3.2 Scalp and Hair Science (Trichology)

Scalp and Hair Science (Trichology)

Core Knowledge: Trichology (from the Greek trichos, meaning "hair," and logos, meaning "the study of") is the scientific study of hair, its diseases, and its care. Every haircut, shave, chemical texturizing treatment, and color formulation relies on an accurate physical and chemical understanding of hair biology.

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Structural Anatomy of Hair: Root vs. Shaft

A mature strand of human hair is divided into two primary anatomical divisions:

                                  HUMAN HAIR STRUCTURE
                                           │
                   ┌───────────────────────┴───────────────────────┐
                   ▼                                               ▼
           THE HAIR ROOT                                   THE HAIR SHAFT
   (Located beneath skin surface;                 (Projects above skin surface;
    living germinative structure)                  non-living, keratinized fiber)
   • Hair Follicle                                • Cuticle (Outer scales)
   • Hair Bulb                                    • Cortex (Fibrous protein core)
   • Dermal Papilla                               • Medulla (Central core/pith)
   • Arrector Pili Muscle
   • Sebaceous Gland

The Hair Root Complex

The hair root is the portion of the hair located beneath the surface of the epidermis, enclosed within the hair follicle in the dermis. It consists of five essential structures:

1. Hair Follicle: A tube-like pocket or invagination of the epidermis that extends downward into the dermis (and occasionally into the subcutaneous tissue). Follicles cover the entire body except the palms of the hands, soles of the feet, lips, and portions of the genitalia. The angle and curvature of the follicle determine the hair's natural growth pattern:
   • Hair Stream: Hair flowing in the same natural direction.
   • Whorl: Hair that forms a circular or spiral pattern, commonly at the crown.
   • Cowlick: A tuft of hair that stands straight up, common at the front hairline or crown.
2. Hair Bulb: The thickened, club-shaped structure that forms the lowest part of the hair root. It fits over and envelops the dermal papilla.
3. Dermal Papilla: A small, cone-shaped elevation located at the base of the hair follicle that fits directly into the hair bulb. It contains a rich loop of blood capillaries and sensory nerve endings that deliver essential oxygen and nutrients to the rapidly dividing cells of the hair matrix. The dermal papilla is the biological "mother of the hair"; if it is destroyed, the follicle can never produce another hair strand.
4. Arrector Pili Muscle: A tiny, involuntary smooth muscle bundle attached to the base of the hair follicle and the dermal papillary layer. When stimulated by cold, fear, or sympathetic nervous arousal, it contracts, pulling the hair upright and causing "goosebumps" (piloerection).
5. Sebaceous (Oil) Glands: Sac-like exocrine glands connected to the hair follicle canal. They secrete sebum, an oily substance that lubricates the hair and skin, prevents excessive moisture loss, and forms the protective acid mantle (pH 4.5–5.5).

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Microscopic Structure of the Hair Shaft

The hair shaft is the non-living, fully keratinized portion of the hair that extends above the surface of the skin. A cross-section of the hair shaft reveals three concentric layers:

                               LAYERS OF THE HAIR SHAFT
                                           │
                   ┌───────────────────────┼───────────────────────┐
                   ▼                       ▼                       ▼
                CUTICLE                 CORTEX                  MEDULLA
          (Outer scale layer)     (Middle protein core)    (Innermost center)
          • Transparent scales    • 90% of hair weight     • Round cells & air
          • Protects cortex       • Melanin pigment        • Often absent in
          • Opens in alkali       • All 3 side bonds         fine/blonde hair
          • Closes in acid        • Perm/color action

1. The Cuticle

The cuticle is the outermost layer of the hair shaft. It consists of a single layer of overlapping, transparent, scale-like cells (imbrications) that point upward toward the hair ends (distal tip), similar to shingles on a roof.

• Protective Barrier: It shields the inner cortex from chemical, thermal, and mechanical damage.
• Chemical Response: Acidic products (pH 4.5–5.5) cause the cuticle scales to contract, harden, and lie flat, reflecting light to create shine. Alkaline solutions (pH > 7.0) cause the cuticle to swell and open, allowing chemical formulations to penetrate the cortex.

2. The Cortex

The cortex is the fibrous middle layer comprising approximately 90% of the total weight and tensile strength of the hair shaft. It is formed by elongated polypeptide chains packed with alpha-keratin microfibrils.

• Melanin Pigment: The cortex houses all natural hair pigment granules:
  • Eumelanin: Provides dark brown and black tones.
  • Pheomelanin: Provides red and yellow/blonde tones.
• Elasticity and Strength: The cortex contains all cross-linking side bonds. All permanent chemical services (permanent waves, curl reformations, chemical relaxers, permanent hair color, and lighteners) take place within the cortex.

3. The Medulla

The medulla is the innermost central core of the hair shaft, composed of round cells and air spaces. It is often called the "pith" or "marrow" of the hair.

• It is frequently absent or fragmented in very fine hair, naturally blonde hair, and vellus hair.
• It plays no functional role in barbering chemical services, hair strength, or physical styling.

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Chemical Composition of Hair Keratin: COHNS Elements & Side Bonds

Hair is made of a fibrous protein called keratin. Living cells in the follicle bulb divide, absorb amino acids, synthesize keratin protein, lose their nuclei, and dehydrate in a process called keratinization before emerging above the scalp.

The COHNS Elements

Human hair keratin is composed of five fundamental chemical elements, known by the acronym COHNS:

                             THE COHNS ELEMENTS IN HAIR
  ┌───────────────┬─────────────────┬──────────────┬──────────────┬──────────────┐
  │  Carbon (C)   │   Oxygen (O)    │ Nitrogen (N) │ Hydrogen (H) │  Sulfur (S)  │
  │     51%       │       21%       │     17%      │      6%      │      5%      │
  └───────────────┴─────────────────┴──────────────┴──────────────┴──────────────┘

• Carbon (C): 51%
• Oxygen (O): 21%
• Nitrogen (N): 17%
• Hydrogen (H): 6%
• Sulfur (S): 5%

Polypeptide Chains and Side Bonds

Amino acids are linked end-to-end by strong chemical peptide bonds (end bonds) to form long polypeptide chains. If peptide bonds are broken, the hair fiber dissolves and breaks permanently.

Polypeptide chains are woven parallel to each other and cross-linked laterally by three types of side bonds, which account for hair's strength, shape, and elasticity:

                           CROSS-LINKING SIDE BONDS (1/3 Each)
                                           │
                   ┌───────────────────────┼───────────────────────┐
                   ▼                       ▼                       ▼
            HYDROGEN BONDS            SALT BONDS            DISULFIDE BONDS
          • Physical side bond    • Physical side bond   • Strong chemical bond
          • Broken by water/heat  • Broken by pH change  • Broken by thio/alkali
          • Reformed by drying    • Reformed by pH 4.5-5.5• Reformed by oxidation

1. Hydrogen Bonds:
   • Nature: Weak, physical cross-links between neighboring polypeptide chains formed by electromagnetic attraction between opposite charges.
   • How Broken: Easily broken by water (wetting the hair) or heat (blow dryers, curling irons, flat irons).
   • How Reformed: Reformed as the hair dries or cools into its new physical shape.
   • Strength: Accounts for roughly one-third (33%) of the hair's lateral strength.
2. Salt Bonds:
   • Nature: Weak, physical, ionic cross-links dependent on pH, formed between positive and negative amino acid charges on adjacent polypeptide chains.
   • How Broken: Broken by changes in pH (application of acidic or alkaline solutions).
   • How Reformed: Reformed when the hair's pH is neutralized back to its natural acidic range (pH 4.5–5.5).
   • Strength: Accounts for roughly one-third (33%) of the hair's lateral strength.
3. Disulfide Bonds:
   • Nature: Strong, chemical, covalent cross-links formed between the sulfur atoms of two adjacent cysteine amino acids (joining them into cystine).
   • How Broken: NOT broken by water, heat, or mild pH changes. Broken only by chemical reducing agents (such as ammonium thioglycolate waving lotions) or strong hydroxide chemical relaxers.
   • How Reformed: In permanent waving, broken disulfide bonds are reformed into a new permanent curly shape using an oxidizing neutralizing lotion (hydrogen peroxide). In hydroxide relaxing, disulfide bonds are permanently broken and converted to lanthionine bonds (lanthioninization), which can never be reformed.
   • Strength: Accounts for one-third (33%) of the hair's lateral strength but provides its permanent structural shape.

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The Hair Growth Cycle

Human hair grows in continuous, repeating cycles consisting of three distinct phases:

                                THE HAIR GROWTH CYCLE
  ┌─────────────────────────┐   ┌─────────────────────────┐   ┌─────────────────────────┐
  │      ANAGEN PHASE       │──>│      CATAGEN PHASE      │──>│      TELOGEN PHASE      │
  │     (Active Growth)     │   │      (Transitional)     │   │    (Resting/Shedding)   │
  │ • Lasts 2 to 6 years    │   │ • Lasts 1 to 2 weeks    │   │ • Lasts 3 to 6 months   │
  │ • 85% to 90% of hair    │   │ • <1% of hair           │   │ • 10% to 15% of hair    │
  │ • ~0.5 inch/month       │   │ • Bulb detaches         │   │ • 50-100 shed daily     │
  └─────────────────────────┘   └─────────────────────────┘   └─────────────────────────┘

1. Anagen Phase (Active Growth Phase):
   • Matrix cells in the hair bulb rapidly divide via mitosis, synthesizing new hair protein.
   • Lasts 2 to 6 years (and up to 10 years in some individuals).
   • Approximately 85% to 90% of all scalp hair is in the anagen phase at any given time.
   • Scalp hair grows at an average rate of 0.5 inches (1.25 cm) per month (approximately 6 inches per year).
2. Catagen Phase (Transitional Regression Phase):
   • A brief transitional phase signaling the conclusion of active growth.
   • The follicle canal shrinks, the hair bulb detaches from the dermal papilla, and the lower follicle degenerates, forming a club hair.
   • Lasts 1 to 2 weeks.
   • Less than 1% of scalp hair is in the catagen phase at any given time.
3. Telogen Phase (Resting and Shedding Phase):
   • The resting phase where the club hair remains dormant within the follicle until it is shed or pushed out by a newly emerging anagen hair.
   • Lasts 3 to 6 months (approximately 100 days).
   • Approximately 10% to 15% of scalp hair is in the telogen phase.
   • Daily Shedding: The average healthy human scalp sheds 50 to 100 hairs per day as part of normal telogen renewal.

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The Four Essential Hair Analysis Properties

Before performing any haircut, straight-razor shave, chemical service, or scalp treatment, the barber must perform a thorough physical hair and scalp analysis evaluating four critical properties:

                                THE FOUR HAIR PROPERTIES
                                           │
         ┌───────────────────┬─────────────┴───────┬───────────────────┐
         ▼                   ▼                     ▼                   ▼
      TEXTURE             DENSITY               POROSITY           ELASTICITY
  (Strand diameter)   (Strands/sq in)      (Moisture absorption)  (Stretch & return)
  • Coarse            • Low (~1,500)       • Low (Resistant)      • Wet: 50% stretch
  • Medium            • Medium (~2,200)    • Average / Normal     • Dry: 20% stretch
  • Fine              • High (~3,000)      • High (Over-porous)   • Snaps = Low elasticity

1. Hair Texture

Texture refers to the thickness or diameter of an individual hair strand. It is categorized as:

• Coarse Hair: Largest diameter; possesses a thick, strong cuticle and a large cortex. It is resistant to chemical penetration and requires longer processing times or stronger formulations.
• Medium Hair: The standard benchmark texture; presents average diameter and normal chemical response.
• Fine Hair: Smallest diameter; possesses a thin cuticle and lacks a medulla. It is fragile, absorbs chemicals rapidly, and is prone to damage from overprocessing and thermal tools.

2. Hair Density

Density refers to the number of individual hair strands per square inch of scalp area. It determines how thin or thick the hair is overall.

• The average scalp possesses approximately 2,200 hairs per square inch, totaling roughly 100,000 individual hairs on the head.
• Density correlates with natural hair color:
  • Blonde Hair: ~140,000 hairs (highest density, fine strands)
  • Brown Hair: ~110,000 hairs
  • Black Hair: ~108,000 hairs
  • Red Hair: ~80,000 hairs (lowest density, coarse strands)
• Density dictates subsection parting thickness and product volume required.

3. Hair Porosity

Porosity is the ability of the hair to absorb moisture, liquids, and chemicals, determined directly by the condition of the cuticle layer.

• Low Porosity (Resistant): Cuticle scales lie flat, compact, and tight. Absorbs moisture slowly and resists chemical penetration. Requires alkaline pre-softening or longer processing times.
• Average / Normal Porosity: Cuticle scales are slightly raised. Absorbs moisture and chemicals evenly and predictably.
• High Porosity (Over-Porous): Cuticle scales are widely raised, damaged, or stripped from chemical overprocessing, environmental exposure, or excessive thermal styling. Absorbs moisture quickly but loses it rapidly. Color takes rapidly but fades quickly.
• Porosity Test: Grasp a small dry strand of hair between thumb and forefinger, sliding fingers from the ends toward the scalp (against the cuticle grain). If the strand feels smooth, porosity is low; if slightly rough, porosity is normal; if very rough, raspy, or squeaky, porosity is high.

4. Hair Elasticity

Elasticity is the ability of hair to stretch beyond its original length and return without breaking. It directly reflects the structural health of the cortex and disulfide side bonds.

• Normal Wet Elasticity: Healthy hair can stretch up to 50% of its original length when wet and return without breaking.
• Normal Dry Elasticity: Healthy dry hair stretches up to 20% of its original length.
• Low Elasticity: Hair feels brittle, stretches very little, and snaps easily under minimal tension. Hair with low elasticity must receive intensive deep conditioning and protein treatments before any chemical services are performed.

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Wave Pattern and Follicle Formation

Wave pattern is the amount of movement in the hair strand, and it is a separate property from texture. Two strands can share an identical diameter and behave completely differently because their follicles are shaped differently.

Three working consequences the examination tests:

1. Shrinkage governs cutting length. The tighter the wave pattern, the more the hair retracts as it dries. Cutting curly and coily hair to a "finished" length while wet leaves the client far shorter than intended — cut it dry, or cut wet with a deliberate length allowance.
2. The bends are the weak points. Each curve in a coily strand is a stress riser where the cuticle lifts and the strand breaks. That is why coily hair tolerates less tension, less heat, and less chemical processing time.
3. Wave pattern is not the same as texture. Texture is the diameter of a single strand — coarse, medium, or fine. Wave pattern is the shape of the strand's path. Fine coily hair and coarse straight hair are both common, and confusing the two produces the wrong chemical formula every time.

[!NOTE] Chemical services follow the follicle. A permanent wave imposes a new pattern on hair whose disulfide bonds are broken and re-formed around a rod, and a chemical relaxer permanently removes an existing pattern through lanthionization. Neither changes the follicle, which is why the natural pattern always returns at the new growth.