The hair shaft is the visible portion of hair that extends beyond the surface of the skin. Its complex, non-living composition of specialized proteins enables it to withstand daily stresses while maintaining its function. Understanding the hair shaft’s architecture is the first step in comprehending the physical characteristics and health of a person’s hair.
Defining the Hair Shaft
The hair shaft is the externally visible component of the hair strand. This part of the hair is distinct from the hair root, which lies below the surface within the hair follicle.
The shaft is composed primarily of dead, highly durable cells that have undergone a process called keratinization. These keratinized cells are densely packed and fused together, forming a strong, flexible fiber. The structural composition of the protein keratin, which is also found in fingernails, makes it resistant to wear and tear.
Since the shaft is composed of non-living material, damage to this section is not biologically reversible and requires protective care.
The Three Layers of the Hair Shaft
The hair shaft consists of three concentric layers: the cuticle, the cortex, and the medulla.
The outermost layer is the cuticle, which acts as the hair’s primary protective shield. It is made of flat, overlapping cells arranged like shingles on a roof, typically pointing toward the hair’s tip. A smooth, flat cuticle reflects light effectively, contributing significantly to the hair’s shine and helping to regulate moisture movement.
Beneath the cuticle lies the cortex, the thickest layer that accounts for the majority of the hair shaft’s mass. This region consists of long, twisted keratin filaments held together by chemical bonds, which provide the hair with its elasticity and strength. The cortex is also where the pigment granules, which determine the hair’s color, are contained.
The innermost core of the hair shaft is the medulla, though it is often absent in finer or thinner hair types. When present, the medulla is a soft, thin core composed of transparent cells and air spaces. Its exact biological function is less understood, but it is considered a core structural element, especially in thicker hair.
How Hair Shaft Structure Determines Appearance
The hair shaft structure determines the hair’s color and texture. Color is determined by the concentration and type of melanin pigment held within the cortex layer. Hair color results from the ratio of two melanin types: eumelanin, which produces brown and black hues, and pheomelanin, which results in red and yellow tones.
The shape of the hair shaft’s cross-section is the primary determinant of the hair’s curl pattern or texture. A nearly circular cross-section produces straight hair. As the cross-section becomes more oval or flat, the hair fiber grows with a curl, resulting in wavy, curly, or coily hair.
The condition of the cuticle layer also impacts the hair’s overall shine and health. When the cuticle scales are tightly closed and smooth, the hair has high luster because light is reflected uniformly. Damage that lifts or chips the cuticle, often caused by chemical treatments or heat styling, exposes the internal cortex and leads to a duller appearance and increased vulnerability to breakage.