the head, as well as in the pubic and armpit regions. One possibility is that head hair was retained because it played a protective role against the harmful effects of UV light on the scalp. Given that our bipedal ancestors traversed the open savannas of equatorial Africa, the head would have been particularly susceptible to direct sunlight exposure.

Regarding pubic and armpit hair, it is plausible that they were retained due to their significance as indicators of sexual maturity. These hair regions may have served as visual cues, signaling reproductive readiness and increasing the chances of successful mating and reproduction.

In summary, while the hypotheses regarding body hair loss provide valuable insights, the preservation of head hair and hair in the pubic and armpit regions likely involves additional factors such as protection against UV radiation and the role of sexual selection.

Evolution of Curly Hair

In addition to the previously mentioned reasons for the evolution of curly hair, increased protection against UV light has been proposed as another potential selective factor. Studies have shown that straight hair permits more light to penetrate the body through the hair shaft and follicle compared to curly hair. In this sense, human hair can be likened to a fiber optic cable, allowing easy passage of light when it is straight but impeding its flow when it is kinked or coiled. This indirect evidence suggests that UV light could have played a role as a selective agent driving the evolution of curly hair, providing greater protection against potentially harmful UV radiation.

Social and Cultural Significance of Hair

Hair holds significant social significance for human beings. It serves as an indicator of biological sex due to its sexually dimorphic distribution. Generally, adult males exhibit more body hair than adult females, with facial hair being a prominent secondary male sex characteristic. Furthermore, hair can provide insights into age. White hair is commonly associated with older age in both males and females, while male pattern baldness is often seen as a sign of aging in males.

Moreover, hair color and texture can serve as indications of ethnic ancestry. The unique combination of hair color and texture can provide clues about an individual’s heritage and ethnic background. Thus, hair carries not only physical characteristics but also cultural and social implications in understanding human diversity and identity.

Hair holds significant cultural significance as well. Hairstyles and hair colors can serve as indicators of social group membership and, unfortunately, can also contribute to the perpetuation of stereotypes. Throughout history and across various societies, head shaving has been utilized as a form of punishment, particularly targeting women. Conversely, in certain cultures, cutting off one’s hair symbolizes liberation from one’s past or a fresh start. Hair can also hold symbolic meaning in mourning rituals within different cultural contexts.

Religion also plays a role in the practices surrounding hair. For instance, in Islam, many women choose to cover their hair with a headscarf as a sign of modesty and religious devotion. Sikh men grow their hair long and cover it with a turban, representing their commitment to their faith. In Amish culture, men grow facial hair only after marriage, specifically sporting a beard while abstaining from growing a mustache, as depicted in figure below.

Image of an Amish man with long beard
This style of facial hair is adopted by most Amish men after they marry.

These cultural practices and beliefs associated with hair highlight its deep-rooted significance in shaping personal and collective identities, as well as its ability to convey social, religious, and symbolic messages within different communities.

Regrettably, hairstyles, colors, and hair characteristics are sometimes used to reinforce stereotypes, particularly when it comes to women. An example of this is the perpetuation of “blonde jokes“, which maintain negative stereotypes despite lacking any factual basis. Many hair-related stereotypes are deeply ingrained and may even go unnoticed by those perpetuating them. Hairstyles are often judged by others based on assumptions regarding gender, sexuality, worldview, and socioeconomic status, even though these inferences are frequently inaccurate.

The biology of human hair
What are your biases? Are they fair?

It is crucial for us to be conscious of our own biases and critically evaluate whether they are justified. Take a moment to examine the collage in the figure above. What are your initial reactions? Are these reactions grounded in factual information? Do you unknowingly harbor unfair biases? It is essential to challenge and confront these biases to foster a more inclusive and understanding society.

Summary

  • Hair is a filamentous structure that originates from hair follicles located within the dermis of the skin. Composed primarily of keratinocytes, tightly packed cells filled with keratin, hair serves various functions in the human body. Nearly the entire surface of the human body is adorned with hair follicles, except for certain regions like mucous membranes, lips, palms, and soles.
  • The hair root resides within the hair follicle and represents the living portion of the hair. It is responsible for hair growth and renewal. On the other hand, the visible portion of the hair, extending above the skin’s surface, is known as the hair shaft. Composed of non-living cells, the hair shaft lacks biochemical activity. It is through the hair shaft that we perceive the length, color, and texture of our hair.
  • Hair growth initiates within a follicle as stem cells undergo division, giving rise to new keratinocytes. This process sets the foundation for the potential length that an individual hair can reach.
  • The hair shaft, extending beyond the scalp, exhibits three distinct zones. The outermost zone is referred to as the cuticle, serving as a protective layer. Next, we have the cortex, situated in the middle, which forms the main bulk of the hair shaft and contributes to its strength and structure. Lastly, the innermost zone is known as the medulla, occupying the core of the hair shaft.
  • Hair characteristics such as color, texture, and balding patterns in adult males are influenced by genetic factors. The presence and distribution of melanin, specifically eumelanin and pheomelanin, determine the color of the hair. Texture encompasses various aspects such as the curl pattern, thickness, and overall consistency of the hair. These visible traits are determined by our genetic makeup and contribute to the diversity and uniqueness of our hair.
  • Head hair serves multiple functions, including providing insulation and safeguarding the scalp from the harmful effects of UV light. Throughout the body, hair holds significant sensory importance, acting as a receptor for detecting movement caused by air currents or physical contact. Particularly, eyelashes and eyebrows serve as protective barriers for the eyes, shielding them from dust, dirt, sweat, and other potential irritants. Additionally, eyebrows contribute to nonverbal communication, aiding in the expression of emotions such as sadness, anger, surprise, and excitement.
  • Humans stand out from other mammals for their notable reduction in body hair over the course of evolution. This loss is believed to be advantageous as it facilitates the rapid evaporation of sweat from less hairy skin, contributing to more efficient cooling of the body. Additionally, the evolution of curly hair among humans is speculated to have occurred as a means of enhancing protection against the harmful effects of UV light. Curly hair may have provided an additional barrier, limiting the amount of direct sunlight that reaches the scalp and skin. These adaptations demonstrate the complex interplay between environmental factors and the evolution of hair characteristics in humans.
  • Hair holds significant social significance for human beings, serving as an indicator of various aspects such as biological sex, age, and ethnic ancestry. It plays a role in distinguishing between males and females, with differences in hair patterns and distribution. Furthermore, hair characteristics like color, texture, and style can be associated with specific age groups or stages of life. Additionally, hair has cultural significance, with hairstyles often reflecting social group membership or cultural identity. It is a form of expression and can be influenced by societal norms, fashion trends, and personal choices.
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References

  1. Blausen.com staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.
  2. Brainard, J/ CK-12 Foundation. (2016). Figure 7 This style of facial hair is adopted by most Amish men after they marry [digital image]. In CK-12 College Human Biology (Section 12.5) [online Flexbook]. CK12.org. https://www.ck12.org/book/ck-12-college-human-biology/section/12.5/
  3. Sony Pictures Animation. (2019, December 5). Hair love | Oscar®-winning short film (Full) | Sony Pictures Animation. YouTube. https://www.youtube.com/watch?v=kNw8V_Fkw28
  4. TED-Ed. (2015, August 25). Why do some people go bald? – Sarthak Sinha. YouTube. https://www.youtube.com/watch?v=8diYLhl8bWU
  5. TEDx Talks. (2015, February 4). Why do we care about hair | Naomi Abigail | TEDxBaDinh. YouTube. https://www.youtube.com/watch?v=hDW5e3NR1Cw

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