The hair shaft structure visible above the surface of the skin is formed under the skin in the hair follicle: 1 - epidermis; 2 - dermis; 3 - hipodermis (subcutaneous tissue); 4 - sebaceous gland; 5 - bulge; 6 - cuticle (outer layer); 7 - cortex (middle layer); 8 - medulla (inner layer); 9 - sebaceous gland.

 

The hair fiber structure consists of three main layers: the inner layer - medulla (1), medium - cortex (2) and outer - cuticle (3).

 

1 - Hair fiber medulla - in some of the terminal hairs, the cortex has a central hollow core, called the medulla. The medulla is easily identified as a pale-staining, sometimes discontinuous line of cuboidal cells. In some animals, the air within the medulla plays a role in the regulation of body temperature. Humans always have an amorphous medulla, if present. In human hair, the medulla may be as much as one third of the diameter of the hair fiber, and may be continuous, discontinuous, or fragmental. In coarse hairs it is usually continuous or fragmental, whereas in fine hairs it appears discontinuous or absent. Some authors are of the opinion that even though some adult hairs appear non-medullated when observed under the light microscope, when viewed under polarized light, all hairs, with the exception of the very fine ones, shows a fragmental or discontinuous medulla. The medulla may be only one or two cells in diameter, but it is present nonetheless. The type of medulla present can vary even within the same hair. The medulla is composed of large, loosely connected keratinized cells. Large intra and intercellular air spaces in the medulla determine to a large extent the sheen and color tones of the hair by influencing the reflection of light. That is why hair color looks a lot different in sunlight than it does in the shade.

2 - Hair fiber cortex - cortical cells constitute the bulk of a hair, and it is the cortex that gives a hair fiber its eventual shape, resilience, elasticity and curl. The cortex as the main body of the hair is composed of elongated and fusiform (spindle-shaped) cells. Within each cortical cell are bundles of hair protein called fibrils, running parallel to the fiber axis, and between the fibrils is a softer material called the matrix. The cortex may contain cortical fusi, pigment granules, and/or large oval-to-round-shaped structures called ovoid bodies. Cortical fusi in are irregular-shaped airspaces of varying sizes. They are commonly found near the root of a mature human hair, although they may be present throughout the length of the hair. In the living portion of the hair root the fusi are filled with fluid; as the hair grows and dries out, air replaces the fluid. Pigment granules are small, dark, and solid structures that are granular in appearance and considerably smaller than cortical fusi. They vary in color, size, and distribution in a single hair.

3 - Hair fiber cuticle - the cuticle is a single row of cells, which may be identified at the level of the lower bulb. Unlike the cells of the inner sheath, the cuticle cells of the cortex do not show the presence of trichohyalin. Trichohyaline is a structural protein that is produced and retained in the cells of the inner root sheath and medulla of the hair follicle. The cells of the cuticle can also be differentiated from cells destined to become the cortex by their lack of melanin (pigment). Melanin is responsible for the color of hair and an individual’s hair color depends on what kind of melanin the person has. The cells of the cuticle can be recognized in the upper part of the bulb as they sweep upward as a single row from the matrix. About midway in the bulb, these cells are cuboidal in shape. When stained with a basic dye, the microscopic appearance of a histological section of these cells is found to be etched with numerous granules. When the cells reach the upper region of the bulb, they become columnar, with the long axis oriented radially. This orientation is maintained for a short distance above the bulb, and then their outer edges begin to be tipped upward. Since these cells are at least twice as broad as they are high, they become imbricated when the outer sides shift at the tips. Imbrication is a pattern or design having regularly arranged, overlapping edges, as roof tiles or fish scales. As their orientation shifts from a horizontal plane to a vertical one, the cells become flattened. This orientation is completed below the midway mark of the follicle. In the upper half of the follicle, these cells undergo hyalinization, a form of degeneration. Their nuclei disappear, and the mature cuticle adheres to the cells of the cortex. The cells of the cuticle of the hair are interlocked with those of the inner root sheath, firmly anchoring the hair in the follicle. The inner root sheath must grow at the same rate as the hair, or faster as it molds and guides the shaft in its passage outward. A major function of cuticle on the hair shaft is to protect the inner, softer tissue called the cortex, or else the cortex could become frayed and fall apart. The scale pattern of the cuticle in human hairs is routinely imbricate. Cuticles are also classified as coronal when each cell completely surrounds the hair. The free edge of cuticle cells may be simple, dentate (with short ridges), or serrate (saw-edged). Cuticle cells may be elongate, acuminate (gradually tapering to a point), ovate (egg shaped), or flattened. The cuticle cells have different staining properties than the surrounding structures, which may signify chemical differences in the keratins. The keratin of the cuticle of the cortex seems to be different from that of the cuticle of the inner sheath, and it might explain the split that occurs between the hair and the inner sheath. The cuticle is responsible for much of the mechanical strength of the hair fiber. A healthy cuticle is more than just a protective layer, as the cuticle also represents the structure that controls the water content of the fiber. Much of the shine that makes healthy hair so attractive is due to the cuticle.