Hair Biology: Structure, Growth, and Types

The Structure of Hair

Human hair is a complex biological structure primarily composed of a protein called keratin. Each strand of hair originates from a hair follicle, a tiny organ embedded in the skin. A hair strand has two main parts: the hair shaft and the hair root.

The Hair Shaft

The hair shaft is the visible part of the hair that extends above the skin's surface. It is considered "dead" tissue because it lacks nerves and blood vessels. The hair shaft itself consists of three primary layers:

• Cuticle: The outermost layer, composed of overlapping, transparent, scale-like cells. In healthy hair, these scales lie flat, creating a smooth surface that reflects light and provides protection. Damage to the cuticle, such as from heat styling or chemical treatments, can cause these scales to lift, leading to rough texture, frizz, and dullness.
• Cortex: The thickest layer, located beneath the cuticle. The cortex is made up of elongated, keratinized cells that contain melanin, the pigment responsible for hair color. It provides hair with its strength, elasticity, and determines its texture (straight, wavy, curly).
• Medulla: The innermost layer, present in some but not all hair types. The medulla is a soft, often discontinuous core that plays a less understood role, though it may contribute to the hair's overall thickness and insulation.

The Hair Root

The hair root is the part of the hair enclosed within the follicle, beneath the skin's surface. This is the living part of the hair, where cell division and growth occur. Key components of the hair root include:

• Hair Bulb: The base of the hair root, a bulb-shaped structure that surrounds the dermal papilla.
• Dermal Papilla: A small, cone-shaped indentation at the base of the hair bulb, containing blood vessels that supply nutrients essential for hair growth and nerves.
• Hair Matrix: Located within the hair bulb, above the dermal papilla. This is where active cell division (mitosis) takes place, producing new hair cells that push older cells upward, forming the hair shaft.
• Arrector Pili Muscle: A tiny muscle attached to the hair follicle. When it contracts, it pulls the hair upright, causing "goosebumps."
• Sebaceous Gland: A gland that produces sebum, an oily substance that lubricates the hair and skin, providing protection and moisture.

The Hair Growth Cycle

Hair growth is a cyclical process, not a continuous one. Each hair follicle independently undergoes three main phases:

• Anagen (Growth Phase): This is the active growth phase, during which hair cells rapidly divide in the hair matrix, and the hair shaft actively lengthens. This phase can last anywhere from 2 to 7 years, determining the maximum length a person's hair can achieve. The percentage of hairs in the anagen phase at any given time varies per individual but is typically around 85-90%.
• Catagen (Transition Phase): A short, transitional phase lasting approximately 2-3 weeks. During catagen, hair growth stops, the hair follicle shrinks, and the dermal papilla detaches from the hair bulb. About 1% of hairs are in this phase.
• Telogen (Resting Phase): The resting phase, lasting around 2-4 months. During telogen, the hair is completely at rest, and a new hair begins to grow beneath it. At the end of this phase, the old hair is shed, making way for the new hair to emerge. Roughly 10-15% of hairs are in the telogen phase. The shedding of 50-100 hairs per day is considered normal as part of this cycle.

Following the telogen phase, the follicle re-enters the anagen phase, and the cycle repeats.

Hair Types and Characteristics

Hair characteristics are influenced by genetics, ethnicity, and environmental factors. Key characteristics include texture, density, porosity, and elasticity.

Hair Texture

Hair texture refers to the diameter or thickness of individual hair strands:

• Fine: Small diameter, delicate, and prone to breakage. May appear thin even with high density.
• Medium: Most common type, offering a good balance of strength and flexibility.
• Coarse: Large diameter, strong, often able to withstand more styling and chemical processes.

Hair Density

Hair density refers to the number of hair strands per square inch of the scalp. It is independent of hair texture:

• Low Density: Fewer hairs per square inch. Scalp may be easily visible.
• Medium Density: Average number of hairs.
• High Density: Many hairs per square inch, creating a full appearance.

Hair Porosity

Porosity describes the hair's ability to absorb and retain moisture. It is determined by the condition of the cuticle:

Porosity Level	Cuticle Condition	Moisture Absorption/Retention
Low Porosity	Tightly closed, overlapping scales	Resists moisture absorption, but retains it well once inside. Products may sit on top.
Normal Porosity	Slightly raised, balanced scales	Absorbs and retains moisture effectively. Healthy hair often falls into this category.
High Porosity	Gaps or raised, damaged scales	Rapidly absorbs moisture due to gaps, but struggles to retain it, leading to dryness.

Understanding hair porosity is crucial for selecting appropriate haircare products. For example, low porosity hair benefits from lightweight, humectant-rich products, while high porosity hair requires richer, sealing products.

Hair Elasticity

Elasticity is the hair's ability to stretch and return to its original shape without breaking. Healthy hair has good elasticity, indicating strong internal bonds. Hair with poor elasticity is brittle and prone to snapping. It is affected by hydration, protein balance, and damage.