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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.
Functions
of the hair fiber and hair follicle references
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