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Hair Follicle Structure
By Kevin J. McElwee
April 1, 1997
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This FAQ was created and is maintained
by:
Kevin J. McElwee
Taken from - http://www.jax.org/~kjm/index.htm - The Alopecia Areata Site
kjm@aretha.jax.org
The Jackson Laboratory, Bar Harbor, ME
I would like to
personally like to thank Mr. McElwee for creating this great document on Alopecia.
INTRODUCTION
The skin is the
largest organ in the body and of primary importance, integral to the survival of mammalian
life but often overlooked when examining the health and welfare of an individual. The
heterogeneous skin structure is derived from the ectoderm and mesoderm of an embryo giving
rise to the epidermis and dermis respectively. Within these generalised layers are
specialised appendages also derived from the ectoderm and/or mesoderm including sensory
nerves, sweat glands and hair follicles. The skin as a whole rests on subcutaneous tissue
largely composed of a loose mesh of collagen fibre, fat cells and muscle tissue.
The skin has a diverse range of
functions:
In short, the skin acts as a
barrier and is the primary organ through which we interact with the external world.
Significant in many of these
properties is the hair follicle appendage. Hair provides protection against heat loss by
adjustment of hair density through contraction of the arrector pili muscle attached to
each hair follicle. By "fluffing up" the hair layer air can be held adjacent to
the skin to provide a heat trapping, invisible layer. Hair can provide indications of
sexual development through onset of secondary sexual characteristics from development of a
mane on a male lion to beard development in humans. Hair may also be of importance in
attracting mates and may be based on colour - such as silver back mountain gorillas -
distribution or quality, all indicators of the general health and vitality of an
individual. Alternatively, it may aid in camouflage for survival where mute tones or
dappled colour blend with an animal's environment. Hair fibre also helps with protection
forming a tough barrier helping protect the epidermis from minor abrasions and/or from
ultra violet light. Specialised hair such as eyebrows and eyelashes protect the eyes by
channelling or sweeping away fluids, dust and debris. Nasal hair plays an important role
in trapping air borne foreign particles before reaching the lungs. Hair fibre may also
increase the surface area for faster evaporation of sweat from neighbouring glands and
individual hairs can aid sensory function. Consequently the hair follicle is of importance
to the survival of mammals and still maintains significance for the human race - not just
biologically, but also through cosmetic and commercial considerations.
The total number of hair follicles
for an adult human is estimated at 5 million with 1 million on the head of which 100,000
alone cover the scalp. In humans, the only external regions of skin devoid of hair
follicles are the palms of the hands and soles of the feet. The basic hair follicle
structure remains essentially the same throughout the range of mammalian species with
modifications for specialised functions. The hair follicle can be recognised as a separate
entity within the skin with formation and maintenance based on interaction between dermal
and epidermal components. At the heart of each anagen hair follicle lies the dermal
papilla (DP). The dermal papilla (DP) presents as a healthy "pear" shape in
normal hair follicles (in alopecia areata the DP becomes small and the cells densely
packed). As the name suggests, derived from the dermis mesenchyme the DP consists of a
highly active group of cells shown to be capable of inducing follicle development from the
epidermis and production of hair fibre. Under the influence of the DP, epidermal cell
differentiation during anagen produces a keratinized hair fibre and associated products.
The source epidermal cells (called cortical or matrix cells) which lie in the immediate
vicinity of the dermal papilla are a living, actively proliferating group of cells which
differentiate and become keratinized to form the hair cortex (Co) and surrounding hair
cuticle (Hc) of the hair shaft at the centre of which is situated the medulla (M). Cells
Around the hair shaft comprise the inner root sheath (IRS) which can be divided into three
layers the cuticle (Cu), Huxley layer (Hu) and Henle layer (He) based on structure,
patterns of keratinisation and incorporation of a product called trichohyalin. The IRS
breaks down at the level of the sebaceous gland to leave only the hair cortex and
surrounding cuticle to protrude above the epidermis.
Diagram showing the main differentiated layers in a mature anagen hair follicle. The
hair follicle penetrates the dermal layer of the skin composed of fibroblast cells and
collagen connective tissue interspersed with blood vessels, sweat glands and sensory
nerves. The bulb region sits just above the subcutaneous (adipose fat) tissue layer.
Click pics for annotated views of the dermal papilla and the hair layers respectively
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 (not shown). This is the muscle that makes hair stand errect and produces
"goose bumps" when you are cold. Isolating epidermal tissue from the dermis in
anagen, hair shaft producing follicles (between the IRS and ORS), is a basement membrane
(BM) described as the glassy membrane within the hair follicle. In other words, the BM
provides a physical dividing line between cells descendant from embryonic ectoderm
(epidermis) and embryonic mesoderm (dermis). This physical barrier has a role to play in
our immunological protection. For human and other hair follicles, a dermal sheath (DS)
encloses the epidermal component and consists of a thin layer of mesenchymal cells
extending from the stratum papillaris to the dermal papilla (on the outside of the ORS
layer running the full length of the hair follicle - not shown in diagram).
In embryogenesis the establishment of
a DP is vital to the development of all hair follicles and associated modified structures.
The DP is a group of specialised dermal fibroblast cells, derived from the embryonic
mesoderm, which aggregate in the dermis just below the epidermis. For humans this initial
aggregation begins when the embryo is approximately 60 days old . The DP cells at this
stage are only loosely collected and present as long, spindle shaped. Within the skin the
development of a DP marks the site for future development of a hair follicle.
Above the DP an epidermal plug, or
peg, of cells develops and proliferates growing into the dermis to link with the DP. The
mesoderm derived DP and the ectoderm derived epidermal plug apparently
"communicate" with the result of further proliferation of epidermal matrix cells
and differentiation into the various sheath and hair fibre structures. The DP develops
into a more identifiable structure of rounded cells containing organelles vital for
product synthesis, although the cells themselves at this later stage of development are
non-proliferative. Hence, development of a hair follicle requires a continuum through
induction, initiation, elongation and differentiation.
Diagram showing accumulation of dermal papilla cells below undifferentiated epidermis
which is then stimulated to grow down into the dermis as a hair "peg".
Interaction between the hair peg and the dermal papilla cells promotes differentiation
into a mature hair follicle.
It is the DP which directs and
dictates the embryonic generation of a hair follicle and it also retains this instructive
ability throughout the life of the hair follicle. Oliver et al revealed that the
removal of the DP stops hair growth but that the lower third of the dermal sheath is
capable of supplying new cells for regeneration of a new DP by infiltrating and
transforming at the site of the original DP with subsequent hair follicle regrowth. With
removal of more than the lower third of a hair follicle, reformation of a DP is unable to
occur and the hair follicle is effectively permanently destroyed. The DP cells retain
their embryonic functional abilities and are able to induce new hair fibre growth in
mature, adult skin when implanted into previously deactivated hair follicles and in close
association with ORS epidermal cells.
DP cells can also interact with adult
epidermis to induce the development of new hair follicles. In the established hair
follicle the DP cells act in conjunction with epidermal cells via mechanisms similar to
those in embryogenesis to permit hair follicle cycling through hair production and resting
phases. DP cells are almost unique in maintaining their embryogenic regenerative
properties in adults making them potentially attractive for investigation with a view to
gaining an insight on organ/limb regeneration and similar studies.
Under normal circumstances hair growth in
each hair follicle occurs in a cycle. There are three main phases of the hair growth
cycle; anagen, catagen and telogen with anagen further subdivided into proanagen,
mesanagen and metanagen. Anagen is the active growth phase when hair fibre is produced.
Proanagen marks initiation of growth with RNA and DNA synthesis in a follicle which then
quickly progresses through mesanagen to metanagen and maximum follicle length and girth.
In this mature state of proliferation and differentiation the hair follicle consists of a
total of eight concentric layers and melanogenesis occurs within pigmented hair follicles.
Anagen is followed by catagen, a period of controlled regression of the hair follicle.
Ultimately the hair follicle enters telogen, when the follicle is in a so-called resting
state.
Diagram showing regression of a mature anagen hair follicle. On entering catagen the
dermal papilla condenses as the cells become inactive. With a lack of dermal papilla cell
stimulation, the hair fibre and root sheaths stop growing. In telogen the dermal papilla
can become isolated in the dermis and the hair fibre can easily be pulled out (by combing
or brushing). In alopecia areata only growing hair follicles come under attack. Resting
follicles allegedly escape destruction by immune cells. So by truncating the growth phase
hair follicles might escape total destruction.
Anagen is the longest phase with up to
90% of follicles on a normal human scalp in this active hair growth state at any given
time and correspondingly telogen hair follicles comprise up to 10% on the scalp. The
average rate of hair fibre growth is around 0.35mm a day but this rate varies depending on
the site of the hair follicle and the age and sex of the individual. The length of the
anagen growth phase for scalp hair is usually 6-10 years while telogen lasts just 30-90
days and catagen is best estimated at 14-21 days. In most young mammals the anagen growth
phase occurs in a wave like pattern across the skin surface. But the hair follicles of
humans and guinea pigs can run through the normal cycles of growth entirely independently
of neighbouring follicles.
Diagram showing a resting hair follicle returning from resting telogen to growing
anagen. If the old fibre has not already fallen out it is pushed out by the new hair fibre
growing underneath.
Normally this cycle of hair production
and inactivity will continue for the duration of the individual's life but other factors
can influence and inhibit hair production and in some cases lead to physical destruction
of the hair follicle. Factors may include adverse reactions to drugs and cosmetics , or as
a result of scarring, tumours, radiation, the genetics of the individual, hormones and/or
their immune system. And that of course brings us back to alopecia areata where the
growing anagen hair follicle is apparently the target of immunological attack.
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