4.3 Hair Anatomy, Chemical Composition, and Growth Phases

4.3 Hair Anatomy, Chemical Composition, and Growth Phases

Trichology—derived from the Greek trichos (hair) and logos (study)—is the biological science of the structure, functions, and diseases of human hair. For the master barber, hair is the primary medium of daily craft. Every shear cut, clipper fade, straight razor taper, and chemical texture service directly manipulates the microscopic physical and chemical architecture of the hair fiber. Developing an authoritative grasp of hair histology, the molecular composition of keratin, and the biological rhythm of the hair growth cycle equips the practitioner to anticipate hair behavior, protect the structural integrity of the strand, and provide scientifically sound client consultations.

Structures of the Hair Root

A mature strand of human hair is divided into two primary anatomical divisions: the hair root, which lies encased beneath the surface of the skin within the follicle, and the hair shaft, the non-living keratinized portion that projects outward above the epidermis.

The hair root comprises five essential structures situated within the dermis and subcutaneous tissue:

1. Hair Follicle: A tube-like pocket or cylindrical depression in the skin or scalp that encloses and anchors the hair root. Follicles originate in the dermis and frequently extend into the subcutaneous adipose layer. The angle at which the follicle tilts beneath the skin dictates the hair stream (the natural angle and direction of hair growth), as well as natural growth patterns such as whorls and cowlicks. Follicles are distributed over the entire body, excluding the palms of the hands and soles of the feet.
2. Hair Bulb: The lowest, thickened, club-shaped swelling at the base of the hair root. It fits directly over and completely caps the dermal papilla. The lower cellular zone of the bulb contains the hair matrix—a concentrated cluster of active germinative stem cells whose rapid division produces the keratinocytes that form the emerging hair shaft.
3. Dermal Papilla: A small, cone-shaped elevation of vascular connective tissue located at the absolute base of the hair follicle that fits snugly into the hollow cavity of the hair bulb. The dermal papilla contains a dense network of blood capillaries and nerve endings that supply oxygen, amino acids, and vital nutrients to the multiplying cells of the hair matrix. Historically termed the "mother of the hair," the dermal papilla is essential for follicular life; if the dermal papilla is destroyed, the follicle atrophies and can never regenerate a new hair strand.
4. Arrector Pili Muscle: A tiny, involuntary band of smooth muscle fiber attached to the connective tissue sheath of the hair follicle and anchored to the papillary layer of the dermis. Activated by sympathetic nerve impulses in response to environmental cold or emotional stress (fear, excitement), the arrector pili contracts, pulling the hair follicle upright and elevating the surrounding skin to create "goose bumps" (cutis anserina). This muscular contraction simultaneously compresses adjacent sebaceous glands, expressing sebum into the follicular canal.
5. Sebaceous Glands: Sac-like exocrine glands connected directly to the upper third of the hair follicle via a duct. They secrete sebum, an oily substance that lubricates the emerging hair strand and coats the epidermal surface, imparting pliability, moisture retention, and shine.

Microscopic Anatomy of the Hair Shaft

The hair shaft is a fully keratinized, non-living biological filament composed of three concentric layers: the cuticle, the cortex, and the medulla.

1. The Cuticle

The cuticle is the outermost protective armor of the hair shaft. It consists of a single layer of flat, transparent, scale-like cells that overlap one another like shingles on a roof or scales on a fish, with the free scale edges pointing upward toward the hair tip.

• Physical Barrier: The cuticle protects the delicate internal cortex from mechanical abrasion, heat damage, ultraviolet light, and environmental degradation.
• Porosity Regulation: In healthy, untreated hair, cuticle scales lie tightly compressed, creating a smooth, reflective surface that gives hair natural shine and resists moisture penetration (low porosity). Alkaline chemical agents (such as permanent wave solutions, hair relaxers, and oxidative hair colors) cause the cuticle scales to swell and flare open, allowing chemical active ingredients to penetrate into the cortex.
• Acid Balance: Mild acidic rinses and conditioners with a pH between 4.5 and 5.5 cause swollen cuticle scales to contract and lie flat, restoring shine and locking in moisture.

2. The Cortex

The cortex is the thick, fibrous middle layer of the hair shaft, constituting approximately 90 percent of the total hair weight and mass. It is formed from elongated, spindle-shaped cortical cells aligned parallel to the length of the hair strand.

• Keratin Polypeptide Chains: The cortex contains complex helical keratin proteins organized into protofibrils, microfibrils, and macrofibrils. This molecular structure gives hair its extraordinary elasticity, flexibility, and tensile strength.
• Melanin Pigment: The cortex houses all natural melanin granules that determine hair color:
  • Eumelanin: Provides brown and black tones.
  • Pheomelanin: Provides red, ginger, and yellow-blonde tones.
• Chemical Processing Core: Virtually all chemical restructuring services—including cold waving, sodium hydroxide relaxing, and permanent oxidative coloring—take place exclusively within the cortex. Breaking and reforming the structural side bonds of the cortex enables the barber to alter the hair's curl pattern permanently.

3. The Medulla

The medulla is the innermost core or central canal of the hair shaft, often referred to as the "pith" or marrow of the hair. It is composed of rows of rounded, loosely connected cells and air spaces.

• Biological Variation: The medulla is frequently discontinuous, fragmented, or completely absent in very fine hair, naturally blonde hair, and vellus (peach fuzz) hair. Coarse terminal hair and beard hair generally have a prominent, continuous medulla.
• Service Relevance: The medulla plays no active physiological role in haircutting or chemical restructuring services; chemical solutions bypass or ignore the medulla entirely.

Chemical Composition of Hair: The COHNS Elements and Side Bonds

Hair is composed primarily of keratin, a fibrous, sulfur-rich protein synthesized from 21 distinct amino acids. Amino acids are linked end-to-end by strong chemical peptide bonds (also called end bonds) to build long polypeptide chains. Polypeptide chains twist into a helical coil called an alpha-helix, which then cross-links with neighboring chains to establish the three-dimensional structural stability of the hair fiber.

The COHNS Elemental Analysis

Normal human hair keratin is composed of five fundamental chemical elements, easily remembered by the acronym COHNS:

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

While sulfur accounts for only ~5 percent of hair's elemental composition, it plays a vital structural role because sulfur atoms form the covalent disulfide cross-links that give keratin its rigidity and chemical resistance.

The Three Side Bonds of the Cortex

The helical polypeptide chains within the cortex are held together side-by-side by three distinct varieties of chemical side bonds:

1. Hydrogen Bonds: Weak, physical, cross-organizing bonds formed by electrical attraction between opposite electrical charges on adjacent peptide chains. Hydrogen bonds account for approximately one-third of the hair's total structural strength. They are readily broken by water, perspiration, or thermal heat, and reform as the hair dries or cools. Every wet set, blow-dry, or hot-tool style relies entirely on breaking and reforming hydrogen bonds.
2. Salt Bonds: Weak, physical side bonds that result from ionic attraction between positive and negative charges on neighboring acidic and basic amino acid side chains. Like hydrogen bonds, salt bonds account for one-third of hair's strength. They are broken by changes in pH (both strong alkaline and strong acidic solutions) and reform automatically when the hair returns to its natural isoelectric pH range (4.5 to 5.5).
3. Disulfide Bonds: Strong, covalent chemical bonds formed between the sulfur atoms of two adjacent cysteine amino acids, creating a new unit called cystine. Although fewer in number than hydrogen or salt bonds, disulfide bonds account for the remaining one-third of hair strength and provide its permanent curl memory and chemical stability. Disulfide bonds cannot be broken by water, heat, or mild pH changes; they can only be broken by reducing agents (such as ammonium thioglycolate in perms and thio relaxers) or high-pH alkaline hydroxides (which permanently convert disulfide bonds into lanthionine bonds).

The Hair Growth Cycle: Anagen, Catagen, and Telogen

Hair does not grow continuously without interruption. Instead, every hair follicle on the human body operates independently through an asynchronous, repetitive biological cycle consisting of three distinct phases: Anagen, Catagen, and Telogen. Because neighboring follicles reside in different phases simultaneously, humans shed hair gradually rather than molting all at once.

1. Anagen Phase (Active Growth Phase)

The anagen phase is the active vegetative stage during which hair bulb matrix cells undergo rapid mitosis, continuously dividing and pushing upward to become keratinized into new hair shaft tissue.

• Duration: Lasts between 2 to 6 years on the human scalp, determined primarily by genetics, age, and overall health.
• Distribution: At any given moment, approximately 85 to 90 percent of all scalp hairs reside in the anagen phase.
• Growth Rate: Scalp hair grows at an average rate of approximately 1/2 inch (1.25 cm) per month, or roughly 6 inches per year. Growth is generally fastest between the ages of 15 and 30 and slows slightly with advanced age.

2. Catagen Phase (Transitional / Regression Phase)

The catagen phase is a brief, highly organized regression period signaling the end of the active growth stage.

• Follicular Changes: Mitotic cell division in the matrix ceases completely. The hair follicle shrinks to roughly one-sixth of its normal length, the follicular canal narrows, and the lower portion of the follicle detaches entirely from the dermal papilla. The base of the hair root keratinizes into a hard, rounded, white knob known as a club hair.
• Duration: Lasts only 1 to 2 weeks.
• Distribution: Less than 1 percent of all scalp hairs are in the catagen phase at any single time.

3. Telogen Phase (Resting Phase)

The telogen phase is the final, quiescent resting period of the hair follicle cycle.

• Follicular Behavior: The shrunken follicle rests quietly in the upper dermis. The club hair remains loosely seated in the follicle until it is shed naturally during washing, brushing, or combing, or until the follicle re-awakens into a new anagen cycle and the budding new hair pushes the old club hair out of the follicular ostium.
• Duration: Lasts approximately 3 to 6 months on the human scalp.
• Distribution: Approximately 10 to 15 percent of scalp hairs are in the telogen phase at any given time.
• Normal Daily Shedding: It is completely normal and physiological for an adult human to shed between 50 and 100 hairs per day as telogen follicles complete their rest and cycle back into active anagen growth.