2.1 Hair Structure, Composition, and Growth Cycles

2.1 Hair Structure, Composition, and Growth Cycles

Introduction to Trichology

The scientific study of hair, its diseases, and its care is called trichology, which comes from the Greek words trichos (meaning hair) and logos (meaning the study of). As a licensed cosmetologist in the state of Florida, having a deep, scientific understanding of hair structure, composition, and growth cycles is essential for your professional practice. It enables you to perform services safely, understand how chemical treatments affect hair at the molecular level, and recognize when a scalp condition requires a referral to a medical professional.

Hair Root and Hair Shaft Structures

A mature strand of human hair is divided into two principal parts: the hair root and the hair shaft.

1. The hair root is the portion of the hair located below the skin's surface. This is the living part of the hair structure where cells are formed, nourished, and grow.
2. The hair shaft is the portion of the hair that projects above the epidermis. It is composed of non-living, keratinized protein.

Structures of the Hair Root

The hair root is housed within the skin and consists of five main structures that work together to produce and support hair growth:

• Hair Follicle: The tube-like pocket or depression in the skin or scalp that contains the hair root. Hair follicles are distributed all over the 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, wrapping around the hair root.
• Hair Bulb: The lowest part of a hair strand. It is a thickened, club-shaped structure that forms the lower part of the hair root. It fits over and covers the dermal papilla.
• Dermal Papilla: A small, cone-shaped elevation located at the base of the hair follicle that fits into the hair bulb. The dermal papilla contains the blood and nerve supply that provides the essential nutrients needed for hair growth and regeneration. Often referred to as the 'mother of the hair,' if the dermal papilla is destroyed, the follicle will cease to produce hair.
• Arrector Pili Muscle: A tiny, involuntary muscle in the base of the hair follicle. Cold temperatures or strong emotions (such as fear or excitement) cause this muscle to contract, pulling the follicle upright and making the hair stand straight up, which results in what is commonly called 'goosebumps.'
• Sebaceous Glands: These are the oil glands of the skin, connected to the hair follicles. They secrete a fatty or oily substance called sebum, which lubricates the skin and hair, keeping them soft, pliable, and water-resistant.

The Three Layers of the Hair Shaft

The hair shaft is composed of three distinct layers of cells. Understanding these layers is critical for chemical services, as different products target specific layers.

graph TD
    A[Hair Shaft] --> B["Cuticle: Outermost Protective Scale-Like Layer"]
    A --> C["Cortex: Middle Protein Core - 90% of Weight"]
    A --> D["Medulla: Innermost Pith - Absent in Fine Hair"]
    
    C --> E[Melanin Pigment]
    C --> F[Polypeptide Chains & Side Bonds]

Chemical Composition and Keratinization

Hair is composed of protein that grows from cells originating within the hair follicle. As these cells mature, they undergo a process called keratinization. During keratinization, the cells fill with a fibrous protein called keratin, lose their nucleus, move upward, and die. By the time the hair shaft emerges from the scalp, it is completely non-living fiber.

This keratin protein is made up of long chains of amino acids, which are composed of five main elements (often remembered by the acronym COHNS):

• Carbon: 51%
• Oxygen: 21%
• Hydrogen: 6%
• Nitrogen: 17%
• Sulfur: 5%

The Keratinization Process

The process of keratinization begins deep within the follicle at the hair bulb. Here, the hair cells divide rapidly through mitosis. As these new cells are pushed upward, they undergo a series of chemical changes. They absorb keratin proteins and begin to dehydrate. The cell membranes fuse together, and the cells eventually lose their nuclei, organelles, and moisture, leaving behind a highly organized structure of dead protein. This explains why the hair shaft does not have blood vessels or nerves, meaning it cannot repair itself if damaged.

Understanding Peptide and Polypeptide Chains

Proteins are made of long chains of amino acids. These amino acids are joined together end-to-end by strong, chemical peptide bonds (also known as end bonds). A long chain of amino acids linked by peptide bonds is called a polypeptide chain. The cosmetologist must understand that chemical services must never break these peptide bonds, as doing so will permanently destroy the hair shaft, causing it to disintegrate. Chemical texturizers instead target the side bonds (hydrogen, salt, and disulfide bonds) that connect these polypeptide chains parallel to each other.

Side Bonds of the Cortex

The cortex is made of polypeptide chains linked together by three types of cross-bonds or side bonds that give hair its strength and elasticity:

1. Hydrogen Bonds: Weak, physical side bonds that are easily broken by water or heat. They reform as the hair dries or cools. These bonds are responsible for temporary wet-setting styling.
2. Salt Bonds: Weak, physical side bonds that are broken by changes in pH (highly alkaline or acidic solutions). They reform when the pH is restored to normal (4.5 to 5.5).
3. Disulfide Bonds: Strong, chemical side bonds that can only be broken by chemical solutions (such as permanent wave solutions or chemical relaxers) or extreme heat. They cannot be broken by water or heat. Reforming these bonds is what permanently changes the hair's shape.

Hair Growth Cycles

Hair growth occurs in repeating cycles rather than continuously. The average rate of hair growth on the human scalp is about 0.5 inches (1.25 cm) per month. The three phases of the hair growth cycle are:

• Anagen Phase: The active growth phase. New hair cells are constantly produced in the follicle. Approximately 90% of scalp hair is in this phase at any given time. It lasts from 2 to 6 years, determining the maximum length a person's hair can grow.
• Catagen Phase: The brief transition period between growth and resting. The follicle canal shrinks and detaches from the dermal papilla, forming a 'club hair.' Less than 1% of scalp hair is in this phase, which lasts only 1 to 2 weeks.
• Telogen Phase: The resting phase. The fully grown hair remains in the follicle until it is shed or pushed out by new anagen growth. Approximately 9% to 10% of scalp hair is in this phase, which lasts about 3 to 6 months.

On average, a healthy person loses 35 to 40 hairs per day as part of the natural hair shedding cycle.