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Healthy hair structure comprises several distinct regions and is made of specialized proteins and chemical bonds critical for function and resilience.
Keratin
Keratin is the primary fibrous protein of hair, nails, and skin’s outer layer, conferring strength, durability, and structure. It forms intermediate filaments, stabilized by disulfide and hydrogen bonds, protecting the cortex and resisting chemical and physical stress. Hair keratins are produced by hair follicle cells and make up the bulk of the shaft.
Melanin
Melanin determines hair color by its presence and type in the hair cortex, synthesized by melanocytes. There are two primary forms: eumelanin (brown/black) and pheomelanin (red/yellow), and their ratios produce the enormous spectrum of human hair shades. Melanin also serves a UV-protective role and declines with age, causing graying.
Cuticle
The cuticle is the outermost layer of the hair shaft, composed of overlapping flat cells like roof shingles. Its main role is to protect inner structures, retain moisture, and provide shine and smoothness. Damage to the cuticle leads to dullness, frizz, split ends, and increased porosity.
Cortex
The cortex is the main body of the hair, comprising elongated keratinized cells and the majority of pigment (melanin). The cortex provides tensile strength, elasticity, flexibility, and is responsible for the hair’s ability to hold color.
Medulla
The medulla is the innermost, sometimes absent, portion of the hair shaft, consisting of loosely packed, soft, often hollow cells and air spaces. Its function is not fully understood, though it may contribute to thickness and reflectivity.
Sebum (Natural Oil Produced by Scalp)
Sebum, a waxy substance secreted by sebaceous glands in scalp hair follicles, conditions the scalp and hair, provides barrier protection, imparts shine, and supports microbial balance. Excess or deficiency contributes to scalp conditions such as dandruff, seborrheic dermatitis, and folliculitis.
Disulfide Bonds (S-S Bonds)
Disulfide bonds are strong covalent links between cysteine amino acids in keratin, primarily responsible for hair’s strength and permanent shape. Chemical treatments such as relaxing or perming alter disulfide bonds, changing hair’s texture.
Hydrogen Bonds
Hydrogen bonds are weaker, temporary bonds between keratin chains and are responsible for hair’s flexibility and reversibility of styles following wetting and drying. They are easily disrupted by water or heat and reformed when hair dries or cools.
Salt Bonds
Salt bonds, or ionic bonds, are electrostatic attractions between oppositely charged amino acid side chains in keratin, contributing to hair’s structure and about a third of its strength. They can be disrupted by changes in pH.
Hair Types & Texture Categories
Understanding natural hair types informs care and management strategies.
Straight
Straight hair (Type 1) lies flat due to its round follicle, appears smooth and glossy, and is prone to oiliness since sebum travels easily along the strand . It is resistant to curling and commonly seen in East Asian backgrounds.
Wavy
Wavy hair (Type 2) forms gentle S-shaped patterns. It combines some of the advantages of both straight and curly hair in terms of versatility but can be prone to frizz and requires moderate moisture
Curly
Curly hair (Type 3) spirals into well-formed ringlets or spirals. The follicle is oval, and sebum has difficulty traveling down the strand, making curls drier and more susceptible to frizz and shrinkage.
Coily (Kinky)
Coily or kinky hair (Type 4) forms tight, dense, springy coils or zigzag patterns. It is the most fragile and prone to shrinkage and breakage, with high density but less visible volume.
Fine Hair
Fine hair has a small diameter and is soft, often appearing flat or limp and prone to oiliness; it requires gentle care and lightweight styling products.
Medium Hair
Medium hair is neither too fine nor too thick, provides relative versatility in styling, and typically maintains a healthy shine and bounce.
Thick Hair
Thick hair has a wide diameter, offering volume and body but can be heavy and harder to manage or style, sometimes prone to dryness and frizz
Oily Hair
Oily hair results from an overactive scalp with copious sebum, leading to greasy appearance and limpness, particularly at the roots.
Dry Hair
Dry hair lacks adequate moisture retention, resulting in roughness, breakage, and dullness, often due to cuticle damage or reduced sebum
Normal Hair
Normal hair balances sebum production and moisture, appears shiny and healthy, and requires basic maintenance without special management.
Color-Treated Hair
Color-treated hair is chemically altered and typically more porous and prone to dryness and damage, needing specialized moisturizing and strengthening products.
Chemically Treated Hair (Permed, Relaxed)
Hair that is permed or relaxed has undergone chemical alteration of disulfide bonds, making it more fragile, porous, and susceptible to breakage, requiring restorative care.
Damaged Hair
Damaged hair exhibits breakage, split ends, dullness, increased tangling, and loss of elasticity due to mechanical, chemical, or environmental insults.
Thinning Hair
Thinning hair involves a reduction in scalp hair density often due to androgenetic alopecia or telogen effluvium.
Textured Hair (African, Afro-Caribbean, etc.)
Textured hair describes various tight curl patterns common among individuals of African descent requiring moisture-rich products and gentle handling due to its fragility and shrinkage propensity.
This compilation serves as a foundation for understanding the complex interplay of scalp/hair conditions and the structural diversity that underpins hair health and aesthetics. For personalized guidance, it is advised to consult a qualified healthcare or hair care professional.
Elasticity
Elasticity refers to hair’s capacity to stretch under tension and return to its original length without breaking, directly reflecting the health of keratin proteins and the hair’s moisture-protein balance. Healthy hair with good elasticity can stretch up to 30–50% of its original length (or nearly double in optimal conditions), snapping back without damage. Reduced elasticity is a hallmark of structural impairment, indicating loss of cuticle protection or degradation of the cortex (where keratin and moisture reside). Poor elasticity results in increased breakage, split ends, decreased curl definition, and difficulty retaining styles. Heat styling, frequent coloring/chemical treatments, chronic dryness, and protein or moisture imbalances are common causes of diminished elasticity. Testing elasticity at home involves stretching a clean, damp hair and observing whether it rebounds or breaks; immediate snapping signals poor elasticity and the need for hydrating and fortifying treatments. Improving elasticity involves using moisturizing, protein-enriched treatments, and minimizing harsh styling and chemical exposures.
Porosity (Low, Normal, High)
Porosity describes the hair’s ability to absorb and retain moisture, determined primarily by the condition and spacing of cuticle cells on the hair’s surface. It is categorized as low, normal (medium), or high, each bearing different implications for hair health and care needs:
Low Porosity: The cuticle layers are tightly packed, resulting in a smooth, almost impermeable surface that resists moisture and product penetration. Such hair tends to hold onto moisture once absorbed but can be prone to buildup, may feel heavy after product use, and requires lightweight, water-based formulations and occasional heat-assisted treatments to improve absorption.
Normal (Medium) Porosity: The cuticles are less tightly packed than low-porosity hair but not as open as high-porosity hair, allowing efficient moisture entry and retention. This hair type is generally the healthiest responding well to treatments, holding styles, retaining color, and displaying optimal shine and elasticity
High Porosity: Cuticles are widely spaced, lifted, or have gaps stemming from damage by heat, chemicals, or environmental exposure. High-porosity hair quickly absorbs moisture (and often water or products), but it loses hydration just as rapidly, leading to chronic dryness, frizz, split ends, and dullness. This type requires extra protection, regular use of oils, butters, leave-ins, protein treatments, and sealing strategies to maintain hydration and reduce further damage. Porosity is both genetically determined and affected by cumulative hair trauma.
Simple at-home tests (e.g., strand float test, spray test) can be used to assess porosity and guide the adjustment of hair care routines to match repair requirements.
