3.2 Hair Shaft Layers, Follicle Anatomy & Chemical Bonds
Key Takeaways
- Trichology is the scientific study of hair, its diseases, and care, dividing hair into the root (below skin surface) and shaft (above skin surface).
- The hair shaft consists of three distinct layers: the protective cuticle, the fibrous cortex (containing 90% of hair weight and melanin), and the central medulla.
- All chemical salon services—including permanent coloring, chemical relaxing, and permanent waving—take place exclusively within the cortex.
- Hair strength relies on three side bonds: physical hydrogen bonds (broken by water/heat), physical salt bonds (broken by pH changes), and chemical disulfide bonds (broken only by chemical reducing agents or hydroxides).
- The hair growth cycle progresses through three distinct phases: Anagen (active growth, 2-6 years), Catagen (transition, 1-2 weeks), and Telogen (resting, 3-6 months).
3.2 Hair Shaft Layers, Follicle Anatomy & Chemical Bonds
Trichology is the scientific study of hair, its diseases, and care. For cosmetologists, understanding the internal and external anatomy of hair—along with its chemical composition—is vital for executing successful color, perm, and chemical relaxing services without causing structural failure or hair breakage.
Hair Divisions: Root & Shaft
A mature human hair strand is divided into two major parts: the hair root (located beneath the surface of the epidermic skin) and the hair shaft (the non-living portion projecting above the skin surface).
Structures of the Hair Root
Hair Shaft (Visible)
~~~~~~~~~~ SKIN SURFACE ~~~~~~~~~~
| |
| Fol- | <-- Hair Follicle (45-degree angle pocket)
| licle |
+----+ +----+
| Sebaceous Gland |
+----+ +----+
| Hair | <-- Arrector Pili Muscle attached
| Bulb | <-- Club-shaped base
( Dermal ) <-- Dermal Papilla (Blood & nerve supply)
=======
- Hair Follicle: A tube-like depression or pocket in the skin or scalp that contains the hair root. Follicles extend down into the dermis and are slanted at angles ranging from 45 degrees, creating natural growth patterns (whorls and cowlicks).
- Hair Bulb: The lowest area of the hair strand—a thickened, club-shaped structure that fits over and covers the dermal papilla.
- Dermal Papilla: A small, cone-shaped elevation located at the base of the hair follicle. It contains the blood vessel and nerve supply that provides essential nutrients needed for cell mitosis and hair growth. If the dermal papilla is destroyed, hair growth ceases.
- Arrector Pili Muscle: A small, involuntary smooth muscle attached to the side of the hair follicle. Cold temperature or fear causes this muscle to contract, pulling the follicle upright and creating "goosebumps."
- Sebaceous Glands: Oil glands connected to the follicle that secrete sebum to keep the hair lubricated and pliable.
Microscopic Layers of the Hair Shaft
The hair shaft consists of three primary concentric layers:
| Hair Shaft Layer | Microscopic Structure | Function & Cosmetology Significance |
|---|---|---|
| Cuticle | Outermost layer formed of transparent, scale-like overlapping cells (resembling shingles on a roof). | Protects the inner cortex. Controls hair porosity. A healthy cuticle lies flat, giving hair shine and smoothness. |
| Cortex | Middle fibrous protein layer composed of elongated polypeptide chains. Contains ~90% of total hair weight. | Contains melanin (natural pigment), provides strength, elasticity, and structure. All chemical processing (perms, relaxers, permanent color) takes place here. |
| Medulla | Innermost core consisting of round, loosely connected cells. | Often absent in fine or naturally light blonde hair. Plays no active role in chemical salon services. |
Chemical Composition of Hair & Keratinization
Hair is composed primarily of keratinized protein (roughly 91% protein, 6% to 8% water, and trace lipids/minerals). As living hair cells form in the follicle bulb, they push upward, mature, fill with keratin protein, lose their nuclei, and die—a process called keratinization.
The COHN Elements
The primary chemical elements that compose hair protein are commonly remembered by the acronym COHN:
- Carbon (C): 51%
- Oxygen (O): 21%
- Hydrogen (H): 6%
- Nitrogen (N): 17%
- Sulfur (S): 5%
Amino acids are linked together end-to-end by strong peptide bonds (end bonds) to form long polypeptide chains. Cosmetologists must never break peptide bonds with harsh chemicals, as breaking end bonds destroys the polypeptide chain, resulting in chemical haircutting (hair disintegration).
Chemical Side Bonds of the Cortex
The long polypeptide chains of the cortex are linked crosswise by three types of side bonds. These side bonds account for hair's elasticity, tensile strength, and structural shape during styling and texturing.
POLYPEPTIDE CHAIN ======== [ HYDROGEN BOND ] ======== POLYPEPTIDE CHAIN
======== [ SALT BOND ] ========
======== [ DISULFIDE BOND] ========
1. Hydrogen Bonds
- Type: Physical, weak cross-link side bond.
- Mechanism: Easily broken by water or thermal heat (curling irons, blow drying).
- Re-formation: Reformed as the hair cools or dries completely into a new shape.
- Strength: Account for 1/3 of the hair's total cross-bond strength due to their vast quantity.
2. Salt Bonds
- Type: Physical, weak ionic cross-link side bond between adjacent polypeptide chains.
- Mechanism: Depend on pH balance; easily broken by strong acidic or alkaline solutions.
- Re-formation: Reformed when the hair's pH returns to its natural balance (4.5 to 5.5).
- Strength: Account for 1/3 of the hair's total cross-bond strength.
3. Disulfide Bonds
- Type: Chemical, strong covalent side bond formed between two sulfur (S) atoms of adjacent cysteine amino acids.
- Mechanism: Cannot be broken by water, wet heat, or physical styling. Broken ONLY by chemical reducing agents in permanent wave solutions (thioglycolate compounds) or high-pH chemical relaxers (sodium hydroxide / lye).
- Re-formation: Reformed through chemical neutralization (oxidation) in perms, or permanently converted to lanthionine bonds during hydroxide relaxing.
- Strength: Account for 1/3 of the hair's total strength, providing structural integrity.
The Hair Growth Cycle
Hair growth occurs in repeating cycles consisting of three main phases:
- Anagen Phase (Active Growth): The growth phase during which new hair cells are produced rapidly in the follicle bulb. Lasts between 2 to 6 years (sometimes up to 8 years). Approximately 85% to 90% of scalp hair is in the anagen phase at any given time.
- Catagen Phase (Transition): The brief transitional stage between growth and resting. The follicle shrinks, detaches from the dermal papilla, and the hair bulb disappears. Lasts 1 to 2 weeks.
- Telogen Phase (Resting): The final resting phase during which the hair is shed or pushed out by new anagen growth starting beneath it. Lasts 3 to 6 months (roughly 100 days). Approximately 10% to 15% of scalp hair is in telogen.
Normal Shedding Rates
- The average natural shedding rate is 50 to 100 hairs per day.
- Factors affecting growth cycle duration include age, genetics, hormonal fluctuations, nutrition, and general health status.
Which physical side bond in the cortex accounts for one-third of hair strength and is easily broken by water or thermal heating tools?
All professional chemical services—including hair coloring, permanent waving, and chemical relaxing—take place in which layer of the hair shaft?
During which phase of the hair growth cycle does the hair follicle shrink and detach from the dermal papilla?