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Hair
follicle anatomy
The mature anagen hair may be divided into vertical or concentric
(horizontal) compartments. The major vertical divisions of the
hair include (from superficial to deep) are:
- The upper follicle consisting of the infundibulum and the
isthmus
- The middle portion of the hair follicle comprising of the
bulge
- The lower follicle consisting of the suprabulbar and the bulbar
areas
The upper and middle parts of the follicle are permanent, but
the lower follicle regenerates with each turn through the hair
follicle cycle.
The major compartments of the hair from outermost to innermost
include the:
- Connective tissue sheath
- Outer root sheath
- Inner root sheath
- Cuticle
- Hair shaft cortex
- Hair shaft medulla
Infundibulum
The infundibulum extends from the epidermis to the opening of
the sebaceous gland duct. The epithelium of the infundibulum is
continuous with the epidermis, thus its cells can regenerate the
epidermis and replenish it after wounding or injury. The lumen
(inner cavity) of the infundibulum normally contains the hair
shaft, keratin material and the natural oil produced by the sebaceous
glands of the follicles, sebum. The infundibulum includes the
upper part of the hair canal.
The sebaceous gland
The sebaceous gland secretes sebum - a semi-fluid secretion consisting
chiefly of fat, keratin, and cellular material. The cells at the
periphery of the gland are small, nucleated, and devoid of lipid
(fat). These cells divide and are displaced into the center of
the gland where they produce and accumulate lipid. They then lose
their sub-cellular organelles, disintegrate and release their
contents into the sebaceous duct.
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 sebaceous
duct empties into the hair canal at the border of the isthmus
and infundibulum. In addition to its role in producing sebum,
the sebaceous gland may also be important in normal hair shaft
outgrowth.
The isthmus
The isthmus extends from the opening of the sebaceous gland duct
to the insertion of the arrector pili muscle. The outer root sheath
of the isthmus, as compared to the epithelium of the infundibulum,
has no granular layer and its cells possess pale cytoplasm due
to an increased amount of glycogen.
Arrector pili muscle
The arrector pili muscle is connected to the epidermis at the
other end. The contraction of the muscle pulls on both the hair
to make it erect and pulls on the skin making a bumpy surface.
Bulge
The inferior most portion of the isthmus contains the bulge area
of the hair follicle. The bulge region is believed to be the storage
area for hair follicle stem cells. Hair follicles go through a
cycle of growth, transition and rest. With each successive effort
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 to be the bulge region.
Although the bulge area is cylindrical and often difficult to
identify in adult anagen (period of growth) hair follicles, it
becomes quite prominent during telogen (period of inactivity).
The suprabulbar region
The suprabulbar area of the follicle is the region below the
isthmus and above the bulb. This is comprised of four layers
from outermost to innermost: dermal sheath, outer root sheath,
inner root sheath, and hair shaft. In this region, the three inner
root
sheath
layers
viz. the Henle's layer, Huxley's layer, and the cuticle, completely
keratinize and become impossible to differentiate from one another.
The perifollicular sheath
The perifollicular sheath (dermal sheath) envelops the epithelial
components of the hair follicle and consists of an outer layer
of cells called
the connective tissue sheath and an inner basement membrane called
the hyaline or vitreous (glassy) membrane. This basement membrane
of the follicle is continuous with the interfollicular basement
membrane and is most prominent around the outer root sheath at
the bulb in anagen follicles. During catagen, the basement membrane
in the lower portion of the follicle thickens and then disintegrates.
The outer root sheath
The outer root sheath forms a non-keratinizing region at the
periphery of the follicle and is continuous with the epidermis.
It extends all the way to the tip of the bulb, around which it
is composed of two layers of greatly flattened cells. The outer
root sheath contains many vacuoles, Golgi complexes (A golgi complex
is a complex of parallel, flattened vesicles and vacuoles that
lies adjacent to the nucleus of a cell and is concerned with the
formation of secretions within the cell), smooth and rough endoplasmic
reticulum, mitochondria, (containing genetic material) and so
on. The outer root sheath also contains great amounts of glycogen
in the lower part of the follicle. Glycogen is the source of the
energy for protein synthesis during hair growth, and the presence
of glycogen in the outer root sheath suggests the existence of
an energy-consuming activity in these cells.
The inner root sheath
The inner root sheath extends from the base of the bulb to the
isthmus. The lower inner root sheath is characterized by large
eosinophilic cytoplasmic inclusions called trichohyalin granules.
Trichohyaline is a structural protein that is produced and retained
in the cells of the inner root sheath and medulla of the hair
follicle. Keratin fibers are also produced in the inner root sheath.
Based on structure, patterns of keratinization and incorporation
of trichohyaline, the inner root sheath actually consists of three
different cell lineages; which are (from outermost to innermost)
-
Henle's layer is one cell layer thick and is the first to develop
trichohyaline granules and the first to cornify.
- Huxley's layer is two to four cell layers thick and cornifies
above Henle's layer at the region known as Adamson’s
fringe.
- The inner root sheath cuticle is one cell layer thick,
develops a few trichohyaline granules, and begins to keratinize
below
Adamson's fringe. The cells of the inner root sheath cuticle overlap one
another with their free edges oriented towards the deep
portion of the follicle. These cells are in opposition to the
cells
of cuticle of the hair shaft that are oriented upwards, thereby anchoring
the hair shaft in place. Thus, the fully cornified inner
root sheath anchors and directs the growth of the emerging hair
shaft.
The hair shaft
The hair shaft is the part of the hair that can be seen above
the scalp. It consists mainly of dead cells that have turned into
keratins and binding material, together with small amounts of
water. The hair shaft is composed of three parts from outermost
to innermost: the cuticle, the cortex, and the medulla (present
in large terminal hairs).
-
The cuticle is a single row of cells, which may be identified
at the level of the lower bulb by its lack of trichohyaline
granules (in contrast to cells of the inner root sheath) and
melanin (in
contrast to cells destined to become the cortex). These cells
are cuboidal initially, then columnar as they extend up the
bulb, and then flatten to form an overlapping pattern with
their free
edges oriented upwards. 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.
- The cortex makes up most of the hair shaft, and is the component
that gives hair its special qualities such as elasticity and
curl. The cortex is packed with strands of keratin, lying along the
length of the hair. These keratin fibers are made of the low-sulphur
keratins, and are compressed into bundles of larger fibers.
The cortex also contains granules of the hair pigment melanin.
- 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.
The hair follicle bulb
The bulb is the deep, bulbous portion of the follicle that surrounds
the dermal papilla. The bulb contains the matrix cells, the living,
actively proliferating group of cells, which differentiate and
become keratinized to form the hair cortex. This rate of proliferation
is one of the highest of any tissue in the body. As they grow
and develop, these cells steadily push the previously formed cells
upwards. When the cells reach the upper part of the bulb they
begin to change and arrange themselves into six cylindrical layers,
one inside the other. The inner three layers of cells become the
actual hair. The outer three layers become the lining of the hair
follicle - the inner root sheath. The cells of the future hair
shaft are positioned at the apex of the dermal papilla and will
form the medulla (in terminal hairs), cortex and hair shaft cuticle.
The dermal papilla
It is the dermal papilla, which directs and dictates the embryonic
generation of a hair follicle. The Dermal papilla presents as
a healthy "pear" shape in normal hair follicles, and
consists of a highly active group of cells shown to be capable
of inducing follicle development from the epidermis. It is comprised
of spindle-shaped cells with cytoplasmic processes called fibroblasts,
collagen bundles, stroma, nerve fibers and a single capillary
loop. It is continuous with the perifollicular sheath (dermal
sheath) of connective tissue, which envelops the lower follicle.
Hair
follicle anatomy references
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