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Home > Hair Physiology > Hair Follicle Anatomy
The hair fiber or the hair shaft is the portion of hair that projects out of the skin of the scalp. It is a cylindrical, keratinized and often pigmented filament. The hair shaft structure consists of dead cells that have been filled with hard keratins and binding materials, combined with small amounts of water.
The hair fiber or shaft structure can be divided into three parts:
Out of the above three the medulla is present only in the exceptionally large terminal hairs where the hair diameter is more than 60 microns.
Hair shaft – the cuticle
The cuticle is the outermost layer of the hair shaft. The cuticle is a single row of cells, which may be identified at the level of the lower bulb. As the cuticle cells sweep upward as a single row from the matrix, they are seen in the upper part of the bulb. After reaching midway in the bulb, these cells are cuboidal in shape. 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. As their orientation shifts from a horizontal plane to a vertical one, the cells become flattened. This orientation is continued till the midway of the follicle. In the upper half of the follicle, these cells undergo a form of degeneration. Their nuclei disappear, and the mature cuticle adheres to the cells of the cortex.
Differentiating cuticle from other cells
The cuticle cells can be differentiated from the other cells as they are devoid of Trichohyalin which is a structural protein that is produced and retained in the cells of the inner root sheath and medulla of the hair follicle. The cuticle cells contain melanin pigments which are not seen in the cortex cells.
Major functions of the cuticle layer
A major function of cuticle layer in the hair shaft is to protect the inner, softer tissue called the cortex. A healthy cuticle, other than being a protective layer, also controls the water content of the hair fiber. Plus the cuticle layer imparts a shinning glossy appearance to the hair.
The cuticle cells also contain different staining properties which signify chemical differences in the keratins.
Hair shaft structure- the cortex
Structurally, the cortex lies in the middle of the hair shaft. The cells in the cortex are elongated and fusiform (spindle-shaped). Within each cortical cell there are bundles of hair proteins which are called as fibrils, in between the fibrils there is a soft material called the matrix where the fibrils are embedded.
These cortical cells give bulk, shape, elasticity and curl to the hair. The cortex in the hair shaft structure also contains cortical fusi, pigment granules and various shaped structures which are called as ovoid bodies.
Cortical fusi 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, the fluid inside the fusi is replaced by air.
Pigment granules are considerably smaller than cortical fusi. They are small, dark, and solid structures that are granular in appearance. They vary in color, size, and even in their mode of distribution in a single hair.
Hair shaft structure-the medulla
The medulla is the inner most layer in some of the terminal hair-shafts. The medulla is composed of large, loosely connected keratinized cells. The composition of the medulla can vary even within the same hair. The medulla is easily identified as a pale-staining, sometimes discontinuous line of cuboidal cells.
In human hair, the medulla may be as much as one third of the diameter of the hair fiber. The formation of medulla cells can be continuous, discontinuous, or fragmental. In coarse hairs it is usually continuous or fragmental, whereas in fine hairs it appears discontinuous or it may be absent altogether.
Large intra and intercellular air spaces in the medulla to some extent determine the sheen and color tones of the hair. That is why there is a difference in the hair color when viewed under sunlight.
The analysis of hair fibers helps in the maintenance of healthy hair. Unlike the hair follicle, the hair shaft is comprised of dead cells without any regenerative properties, but we can take care of our hair to maintain the natural texture and tone with good or natural hair care products, and good nutrition.
It is only with good care that you can have healthy, shining and beautiful hair.