Understanding Hair: From Molecules to Strands

Hair is far more than just a cosmetic feature - it's one of nature's most fascinating biological structures and an engineering marvel that has captured the interest of scientists across multiple disciplines [1, 2]. As a defining characteristic unique to mammals, hair not only serves aesthetic and cultural significance but also plays crucial protective roles for our bodies [1, 3].

From a biological perspective, hair is an incredibly complex structure that can be understood as a sophisticated natural composite material. It protects our scalp from mechanical damage and helps regulate body temperature, while also serving important roles in social and sensory communication [3]. What makes hair particularly intriguing is its hierarchical organisation - from individual protein molecules all the way up to the complete hair fibre we can see and touch [1, 4].

This remarkable structure consists of three main layers - the protective outer cuticle, the central cortex that gives hair its strength, and sometimes a medulla at its core [4]. Each layer has its own unique architecture, and together they create a fibre that is both strong and flexible, capable of withstanding remarkable mechanical forces while maintaining its shape [2].

At the heart of hair's structure lies keratin, a protein that forms complex molecular structures and gives hair its distinctive properties [5]. Understanding how these proteins organise themselves into larger structures not only helps us appreciate the complexity of our hair but also has important implications for hair care, cosmetic treatments, and even forensic science [3, 4].

In this blog post, I'll take you on a journey through the hair structure. We'll begin by exploring keratin proteins - the fundamental building blocks that give hair its unique properties. Then, we'll examine the intricate architecture of hair, layer by layer, from the protective cuticle to the central cortex and medulla. We'll venture inside the hair follicle to understand how hair grows and takes shape, before discussing the various factors that can affect hair structure, from humidity to heat. Finally, we'll look at the practical implications of this knowledge for hair care and styling.

This deep scientific understanding of hair's structure and behavior isn't just academic knowledge - it forms the foundation of modern hair care practices and shapes how we approach hair treatment in professional settings. By the end of this journey, you'll understand why we at The Keratin Lab chose to anchor our identity in the very molecule that makes hair such a remarkable structure.

The Building Blocks: Keratin Proteins

At their most fundamental level, hair fibres are composed primarily of proteins called keratins - making up 65-95% of the hair's weight depending on moisture content [2]. These remarkable proteins belong to the intermediate filament family and come in two distinct varieties: type I (acidic) and type II (basic/neutral) keratins [5].

What makes hair keratins special is that they belong to a specific subclass known as "hard" or "α-keratins", which are distinctly different from the "soft" keratins found in our skin. The key distinction lies in their cysteine content - hair keratins contain approximately 7.6% cysteine compared to only 2.9% in skin keratins [5]. This higher cysteine content allows for more disulfide bonding between protein molecules, creating a tougher, more durable structure [2, 6].

Mockup for skin care cosmetics — α-keratin and β-keratin from Yu, Y., Yang, W., Wang, B., & Meyers, M. A. (2017).

The α-keratin molecules form right-handed helical structures, approximately 1.2 nm (0.0000012mm) in diameter with a periodicity of 0.52 nm (0.00000052mm) per turn [2]. These helical structures can transform under tension into β-sheets (β-keratin) - a different protein configuration that plays a crucial role in hair's ability to stretch [2, 6]. Eight of these α-helical protein chains combine to form what's known as an intermediate filament, measuring about 7.5 nm (0.0000075mm) in diameter [2].

These intermediate filaments don't work alone - they're embedded in a matrix rich in high-sulphur proteins, creating a complex composite structure [7]. The matrix proteins, also known as keratin-associated proteins (KAPs), help bind the intermediate filaments together through extensive disulfide bonding [5].

Understanding this molecular architecture helps explain many of hair's unique properties. The combination of strong protein structures (intermediate filaments) held together by extensive chemical bonds (disulfide bridges) creates a material that is both strong and flexible, capable of withstanding significant mechanical stress while maintaining its structure [2, 6].

The Hair's Architecture: Layer by Layer

The human hair shaft exhibits a sophisticated hierarchical structure, consisting of three main components: the cuticle (outer protective layer), the cortex (main body), and sometimes a medulla (central core) [2, 3].

Let's examine each layer in detail:

Cuticle (Outer Layer):

The cuticle forms the protective armour of the hair shaft, consisting of thin overlapping scales that resemble roof tiles [2, 3]. Each scale is approximately 60 μm (0.06 mm) long and 0.5 μm (0.0005 mm) thick, with about 5-10 scales overlapping to create a total thickness of roughly 5 μm (0.005 mm) [2]. These scales point from root to tip, and their edges can become damaged through daily activities like combing and brushing, particularly in longer hair [2, 3]. The cuticle plays a crucial role in protecting the inner structures and maintaining hair integrity, though interestingly, its damage doesn't significantly affect hair's overall tensile properties [2].

Cortex (Main Body):

The cortex forms the bulk of the hair shaft and is primarily responsible for hair's mechanical properties [2, 3]. It consists of cortical cells that are approximately 100 μm (0.1 mm) long and 1-6 μm (0.001-0.006 mm) thick [2]. These cells contain even smaller structures called macrofibrils, measuring 0.1-0.4 μm (0.0001-0.0004 mm) in diameter [2]. The macrofibrils themselves are composed of intermediate filaments (IFs) embedded in a matrix rich in sulphur content [2, 7]. Within the cortex, there are approximately 20,000 intermediate filaments per cell, organised into intricate patterns [7]. These filaments are connected by non-crystalline molecules and numerous sulphur bonds, which contribute significantly to hair's strength and flexibility [2, 7].

Medulla (Core):

The medulla is a central core that may be present in human hair, though it's not always found [3]. When present, it appears as a vacuolated cellular structure in the centre of the hair shaft [3]. The presence and structure of the medulla can vary significantly among different individuals and even among hairs from the same person [3]. Unlike the cortex, which is crucial for hair's mechanical properties, the medulla's role in human hair is less well understood [3].

This complex, multi-layered architecture creates a remarkably resilient structure. The way these components work together determines many of hair's physical properties, from its strength and flexibility to its appearance and behaviour under different conditions [2, 3].

Inside the Follicle: Where it All Begins

The hair follicle is a fascinating mini-organ that orchestrates the complex process of hair formation and growth [3, 4]. This cylindrical structure extends from the epidermis into the dermis and contains several distinct compartments that work together to produce and shape our hair [3].

The hair and its follicle from Buffoli, B., Rinaldi, F., Labanca, M., Sorbellini, E., Trink, A., Guanziroli, E., Rezzani, R., & Rodella, L. F. (2014).

From the outermost aspect of the follicle, we find the outer root sheath (ORS), which serves as a reservoir of multipotent stem cells, including both keratinocyte and melanocyte stem cells [3]. These stem cells are crucial for hair growth and pigmentation - keratinocyte stem cells give rise to the cells that form the hair shaft structure, while melanocyte stem cells produce the pigment-producing cells that give hair its colour. The ORS is particularly important in the bulge area, located between where the arrector pili muscle attaches and where the sebaceous gland duct enters the follicle [3].

The inner root sheath (IRS) lies beneath the outer root sheath (ORS) and consists of three distinct layers: Henle's layer, Huxley's layer, and the cuticle layer [3]. These layers work together to support and guide the growing hair shaft, with the IRS cuticle layer directly anchoring the hair shaft to the follicle [3].

At the base of the follicle lies the hair bulb, which is the powerhouse of hair production [3, 4]. This region contains the follicular dermal papilla, which is crucial for instructing the hair follicle to grow and determining the size and pigmentation of the hair shaft [3]. The dermal papilla is rich in growth factors and provides essential signals for both hair growth and melanin production [3].

The hair growth cycle consists of three main phases:

Anagen (growth phase): When the follicle is actively producing hair
Catagen (transitional phase): A period of regression
Telogen (resting phase): When the old hair is shed before a new growth cycle begins [3, 4]

During these cycles, the follicle undergoes remarkable changes in structure and activity [4]. The cycling nature of hair growth means that at any given time, different follicles on our scalp are in different phases, with approximately 85% in anagen and 15% in telogen under normal conditions [4].

The shape and orientation of the follicle also play a crucial role in determining whether hair will grow straight or curly [6]. Research has shown that follicle asymmetry and the arrangement of cells within the bulb region contribute significantly to the final form of the hair shaft [6].

Factors Affecting Hair Structure

The structural integrity and behaviour of human hair can be significantly influenced by various environmental and physiological factors:

Humidity and Water Hair fibres respond dramatically to moisture levels in their environment. When exposed to water, hair exhibits a swelling effect, increasing in diameter by approximately 10% [2]. Water acts as both a plasticiser and a swelling agent, reducing the interactions between protein chains whilst increasing the spacing between intermediate filaments [2]. Under high humidity, hair's Young's modulus decreases whilst its extensibility increases, demonstrating how moisture can fundamentally alter hair's mechanical properties [2].

Temperature Effects

Temperature plays a crucial role in hair structure and behaviour. Research has identified several critical temperature thresholds:

A glass transition occurs at approximately 35°C
A significant structural transition takes place around 60°C
Complete melting occurs at about 155°C under saturated conditions [2]

Notably, exposure to high temperatures (60-80°C) can cause irreversible structural changes to the hair. Even after cooling back to room temperature, hair shows permanent alterations in its mechanical properties, suggesting fundamental changes in its molecular organisation [2].

Ethnic Variations

Hair structure varies among different ethnic groups, but these differences are often more nuanced than commonly portrayed. While scientific observations identify structural variations in hair, it’s essential to recognise that these physical differences do not imply distinct biological, ethnic, or racial divisions among humans, nor should they be used to make assumptions about genetic ancestry [8].

Variations can be seen in:

Surface properties of the cuticle
Cross-sectional shapes
Internal organisation of cortical cells [2, 6]

Importantly, these variations exist on a continuous spectrum rather than as rigid categories, with significant overlap across groups [7].

Age-Related Changes

As we age, hair undergoes various structural modifications:

Changes in melanin production and distribution
Alterations in protein structures
Modifications to the cross-sectional area and mechanical properties [3, 4]

Chemical and Mechanical Stresses

Daily grooming practices and chemical treatments can affect hair structure:

Mechanical stresses from combing and brushing can damage the cuticle scales
Chemical treatments can alter the internal protein structures
Excessive heat styling can modify the arrangement of keratin molecules [2, 3]

Understanding these factors is crucial not only for hair care but also for comprehending how hair adapts to various environmental conditions and treatments [2, 3].

Practical Implications

Understanding the intricate structure of human hair has far-reaching implications that extend well beyond pure scientific interest. The complex architecture we've explored - from individual keratin molecules to the complete hair shaft - directly influences how we should approach hair care and treatment in our daily lives.

Consider the simple act of washing and conditioning hair. The overlapping nature of cuticle scales, pointing from root to tip, explains why hair is more vulnerable to damage when brushed against this natural direction. This same structural feature helps us understand why conditioning treatments are more effective when applied in harmony with the scale direction, allowing products to better penetrate and protect the hair shaft [2, 3].

The relationship between hair structure and water is particularly fascinating. When we wash our hair, water acts as a natural plasticiser, temporarily altering the mechanical properties of each strand. This explains why wet hair is more susceptible to damage during brushing and styling. Similarly, understanding how heat affects hair's protein structure helps explain why excessive heat styling can cause permanent damage - once the critical temperature threshold of 60°C is exceeded, irreversible changes occur in the hair's molecular organisation [2].

From a mechanical perspective, human hair is remarkably resilient. Its ability to withstand tensile forces of 150-270 MPa [8] while remaining flexible enough for styling makes it an extraordinary natural material [2]. This balance of strength and flexibility comes from its hierarchical structure - the organisation of keratin proteins into intermediate filaments, then into macrofibrils, and finally into cortical cells.

Mockup for skin care cosmetics. — Hierarchical structure in human hair from Yu, Y., Yang, W., Wang, B., & Meyers, M. A. (2017).

These structural characteristics have applications beyond personal care. In forensic science, the distinct patterns of the cuticle scales and the arrangement of cortical cells can provide valuable investigative information [7]. However, it's important to note that such analyses must be interpreted cautiously, given the significant variation that can exist even among hairs from the same individual.

Perhaps most intriguingly, hair structure can serve as a window into overall health. Changes in hair growth patterns or structural modifications can signal underlying nutritional deficiencies or hormonal changes [3, 4]. This connection between hair structure and health continues to drive research into both diagnostic tools and therapeutic approaches.

The Science Behind The Keratin Lab

The hair structure we've explored brings us back to why we named our salon 'The Keratin Lab'. This name wasn't chosen to promote any particular treatment or service, but rather reflects our dedication to understanding hair at its most fundamental level - starting with its key building block, keratin [2, 5].

Just as scientists in a laboratory approach each experiment with meticulous attention to detail and understanding that every variable matters, at The Keratin Lab, we approach each client's hair with the same scientific mindset. The research shows us that hair structure varies not only between individuals but even among the hairs on a single person's head [2, 3]. This understanding fundamentally shapes our philosophy: there can be no one-size-fits-all approach to hair care.

The scientific literature teaches us that hair responds differently to environmental conditions, chemical treatments, and mechanical stresses based on its unique structural characteristics [2]. When we analyse a client's hair, we're considering multiple factors: the cuticle condition, the cortical structure, the way it responds to moisture, its mechanical properties, and how these characteristics interact with the client's lifestyle and desired outcomes [2, 3].

Our approach is grounded in this scientific understanding. Rather than following trends or applying standardised treatments, we analyse each client's hair structure and condition to develop individualised care strategies. This might mean different approaches to cutting, varying product recommendations, or customised treatment protocols - all based on the scientific principles of hair structure and behaviour [3, 4].

The 'Lab' in our name represents more than just a space - it embodies our commitment to continuous learning and evidence-based practice. As new research emerges and our understanding of hair structure and behaviour evolves, so too do our methods and approaches [4, 7]. We're not just working with hair; we're working with a sophisticated biological material that deserves to be treated with scientific precision and understanding.

Reference

Wang, B., et al., Keratin: Structure, mechanical properties, occurrence in biological organisms, and efforts at bioinspiration. Progress in materials science, 2016. 76: p. 229-318.
Yu, Y., et al., Structure and mechanical behavior of human hair. Materials Science and Engineering: C, 2017. 73: p. 152-163.
Buffoli, B., et al., The human hair: from anatomy to physiology. International journal of dermatology, 2014. 53(3): p. 331-341.
O'Sullivan, J.D., et al., The biology of human hair greying. Biological Reviews, 2021. 96(1): p. 107-128.
Yu, J., et al., Human hair keratins. Journal of investigative dermatology, 1993. 101(1): p. S56-S59.
Thibaut, S., et al., Human hair keratin network and curvature. International journal of dermatology, 2007. 46: p. 7-10.
Boulos, R.A., et al., Unravelling the structure and function of human hair. Green Chemistry, 2013. 15(5): p. 1268-1273.
Technical note: In scientific terms, human hair's tensile strength is measured at 150-270 MPa (Megapascals). This measurement represents the force per unit area that hair can withstand before breaking. A Pascal is the SI unit of pressure, equal to one newton per square metre, and a Megapascal is one million Pascals. These precise measurements allow scientists to compare hair's strength with other biological and synthetic materials.

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The Science Behind The Keratin Lab