3.1 Structure, Composition & Growth of Hair

Introduction to Trichology & Divisions of Hair

Trichology is the scientific study of hair, its diseases, and its clinical and aesthetic care. The term originates from the Greek word trichos (meaning "hair") and the suffix -ology (meaning "the study of"). In professional cosmetology, a comprehensive understanding of trichology is essential because virtually every service performed in the salon—from precision haircutting and thermal styling to permanent waving, chemical relaxing, and oxidative hair coloring—directly impacts the biological and chemical integrity of the hair fiber.

A mature strand of human hair is biologically divided into two distinct components:

1. The Hair Root: The portion of the hair structure located beneath the surface of the epidermis. It is embedded within the hair follicle and is the living, metabolically active zone where cell division, keratinization, and nourishment take place.
2. The Hair Shaft: The portion of the hair that projects above the epidermis. The hair shaft is fully keratinized, non-living tissue. Because the hair shaft contains no blood vessels or nerve endings, any structural alteration, repair, or damage must be addressed through topical chemical and physical salon procedures.

Structures of the Hair Root

Five essential anatomical structures are located within the dermis and subcutaneous layers that generate, support, and regulate the hair fiber:

• Hair Follicle: A tube-like pocket or depression in the skin or scalp that contains and encases the hair root. Follicles are distributed across the entire human body with the exception of the palms of the hands and the soles of the feet. The follicle extends downward from the epidermis into the dermis, and in some coarse scalp hair, into the subcutaneous tissue. The angle at which the follicle slopes into the scalp determines the natural hair stream, directional grain, and growth patterns (such as cowlicks and whorls).
• Hair Bulb: The thickened, club-shaped structure that forms the lowest, deepest portion of the hair root. The bulb is hollowed at its base and fits snugly over and covers the cone-shaped dermal papilla.
• Dermal Papilla: A small, cone-shaped elevation situated directly at the base of the hair follicle that fits into the hollow base of the hair bulb. The dermal papilla is universally referred to as the "mother of the hair" because it contains the maternal blood supply, capillary network, and nerve endings that deliver oxygen, amino acids, vitamins, and hormones required for cellular division and sustained hair growth. If the dermal papilla is destroyed through trauma, disease, or electrolysis, the follicle will never regenerate a new hair strand.
• Arrector Pili Muscle: A small, involuntary band of smooth muscle tissue attached to the base of the hair follicle and the dermal tissue. When stimulated by sudden cold, emotional fright, or neurological stress, the arrector pili muscle contracts, pulling the hair follicle upright and depressing the skin around it, creating the familiar phenomenon known as "goose bumps" (cutis anserina).
• Sebaceous Glands: Microscopic oil glands connected directly to the upper portion of the hair follicles. The sebaceous glands secrete sebum, an oily lipid substance composed of triglycerides, waxes, and squalene. Sebum travels up the follicular canal to the skin surface, lubricating both the hair shaft and the scalp, preventing trans-epidermal moisture loss, maintaining hair flexibility, and forming the protective acid mantle (with a natural pH of 4.5 to 5.5).

Structures of the Hair Shaft: The Three Layers

When examined in cross-section under a microscope, the hair shaft is composed of three concentric, specialized layers:

+-------------------------------------------------------------+
|                      1. CUTICLE                             |
|  - Outermost protective layer                               |
|  - Overlapping scale-like cells (shingle roof arrangement)   |
|  - Swelled by alkaline pH (>7.0), sealed by acidic pH       |
+-------------------------------------------------------------+
|                      2. CORTEX                              |
|  - Middle fibrous protein core (~90% of total hair weight)  |
|  - Polypeptide chains, hydrogen/salt/disulfide side bonds   |
|  - Houses natural melanin pigment (eumelanin & pheomelanin) |
|  - Responsible for elasticity, tensile strength & texture   |
+-------------------------------------------------------------+
|                      3. MEDULLA                             |
|  - Innermost central core of loosely packed round cells     |
|  - Often absent in fine, blonde, or vellus hair             |
|  - Plays no functional role in salon chemical services      |
+-------------------------------------------------------------+

1. The Cuticle

The cuticle is the outermost layer of the hair shaft. It consists of a single layer of transparent, scale-like, overlapping cells pointing toward the hair ends, similar to shingles on a roof or scales on a fish. The cuticle serves as the protective shield for the delicate internal cortex:

• A healthy, compact cuticle lies flat and reflective, imparting natural shine, smoothness, and water-repellent protection.
• The cuticle is altered by pH: alkaline solutions (pH above 7.0, such as permanent waving lotions, hair lighteners, and chemical relaxers) soften and lift the cuticle scales, allowing active chemical agents to penetrate into the cortex. Conversely, acidic solutions (pH below 7.0, such as post-color acid rinses and neutralizing shampoos) contract, harden, and seal the cuticle scales flat against the cortex.

2. The Cortex

The cortex is the middle fibrous layer of the hair shaft, making up approximately 90% of the hair's total weight and bulk. It is composed of elongated, keratinized polypeptide chains wound tightly into fibrous bundles (protofibrils, microfibrils, and macrofibrils):

• The cortex provides the hair with its elasticity, natural flexibility, and exceptional tensile strength.
• The cortex houses all natural melanin pigments:
  • Eumelanin: Granular pigment producing dark brown to black tones.
  • Pheomelanin: Diffuse pigment producing red, copper, and golden-blonde tones.
• Crucial Exam Rule: All permanent structural alterations—including permanent hair coloring, decoloring (lightening), permanent waving, and chemical relaxing—take place entirely within the cortex. Without a healthy cortex, hair cannot sustain chemical services.

3. The Medulla

The medulla is the innermost core of the hair shaft, composed of loosely connected, spherical cells and microscopic air pockets. Key facts for licensing exams include:

• The medulla is often completely absent in naturally fine hair, naturally light-blonde hair, and fine vellus body hair.
• Coarse, thick hair and thick beard hair almost always contain a well-developed, continuous medulla.
• The medulla plays no significant structural or chemical role in salon chemical waving, relaxing, or hair coloring services.

Chemical Composition of Hair & COHNS Elements

Human hair is composed of roughly 90% keratin, a tough, fibrous structural protein synthesized by living cells called keratinocytes within the hair follicle. Keratin is made up of complex chains of amino acids. The fundamental chemical elemental composition of human hair is universally memorized using the acronym COHNS:

Peptide Bonds (End Bonds)

Amino acids are linked together end-to-end by strong chemical covalent bonds known as peptide bonds (also called end bonds). When hundreds of amino acids link together end-to-end via peptide bonds, they form long, spiral chains called polypeptide chains.

[!IMPORTANT] Critical State Board Warning: Peptide (end) bonds should never be broken during routine salon services. Breaking peptide bonds disintegrates the polypeptide chain itself, causing catastrophic chemical melting, structural collapse, and irreversible dissolution of the hair strand.

Side Bonds of the Cortex

The cortex consists of millions of parallel polypeptide chains held together laterally by cross-linking side bonds. These side bonds are responsible for 100% of the hair's lateral elasticity, bounce, and physical strength. There are three types of side bonds:

+-----------------------------------------------------------------------------------------+
|                                 THREE CORTEX SIDE BONDS                                 |
+-------------------+-------------------+-------------------------------------------------+
| Bond Type         | Bond Strength     | How It Is Broken & Reformed                     |
+-------------------+-------------------+-------------------------------------------------+
| 1. Hydrogen Bond  | Weak Physical     | Broken by: Water (wetting) or Heat (thermal)    |
|                   | (~1/3 of strength)| Reformed by: Drying or Cooling into new shape   |
+-------------------+-------------------+-------------------------------------------------+
| 2. Salt Bond      | Weak Physical     | Broken by: Changes in pH (acidic / alkaline)    |
|                   | (~1/3 of strength)| Reformed by: Restoring normal pH (4.5 to 5.5)   |
+-------------------+-------------------+-------------------------------------------------+
| 3. Disulfide Bond | Strong Chemical   | Broken by: Chemical reduction (perms, relaxers) |
|                   | (~1/3 of strength)| Reformed by: Chemical oxidation (neutralizers)  |
+-------------------+-------------------+-------------------------------------------------+

1. Hydrogen Bonds

A hydrogen bond is a weak, physical, ionic cross-link between adjacent polypeptide chains. Although individually weak, hydrogen bonds exist in such vast quantities that they account for approximately one-third of the hair's total structural strength.

• Hydrogen bonds are easily broken by water (when hair is wet during shampooing) or by thermal heat (such as blow dryers, curling irons, and flat irons).
• Hydrogen bonds reform automatically when the hair dries or cools into the newly set physical shape. This mechanism is the scientific basis for all temporary wet setting, roller setting, blow-drying, and thermal iron styling.

2. Salt Bonds

A salt bond is also a weak, physical, ionic cross-link between opposing positive and negative electrical charges on adjacent amino acid side chains. Salt bonds account for approximately one-third of the hair's total strength.

• Salt bonds are broken by changes in pH—specifically when hair is exposed to strong alkaline solutions (such as alkaline perm lotions or hair color) or strong acidic solutions.
• Salt bonds reform automatically when the hair's natural pH balance is restored to the normal range of 4.5 to 5.5.

3. Disulfide Bonds

A disulfide bond is a strong, covalent chemical side bond that is vastly different from physical hydrogen or salt bonds. A disulfide bond joins the sulfur atoms of two neighboring cysteine amino acids to create a single cross-linked molecule called cystine.

• Disulfide bonds account for approximately one-third of the hair's total structural strength.
• Disulfide bonds cannot be broken by water, normal drying, or thermal heat under normal styling conditions.
• Disulfide bonds are broken exclusively by chemical reduction (using reducing agents such as ammonium thioglycolate [thio] in permanent waves, or hydroxide relaxers such as sodium hydroxide) and extreme thermal destruction (temperatures exceeding 450°F / 232°C).
• In chemical texture services (permanent waving), broken disulfide bonds are chemically reformed into their new wavy configuration using an oxidizing neutralizer (hydrogen peroxide). In hydroxide relaxing, disulfide bonds are permanently converted to lanthionine bonds and can never be reformed.

The Hair Growth Cycle: Anagen, Catagen & Telogen

Hair does not grow continuously without pause; rather, each individual hair follicle cycles independently through three distinct, repetitive biological phases:

      [ 1. ANAGEN PHASE ] ---> [ 2. CATAGEN PHASE ] ---> [ 3. TELOGEN PHASE ]
      - Active growth           - Transitional phase       - Resting & shedding
      - 2 to 6 years            - 1 to 2 weeks             - 3 to 6 months
      - ~90% of scalp hair      - <1% of scalp hair        - ~10% of scalp hair

1. Anagen Phase (Active Growth Phase)

• The anagen phase is the active phase of cellular division and follicle growth during which new hair is manufactured.
• Follicle matrix cells around the dermal papilla divide rapidly, synthesizing keratin and pushing older cells upward.
• At any given time, approximately 85% to 90% of all scalp hair is in the anagen phase.
• The anagen phase lasts between 2 to 6 years (and in some individuals with exceptional genetics, up to 7 or more years).
• Normal scalp hair grows at an average rate of 0.5 inch (1.25 cm) per month, which equates to approximately 6 inches per year. Scalp hair growth is slightly faster in warm summer months and in individuals between 15 and 30 years of age.

2. Catagen Phase (Transitional Phase)

• The catagen phase is a brief transitional resting period marking the end of the active growth cycle.
• During catagen, the follicle canal shrinks, the follicle matrix ceases dividing, and the lowest part of the hair root detaches from the nourishing dermal papilla.
• The base of the detached hair bulb shrinks into a hard, club-like structure called a club hair.
• The catagen phase lasts only 1 to 2 weeks and represents less than 1% of all scalp hairs at any single point in time.

3. Telogen Phase (Resting & Shedding Phase)

• The telogen phase is the final resting and shedding phase of the hair cycle.
• The club hair remains quiescent in the resting follicle until it is naturally shed during brushing, combing, or shampooing, or until it is physically pushed out of the follicle canal by a newly emerging anagen hair beginning beneath it.
• The telogen phase lasts approximately 3 to 6 months.
• Roughly 10% to 15% of all scalp hairs are in the telogen phase at any given time.
• Normal Daily Shedding: It is completely normal and healthy for an individual to shed between 50 and 100 hairs per day. Daily shedding within this range is a normal consequence of the continuous renewal of the telogen phase.