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functions of the hair fiber and hair follicle

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The hair fiber and hair follicle

Hair is the only body part of human beings that can be changed to influence social interaction. Apart from the biological function that hair has, like protection from elements and distribution of sweat glands, hair also plays an important psycho emotional role in the lives of humans. Excess hair or lack of it may cause major psychosocial trauma in some. A study of hair biology has therefore gained momentum and an attempt to control hair growth through therapeutic intervention has become a major challenge.

Hair in mammals is composed of the hair follicle and the hair shaft. Follicles are formed only once in the lifetime of an individual. The hair follicle is formed as a small fingerlike structure from the embryonic epidermis. The embryonic epithelium and mesenchyme of mammals has in them the necessary elements that induce the formation of follicles. At the pre-natal stage, the primitive epithelium and the underlying mesoderm send out growth signals and the follicles begin forming from the primitive epidermis. Complete hormonal or neural circuits are not required for the hair follicles to begin forming. This has also been proved when hair follicles have been found to grow in organic cultures of embryonic skin.

Once the necessary signals are received, the epithelium grows downwards into the dermis, forming a small plug that joins with the dermal papilla at the base. The dermal papilla is a mesenchymal condensation composed of specialized fibroblasts. The fibroblasts control the matrix cells and in this way determine the size of the hair. A mammal is born with a fixed number of follicles, which does not change throughout its lifetime. About 5 million hair follicles are formed in mammals at birth. The distribution of the hair follicles and their spacing is genetically predetermined and does not change throughout the lifetime of the mammal.

The follicles in different parts of the body are different and produce hair fibers of different size, shape and color. The fibers are coarse when they form the body hair or scalp hair. These are known as terminal hair. The finer downy hair on the face is known as vellus hair.

The hair follicles are formed as an epithelial finger. The matrix cells at the base of the follicle differentiate and push upwards, forming three enclosed cylinders. The outermost cylinder separates the hair structure from the rest of the dermis and it is called the Outer Root Sheath or ORS. As the matrix cells differentiate upward, they are given a shape by the rigid middle cylinder or the Inner Root Sheath or IRS. The dimensions and curvature of the inner root sheath play a role in determining the final shape of the hair shaft. The IRS also guides the shaft upwards as the IRS and the hair fiber move outwards together.

At the base of the follicle is the dermal papilla, which is a condensed mesenchymal mass composed of specialized fibroblasts. The fibroblasts in the dermal papilla determine the length of the hair because they control the number of matrix cells.

On the outer root sheath, we also find the “bulge”. This is a cluster of cells biochemically different from the others, having properties of epithelial stem cells. The bulge region is also rich in nerve endings and contains neuropeptides that control the proliferation of follicles.

The mature hair follicle goes through a cycle of growth (anagen), regression (catagen), rest (telogen) and shedding (exogen). This is a regenerative and developmental process with the follicle undergoing reform and remodeling.


Functions of the hair fiber and hair follicle

The main function of the hair follicle is to produce a hair shaft or fiber. The shaft is the physically visible portion of the hair and plays a significant role both biologically and psychologically.

The hair shaft is composed of three parts – the cuticle, the cortex and the medulla. The cuticle is a single row of cells, which forms the outermost layer of the shaft. The cortex forms the bulk of the hair. This contains melanin, which is responsible for hair pigmentation. The medulla is the central portion of terminal hairs.

The hair in different parts of the body has an ornamental function-specially in humans. The appearance of a person can be changed by changing the hair pattern and so hair plays an important role in social communication. The hair fibers also form a protective layer on the surface of the epidermis, protecting it from injury, insect bites and electromagnetic rays.

Hair responds to external stimuli and any movement of the fiber is picked up by the follicle to transmit a message to the nervous system. The hair fibers can therefore be said to act like “antennas” to receive sensory signals.

Hair also plays a part in controlling the body heat by providing insulation against sudden heat loss or gain.

The main function of the hair follicle is of course to produce the hair fiber. The form and shape the hair takes depends on the shape of the hair follicle. A straight follicle produces straight hair whereas a curved follicle will produce curly hair. Each part of the follicle plays a distinct role in the growth of hair.

One major function of the hair follicle is to moor the hair shaft. Trichilemmal keratinization of the outer root sheath in the telogen stage of hair cycling forms an adhesive at the base of the shaft. The shaft is embedded in this trichilemmal keratinous mass. The cells surrounding the area where the hair club is attached have been found to be rich in desmosomes and keratinK14. Studies on mice have shown that in specimens lacking DG3 hair is not properly anchored, proving the importance of these enzymes in providing a tight anchorage to the hair.

The outer root sheath of the hair follicle is a reservoir of sebaceous glands. The cells of the outer root sheath also secrete keratins, adhesion molecule, cytokines and other growth factor receptors that help to regenerate and repair the epidermal layer after injury. The epithelial cells in the upper outer root sheath can restore ulcerated wounds while the Langerhans cells can restore the cells lost due to ultraviolet radiation.

The dermal papilla at the base of the follicle with its dome like shape provides a supportive base to the basal lamina. This basal lamina structure is under considerable pressure due to rapid cell division. The dome shape also enables it to accommodate a larger number of matrix cells.

The hair follicle often acts as a sensory organ for the skin. Mechanical stimuli above the skin are detected by the skin and when the hair moves even slightly, neuroreceptors in the follicles transmit a message to the nervous system. The Langerhan’s cells and Merkel cells are the main neurosecretory cells responsible for this. The Langerhan’s cells also activate the immune system when required.

Finally, the hair follicle cycle influences the surrounding skin environment and a study of hair biology is therefore central to the study of the biology of skin itself.

It is of little wonder therefore that the study of hair biology started by Chase many years ago has gained such importance. Many areas of the growth and cycling of hair, still remain a challenge and with further understanding of the pathology, new therapies for disorders of hair growth should emerge.


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