Hair shaft

The hair shaft is the visible, fully keratinized portion of the hair fiber that extends above the skin surface. It is a filamentous biomaterial composed primarily of terminally differentiated keratinocytes produced by the hair follicle. The hair shaft is essentially a "dead" biocomposite structure, meaning its cells have undergone complete cornification and are no longer metabolically active.

Structural Composition

The hair shaft is organized into three concentric layers:

• Cuticle: The outermost protective layer, consisting of thin, flat, overlapping cells arranged like roof shingles. It constitutes approximately 10% of the total fiber weight and serves as a barrier protecting the inner structures.

• Cortex: The bulk of the hair shaft, comprising 86–90% of its mass. It is made up of axially aligned, spindle-shaped cells packed with macrofibril bundles — a composite of intermediate filaments (keratins) embedded in an amorphous matrix of keratin-associated proteins. The cortex determines the hair's mechanical strength, elasticity, and color, as melanosomes (pigment granules containing melanin) are dispersed throughout this layer.

• Medulla: An optional, centrally located region consisting of a continuous or discontinuous column of loosely packed cells and air spaces. The medulla becomes more prevalent as fiber diameter increases and is often absent in fine hairs.

These layers are bound together by the cell membrane complex (CMC), which consists of lipid-rich intercellular material that holds the cells and layers in structural continuity.

Formation