3.1 Trichology & Hair Shaft Structure

Trichology & Hair Shaft Structure

Exam Focus: Trichology is a core pillar of state licensing examinations. Candidates must master the anatomical components of both the hair root (follicle, bulb, dermal papilla, arrector pili, sebaceous glands) and the hair shaft (cuticle, cortex, medulla), as well as the chemical makeup of hair protein, amino acid chains, and side bonds (hydrogen, salt, disulfide).

Trichology is the scientific study of hair, its diseases, and its care. The word originates from the Greek words trichos (meaning hair) and ology (meaning the study of). A full understanding of trichology enables licensed cosmetologists to analyze hair condition, select appropriate chemical services, preserve hair integrity, and prevent damage or injury to the client's scalp and hair shaft.

Full-grown human hair is divided into two principal parts:

1. The Hair Root: The portion of the hair located below the surface of the epidermis (outer layer of skin).
2. The Hair Shaft: The non-living portion of the hair that projects above the epidermis.

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Structures of the Hair Root

The hair root is embedded within the skin and consists of five key anatomical structures that work synchronously to nourish, produce, and support the hair strand:

The Dermal Papilla: "Mother of Hair"

The dermal papilla is often referred to by trichologists as the "mother of hair" because it provides the vital link between the circulatory system and the hair matrix. If the dermal papilla is destroyed or severed from its blood supply, the hair follicle can no longer produce new hair cells, resulting in permanent hair loss at that site.

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Structures of the Hair Shaft

The hair shaft is composed of keratinized, non-living protein cells arranged in three distinct concentric layers:

[ Hair Shaft Layer Architecture ]
+-------------------------------------------------------+
|  1. Cuticle  --> Overlapping shingle-like scale cells  |
|  2. Cortex   --> Polypeptide chains, melanin, 90% wt   |
|  3. Medulla  --> Innermost core (soft, round cells)    |
+-------------------------------------------------------+

1. The Hair Cuticle

The cuticle is the outermost layer of the hair shaft. It consists of a single overlapping layer of transparent, scale-like cells that point toward the free end of the hair strand, resembling shingles on a roof or scales on a fish.

• Protection: The primary function of the cuticle is to form a protective barrier for the delicate inner structure of the cortex.
• Porosity Control: A tight, compact cuticle creates low porosity and high shine because light reflects smoothly off the surface. A damaged, lifted, or rough cuticle creates high porosity, dullness, and vulnerability to chemical over-processing.
• Chemical Penetration: Alkaline solutions (such as permanent wave solutions, hair relaxers, and oxidative lighteners) raise the cuticle scales, allowing chemicals to penetrate deep into the cortex.

2. The Hair Cortex

The cortex is the middle layer of the hair shaft and represents approximately 90% of the total weight of the hair strand.

• Protein Structure: Composed of fibrous protein chains formed by elongated keratinized cells.
• Elasticity & Strength: All natural elasticity, tensile strength, and structural flexibility reside within the cortex.
• Melanin Pigment: Natural hair pigment (eumelanin and pheomelanin) is deposited exclusively within the cortex.
• Chemical Reactions: All chemical changes—including permanent waving, chemical relaxing, hair coloring, and lightening—take place entirely within the cortex layer.

3. The Hair Medulla

The medulla is the innermost layer of the hair shaft, composed of round, loosely packed soft cells.

• Presence: The medulla is frequently absent in very fine hair, naturally blonde hair, and infant hair. It is usually present in coarse hair, bearded hair, and resistant hair types.
• Function: The medulla plays no active role in cosmetology services, hair care treatments, or chemical processing.

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Chemical Composition of Hair & Keratinization

Hair is primarily constructed from keratin, a fibrous, tough protein that also makes up human nails and the outer layer of skin.

Keratinization Process

Living cells originate in the hair bulb, surrounded by nutrient-rich blood from the dermal papilla. As new cells form, they undergo keratinization:

1. Cells move upward through the hair follicle.
2. Cells fill with keratin protein and lose their cell nucleus.
3. The cells dry out, flatten, harden, and die.
4. By the time the hair strand emerges through the surface of the scalp, it is completely non-living keratinized protein.

The COHNS Elements

Hair protein is composed of five fundamental chemical elements, commonly remembered by the acronym COHNS:

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Amino Acids, Polypeptide Chains & Side Bonds

Proteins are built from long chains of amino acids (units of structure linked end-to-end like beads on a necklace). The strong chemical bonds that join amino acids together end-to-end are called peptide bonds (or end bonds). Long chains of amino acids linked by peptide bonds are called polypeptide chains.

CRITICAL WARNING: Cosmetologists must NEVER break peptide bonds with chemical services. If peptide bonds are broken, the polypeptide chain breaks down, causing the hair shaft to disintegrate and dissolve.

Polypeptide chains run parallel to one another within the cortex and are cross-linked by three distinct types of side bonds:

Polypeptide Chain  <=====================================>
                       ||              ||              ||
                    Hydrogen          Salt         Disulfide
                      Bond            Bond            Bond
                       ||              ||              ||
Polypeptide Chain  <=====================================>

1. Hydrogen Bonds

• Type: Physical, weak side bond resulting from an attraction between opposite electrical charges.
• Mechanism: Easily broken by water or thermal heat (blow-drying, curling irons, flat irons).
• Reforming: Reformed as the hair cools or dries into a new shape.
• Contribution: Although individually weak, there are millions of hydrogen bonds in hair, accounting for approximately one-third (1/3) of the hair's total cross-linked strength.

2. Salt Bonds

• Type: Physical, weak side bond between adjacent polypeptide chains.
• Mechanism: Broken by changes in pH—specifically by strong alkaline or strong acidic solutions.
• Reforming: Reformed when the hair's pH is restored to its normal acidic range (pH 4.5 to 5.5).
• Contribution: Accounts for approximately one-third (1/3) of the hair's total strength.

3. Disulfide Bonds

• Type: Chemical, strong side bond that joins the sulfur atoms of two neighboring cysteine amino acids to create a cystine unit.
• Mechanism: CANNOT be broken by water or heat. Broken ONLY by chemical treatments such as permanent waving solutions (thio/alkaline waves), chemical relaxers (hydroxide or thio), and heavy chemical lighteners.
• Reforming: Reformed using neutralizers or oxidizing agents during chemical texturing services.
• Contribution: Accounts for approximately one-third (1/3) of total strength and gives hair its permanent shape and structural integrity.