Hair, a protein filament growing from the skin, extends far deeper than what is visible on the surface. Understanding its full structure reveals that much of its complexity and function lies beneath the skin.
The Visible Hair Shaft
The portion of hair that extends above the skin is known as the hair shaft. This visible part is primarily composed of a tough protein called keratin, which is also found in nails. The hair shaft has three main layers. The outermost layer, the cuticle, consists of overlapping cells arranged like shingles, providing protection to the inner structures. Beneath the cuticle lies the cortex, the thickest layer. The cortex is responsible for hair’s strength, elasticity, and color, as it contains melanin granules. Some thicker hairs also possess a central core called the medulla. While the hair shaft is what we typically see, it represents only a fraction of the entire hair structure.
The Hair Follicle: The Root of Hair’s Depth
The depth of hair is determined by the hair follicle, a tunnel-shaped structure embedded within the skin. Hair follicles originate in the epidermis, the skin’s outermost layer, and extend downwards into the dermis. For terminal hairs, such as those on the scalp, the follicle can even reach into the subcutaneous tissue, the deepest layer of the skin.
At the base of the hair follicle is the hair bulb, an enlarged, rounded structure. Within the hair bulb lies the dermal papilla, a small, cone-shaped structure containing blood capillaries and nerve endings. The dermal papilla provides nutrients and oxygen to the hair matrix cells, which produce new hair. Hair follicles are also associated with sebaceous glands, which release sebum that lubricates the hair and skin. The average depth of a scalp hair follicle is approximately 4.16 mm, but it can extend deeper, up to 1 cm, especially for terminal hairs.
Factors Determining Follicle Depth
The depth of hair follicles varies across the human body due to several influencing factors. Genetic predisposition plays a role in determining the number, size, and growth patterns of hair follicles. The specific body location significantly impacts follicle depth; scalp hair follicles are generally much deeper than the fine vellus hair found on other parts of the body. Hair type also dictates how deep a follicle extends. Terminal hairs, which are long, thick, and pigmented (like scalp hair, eyebrows, and eyelashes), have well-developed follicles that reach deeper into the dermis and sometimes the subcutaneous tissue. In contrast, vellus hair grows from shallower follicles, typically extending only into the deep dermis.
Hair Growth Cycle and Follicle Dynamics
Hair follicles do not maintain a static depth; their activity and position within the skin layers change throughout the hair growth cycle. This cycle consists of three primary phases: anagen, catagen, and telogen. The anagen phase is the active growth period, during which the hair follicle is deepest and most active, continuously producing the hair shaft. For scalp hair, this phase can last from two to eight years.
The catagen phase is a short transitional period lasting about two to four weeks. During catagen, the hair follicle regresses, shrinks, and detaches from the dermal papilla, signaling the end of active growth. The telogen phase is a resting period that typically lasts around two to three months. In this phase, the hair follicle is dormant, and the old hair is eventually shed, making way for a new hair to begin the anagen phase.