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Hair fiber
The hair fiber is the core part of any hair
follicle. Epidermal derived cells close to
the DP remain undifferentiated cells, called
matrix cells, that focus on multiplying and
proliferating to produce more cells. Those
cells made in the center of the hair
follicle are destined to become part of the
hair fiber and are called cortex (cortical)
cells. As the cells multiply the constant
stream of production pushes the cells
upwards towards the skin surface. As they
move up the hair follicle they begin to
differentiate into particular cell types.
The cortex cells change from a round into a
flattened appearance. They are squeezed
together into layers (lamella). If the hair
follicle contains melanocyte cells then
melanin pigment is incorporated into the
cortex cells. These cortex cells become
keratinized and harden. As they do so it
becomes impossible for the cells to function
properly and the cells die. The keratinized
cells are then pushed away from the hair
bulb region and upwards as new cells come in
behind. The cortex cells are now part of the
dead keratinized fiber.
Some large hair follicles have a central
strand of cells that are loosely organized
and not packed together. This tube in the
very center of the hair fiber is called the
medulla.
Around the outside of hair fiber we see a
cuticle. The cuticle is made up of more
keratinized cells but they arrange
themselves in a slightly different way to
cortex cells. As the cuticle cells are
produced, they lay over the cortex cells and
flatten into an overlapping roof tile
fashion. Cuticle cells become progressively
flatter as they get older. As with cortex
cells, when they keratinize the cell can no
longer function properly and dies.
Root sheaths
The Outer Root Sheath (ORS) is distinct from
other epidermal components of the hair
follicle being continuous with the
epidermis. The "bulge" region in the ORS is
the site at which the arrector pili muscle
is attached. The arrector pili muscle is
connected to the epidermis at the other end.
This is the muscle that makes hair stand
erect and produces goose bumps in your skin
when you are cold. The contraction of the
muscle pulls on both the hair to make it
erect and pulls on the skin making a bumpy
surface.
The bulge region is believed to be the
storage area for hair follicle stem cells.
Hair follicles go through a cycle of growth
and rest (below). With each renewed attempt
to produce hair fiber, the hair follicle
must obtain a source of cells to form the
matrix cell population that make hair
fibers. The source of these cells is
believed by some dermatologists to be the
bulge region. Other dermatologists suggest
that stem cells are not present in the bulge
region at all and that new matrix cells are
obtained from the root sheath.
Also extending from the ORS is the sebaceous
gland. It consists of a few cells focused on
production of oils (lipids). These cells are
large with their cytoplasm filled with
vacuoles containing lipid. The cells are
often divided into several lobes of the
sebaceous gland connected together by a
sebaceous duct. The duct has a single
opening into the tube where the hair fiber
sits.
The ORS surrounds the hair fiber and inner
root sheath until deep into the dermis. Just
above the bulb region containing the dermal
papilla the ORS tapers and ends so the ORS
does not entirely cover the hair fiber and
inner root sheath. The ORS consists of
several layers of cells that can be
identified with unique ultrastructural
properties.
The inner root sheath (IRS) is produced by
matrix cells sitting above the hair
follicle. While those matrix cells in the
center of a hair follicle proliferate and
produce the hair fiber and cuticle, the
matrix cells towards the periphery of a hair
follicle proliferate and produce the IRS. As
with cells making up the hair fiber, the
cells destined to be IRS gradually become
differentiated and keratinized as they are
pushed away from the bulb region. As
keratinization occurs, the cells die and
form the IRS surrounding and protecting the
development of the hair fiber. The IRS can
be subdivided into several layers. Adjacent
to the hair fiber we see a single cell thick
IRS cuticle layer that closely
interdigitates with the hair fiber cuticle
layer. The next IRS layer is called the
Huxley layer that may consist of up to four
cell layers. Outside of this there a single
cell layer called the IRS Henle layer. The
Henle layer runs adjacent to the ORS layer.
The products of the sebaceous gland are
believed to help break down the IRS. The IRS
does not extend out of the hair follicle.
Only the hair fiber itself protrudes above
the skin surface. The IRS disintegrates at
the level of the sebaceous duct opening. The
debris from the IRS break down mixes with
the sebaceous gland oils and the result is
sebum. Sebum is an oily solid that is
expelled from the hair follicle and normally
scraped or washed away in our general skin
care habits. Sometimes overproduction of
sebum can build up in the hair follicle and
harden into plugs of material called
comedones (blackheads). Sebum is a nutrient
rich material and an ideal habitat for
bacteria. Colonies of bacteria, particularly
one called Propionibacterium acnes
may proliferate in hair follicles using
sebum as a nutrient supply. The bacteria
feeding on the sebum changes its composition
that makes it irritating. The immune system
responds to the antigens in the modified
sebum and the bacteria resulting in
inflammation and acne.
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A Hair Loss
B Type of hair loss
C What causes hair loss
D Sign and symptoms
E Introduction to skin and
biology
F Follicular structure
G Dermal papilla
H Hair fiber
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