Hair Shaft

1. Overview

The hair shaft is the visible, non-living part of the hair that extends above the surface of the skin. It is composed of keratinized cells that originated from the hair matrix in the bulb. As the final product of hair follicle activity, the hair shaft plays important roles in protection, insulation, sensory perception, and social communication. Its properties—such as thickness, shape, texture, and color—are influenced by genetic, hormonal, and environmental factors.

2. Location

The hair shaft is located external to the epidermis, extending from the follicular opening (pore) on the skin’s surface. Specific characteristics include:

• Projects from all hair-bearing areas of the body including the scalp, eyebrows, eyelashes, arms, legs, axillae, pubic region, and face (in males).

• Emerges from the infundibulum of the hair follicle and is completely external after exiting the skin.

• Not present in glabrous (hairless) skin regions such as the palms, soles, lips, and certain genital areas.

3. Structure

The hair shaft is composed of dead, keratinized epithelial cells and has a tripartite structure arranged concentrically:

• Medulla:

  • Central core of loosely packed, soft keratin-filled cells.

  • Often absent in fine or vellus hair.

• Cortex:

  • Middle layer, forming the bulk of the shaft.

  • Contains densely packed keratin and melanin granules, which determine hair strength and color.

• Cuticle:

  • Outermost layer made of overlapping, flattened keratinocytes.

  • Protects inner layers from physical and chemical damage.

The shaft’s cross-sectional shape (round, oval, or flat) influences whether the hair appears straight, wavy, or curly.

4. Function

The hair shaft serves several structural and protective functions:

• Protection: Eyelashes shield the eyes, nasal hairs filter airborne particles, and scalp hair protects against UV radiation.

• Thermal insulation: Traps heat close to the body in cold environments, particularly when piloerection occurs.

• Sensory amplification: Although the shaft itself lacks nerves, movement of the shaft is detected by mechanoreceptors in the follicle.

• Visual and social signaling: Hair color, style, and distribution contribute to social identity, age recognition, and sexual dimorphism.

5. Physiological role(s)

Though biologically inert, the hair shaft contributes to broader physiological processes:

• Expression of health status: Changes in hair texture, color, or brittleness can reflect nutritional deficiencies, hormonal imbalances, or systemic disease.

• Thermoregulatory aid: In conjunction with the arrector pili muscle, raised hair can trap air for warmth in cold environments.

• Barrier reinforcement: With sebum coating, the shaft aids in maintaining skin hydration and microbial defense.

• Hormonal indicator: Androgen-dependent changes in shaft thickness and distribution mark puberty and certain endocrine disorders.

6. Clinical Significance

Abnormalities or damage to the hair shaft may result in or indicate various conditions:

• Hair shaft disorders:

  • Trichorrhexis nodosa: Weak points along the shaft lead to easy breakage.

  • Monilethrix: Inherited beaded hair appearance due to shaft fragility.

  • Pili torti: Twisting of the shaft, associated with congenital syndromes.

• Weathering and damage:

  • Excessive brushing, heat styling, or chemical treatments can erode the cuticle, leading to dull, frizzy, or brittle hair.

• Hair analysis in diagnostics:

  • Forensic and toxicological studies use shaft segments to assess drug use, heavy metal exposure, and hormone levels.

• Color change and aging:

  • Reduction in melanin within the cortex leads to graying with age (canities).

• Infectious conditions:

  • Fungal infections like tinea capitis may invade or surround the shaft, causing fragility and hair loss.