4.1 Trichology: Hair Structure, Composition & Growth Cycles

4.1 Trichology: Hair Structure, Composition & Growth Cycles

Quick Answer: Trichology is the scientific study of hair, its diseases, and care. Hair is divided into the root (below the skin surface within the follicle) and the shaft (above the skin). The hair shaft contains three concentric layers: the outer protective cuticle, the fibrous cortex (providing 80%–90% of weight, strength, pigment, and elasticity), and the innermost medulla. Hair protein (keratin) is linked longitudinally by peptide bonds and cross-linked laterally by three side bonds: hydrogen, salt, and disulfide bonds—each responsible for one-third of the hair's lateral strength. Hair cycles through Anagen (active growth, 3–5+ years), Catagen (transition, 1–2 weeks), and Telogen (resting/shedding, 3–6 months) phases, shedding 50–100 hairs daily under normal conditions.

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Foundations of Trichology

In the cosmetology profession, an exceptional hairstyle, permanent wave, or vibrant color transformation begins with a scientific mastery of the hair fiber itself. The scientific study of hair, its diseases, and its clinical care is known as trichology (derived from the Greek words trichos, meaning "hair," and ology, meaning "the study of").

A mature, healthy strand of human hair is divided into two primary structural divisions:

1. The Hair Root: The portion of the hair located beneath the surface of the epidermis, enclosed within the hair follicle in the dermis.
2. The Hair Shaft: The non-living, fully keratinized portion of the hair that projects outward beyond the skin surface.

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Anatomical Structures of the Hair Root

Five primary structures are situated within the dermis to govern hair development, nourishment, lubrication, and movement:

1. The Hair Follicle

The hair follicle is a tube-like, pocket-shaped depression or cavity in the skin or scalp that encloses the hair root. Follicles extend downward from the epidermis into the dermis (and occasionally into the subcutaneous fatty tissue). Hair follicles are distributed over virtually the entire human body, with the exception of the palms of the hands and the soles of the feet. The angle at which a follicle slopes into the skin dictates the natural direction, growth pattern, and hair stream (such as cowlicks and whorls) of the hair shaft.

2. The Hair Bulb

The hair bulb is the lowest, enlarged area of a hair strand. It forms a thickened, club-shaped structure that fits over and covers the dermal papilla. The bulb contains living, actively dividing germinal matrix cells that produce new hair fibers.

3. The Dermal Papilla

The dermal papilla (plural: papillae) is a small, cone-shaped elevation located at the absolute base of the hair follicle that nests directly inside the cup of the hair bulb. The dermal papilla contains a rich capillary network and nerve endings that supply oxygen, amino acids, and vital nutrients to the multiplying germinal matrix cells. Because the dermal papilla feeds and generates the growing hair, it is widely referred to in cosmetology as the "mother of the hair." If the dermal papilla is destroyed, the follicle ceases to produce hair.

4. The Arrector Pili Muscle

The arrector pili muscle is a tiny, involuntary bundle of smooth muscle fibers attached to the base of the hair follicle. In response to environmental cold, fear, or emotional shock, the sympathetic nervous system stimulates this muscle to contract. The contraction pulls the hair follicle upright, causing the hair strand to stand perpendicular to the skin and creating visible goosebumps (cutis anserina).

5. Sebaceous Glands

The sebaceous glands are microscopic oil glands located in the dermis and connected directly to the hair follicles. These glands secrete an oily lipid substance known as sebum. Sebum travels up the follicular canal to lubricate the hair shaft and scalp surface. When sebum mixes with perspiration, it forms the acid mantle (pH 4.5–5.5), which protects the skin and cuticle from external pathogens and excessive dehydration.

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Anatomical Layers of the Hair Shaft

When viewed in cross-section under a high-power microscope, the hair shaft reveals three distinct concentric structural layers:

[ Cuticle ]  --> Exterior transparent overlapping shingles (Barrier protection)
  [ Cortex ]   --> Middle fibrous core, 80%-90% weight (Strength, pigment, elasticity)
    [ Medulla ]  --> Innermost central core (Pith; absent in fine/blond hair)

1. The Cuticle

The cuticle is the outermost protective layer of the hair shaft. It consists of a single overlapping layer of transparent, scale-like keratinized cells that resemble shingles on a roof or scales on a fish. Each cuticle scale points downward toward the free end (tip) of the hair strand:

• Barrier Protection: A healthy, intact cuticle provides an impermeable shield protecting the delicate internal cortex from friction, chemical penetration, and thermal stress.
• Reflectivity & Shine: In healthy hair, cuticle scales lie smooth, flat, and compact, reflecting light to produce natural gloss and silky texture.
• Chemical Responsiveness: Acidic solutions (pH below 7.0) cause cuticle scales to shrink, tighten, and lie flat. Alkaline solutions (pH above 7.0) soften and swell the cuticle scales, causing them to flare open so chemical waving, relaxing, or permanent hair coloring solutions can access the cortex.

2. The Cortex

The cortex is the middle fibrous protein layer of the hair shaft. It accounts for approximately 80% to 90% of the hair's total weight and tensile strength:

• Fibrous Protein Architecture: The cortex is formed from millions of elongated polypeptide chains organized into protofibrils, microfibrils, and macrofibrils aligned parallel to the length of the hair strand.
• Melanin Pigment: Natural hair color is produced by melanin pigment granules embedded exclusively within the cortex. The two types of melanin are eumelanin (providing dark brown and black tones) and pheomelanin (providing lighter tones of red, copper, and blond).
• Elasticity & Flexibility: The natural wave pattern, elasticity, and bending strength of human hair reside entirely within the cortex.
• Chemical Alteration Target: All permanent chemical services—including permanent waving, chemical relaxers, lighteners, and permanent oxidative hair colors—exert their chemical actions within the cortex.

3. The Medulla

The medulla is the innermost core of the hair shaft. It consists of loosely packed, rounded cells with air spaces (often described as the pith or marrow of the hair):

• Distribution: The medulla is naturally present in coarse, thick terminal hairs and beard hairs, but it is frequently fragmented or completely absent in naturally fine, delicate, or blond hair.
• Salon Significance: The medulla plays no active role in cosmetology chemical texture or hair coloring services; chemical services succeed completely without affecting the medulla.

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Biochemical Composition of Hair: Keratinization & COHNS Elements

Living human hair is composed of approximately 90% protein, specifically a tough, insoluble fibrous protein called hard keratin. Hair also contains trace moisture (roughly 10%–15%), natural lipids, and mineral salts.

The Keratinization Process

Hair originates as living cellular tissue at the dermal papilla. As germinal cells in the hair bulb multiply rapidly through mitosis, older cells are continuously pushed upward along the follicular canal:

1. As these cells move away from their vascular blood supply, they undergo a biological maturation process called keratinization.
2. The cells fill with keratin protein fibers, lose their cellular fluid, shed their living cell nuclei (nucleolysis), and dehydrate.
3. By the time the emerging hair fiber emerges past the surface of the scalp, it is completely non-living, keratinized tissue.

The COHNS Elements

The keratin protein of human hair is composed of five fundamental chemical elements, universally known in cosmetology by the acronym COHNS:

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

These elements combine to form long molecular units known as amino acids. Amino acids are the basic building blocks of all proteins.

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Peptide Bonds vs. Cross-Linking Side Bonds

To understand how hair withstands daily brushing or how chemical relaxers permanently straighten curls, cosmetologists must distinguish between longitudinal end bonds and lateral side bonds.

Peptide (End) Bonds

Amino acids are linked end-to-end like railroad cars on a track. The chemical bond that joins the carboxyl group (-COOH) of one amino acid to the amino group (-NH2) of an adjacent amino acid is called a peptide bond (or end bond):

• A long chain of amino acids linked together by peptide bonds is called a polypeptide chain.
• Polypeptide chains spiral around each other in a coiled helix configuration.
• Critical Safety Rule: Peptide bonds must never be broken during salon chemical services. If peptide bonds are dissolved (by over-processing, excessively caustic chemicals, or severe heat degradation), the polypeptide chains shatter, causing the hair fiber to break, melt, or disintegrate completely.

The Three Lateral Side Bonds

While peptide bonds connect amino acids end-to-end to form the length of the chain, millions of parallel polypeptide chains are held together side-by-side by three types of cross-linking side bonds:

1. Hydrogen Bonds
2. Salt Bonds
3. Disulfide Bonds

Each of these three side bonds accounts for approximately one-third (33.3%) of the hair's overall lateral tensile strength and elasticity.

+-------------------------------------------------------------+
|               HAIR TENSILE STRENGTH BREAKDOWN               |
+-------------------------------------------------------------+
|  [ Hydrogen Bonds: 33.3% ]   --> Physical (Broken by water/heat)
|  [ Salt Bonds: 33.3% ]       --> Physical (Broken by pH shifts)
|  [ Disulfide Bonds: 33.4% ]  --> Chemical (Broken by chemical texture)
+-------------------------------------------------------------+

1. Hydrogen Bonds (Weak Physical Cross-Links)

• Nature: A weak, physical side bond resulting from the attraction between opposite electrical charges (a slightly positive hydrogen atom attracted to a slightly negative oxygen or nitrogen atom on an adjacent chain).
• How They Are Broken: Easily broken by water, moisture, or heat.
• How They Reform: Reformed when the hair dries or cools.
• Salon Application: Wet setting, blow-drying, flat ironing, and curling irons temporarily reform hydrogen bonds to produce temporary curls or sleek straightening. As soon as the hair absorbs environmental humidity or is shampooed, hydrogen bonds collapse back into their natural resting state.

2. Salt Bonds (Weak Physical Cross-Links)

• Nature: A weak, physical side bond resulting from ionic attractions between positive and negative electrical charges on adjacent amino acid side chains.
• How They Are Broken: Broken by changes in pH—exposure to acidic or alkaline solutions.
• How They Reform: Automatically reformed when the hair's pH is restored to its normal, natural acidic level (pH 4.5 to 5.5).
• Salon Application: Applying alkaline shampoo or waving lotion opens salt bonds, softening the hair fiber. Applying an acidic conditioner or neutralizing solution restores natural pH, rebonding the salt cross-links.

3. Disulfide Bonds (Strong Chemical Covalent Cross-Links)

• Nature: A strong chemical, covalent bond joining the sulfur atoms of two adjacent cysteine amino acids to create one cystine molecule.
• How They Are Broken: Disulfide bonds cannot be broken by water, humidity, heat, or normal mechanical tension. They are broken only by chemical reducing agents (such as ammonium thioglycolate waving lotion) or strong alkaline hydroxide relaxers.
• How They Reform: In permanent waving, broken disulfide bonds are chemically reformed into new curly positions through the application of an oxidizing neutralizer (hydrogen peroxide). In hydroxide relaxers, disulfide bonds are permanently broken and converted into lanthionine bonds (lanthionization), which can never be reformed.

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The Triphasic Hair Growth Cycle

Human scalp hair does not grow continuously. Instead, every individual hair follicle cycles through three distinct, repeating biological phases: Anagen, Catagen, and Telogen.

1. Anagen Phase (Active Growth Phase)

• Duration: Typically lasts 3 to 5 years, and in some individuals up to 7 years.
• Biological Activity: Germinal matrix cells in the hair bulb divide rapidly through mitosis, pushing older cells upward into the follicular canal where they keratinize into hard hair fiber.
• Population: Approximately 85% to 90% of all scalp hair is in the anagen phase at any given moment.
• Growth Rate: Scalp hair grows at an average rate of one-half inch (1.25 cm) per month (approximately 6 inches per year). The length of an individual's anagen phase dictates their genetic maximum hair length.

2. Catagen Phase (Transitional Phase)

• Duration: Extremely brief, lasting approximately 1 to 2 weeks.
• Biological Activity: Mitosis ceases completely. The follicle canal shrinks and detaches from the dermal papilla. The lower end of the hair fiber rounds into a club-like shape, creating a club hair.
• Population: Less than 1% of all scalp hair is in the catagen phase at any time.

3. Telogen Phase (Resting & Shedding Phase)

• Duration: Lasts approximately 3 to 6 months.
• Biological Activity: The hair follicle rests in a dormant state. The club hair remains loosely anchored in the follicle until it is shed naturally during washing or brushing, or pushed out by a new anagen hair emerging beneath it.
• Population: Approximately 10% to 15% of scalp hair is in the telogen phase.
• Cycle Renewal: Once the telogen phase completes, the follicle regenerates, reconnects with the dermal papilla, and enters a brand-new anagen phase.

Physiological Daily Shedding Rates

Because each follicle operates on its own independent biological clock, hair shedding occurs gradually and evenly across the scalp:

• The normal physiological rate of daily hair shedding is 50 to 100 hairs per day.
• Daily shedding within this range is entirely normal and does not represent alopecia or permanent hair loss.
• When shedding noticeably exceeds 100 hairs per day over an extended period, clinical conditions such as telogen effluvium or androgenic alopecia should be investigated.