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Relation of hair growth to the whole skin
The hair is an appendage of the skin and the hair
follicle together with the sebaceous gland forms a part of the
skin tissues. On
receiving signals from the embryonic epithelium and mesenchyme,
the hair follicle begins to grow from the primitive epidermis
in the prenatal stage. A specialized cluster of dermal cells under
the epithelial layer stimulates the stem cells to grow a hair
follicle. The follicles differ in size and shape from site to
site on the body of the mammal. There is therefore a close connection
between the hair follicle and the surrounding skin environment
and any change in one affects the other. Chase first realized
the importance of hair growth in skin biology and undertook the
study of hair dynamics in relation to the biology of the skin
itself.
Influence of the skin environment on hair growth
The surrounding epidermis has always played a role in hair growth.
Plucking of club hair fibers have been found to stimulate growth
in the surrounding scalp area by causing the resting follicles
in the plucked area to go into anagen. The exact mechanism for
this is yet unknown
In mechanical injury or irradiation wounds, the adjacent epidermis
tends to produce an epithelial tongue that covers the wound. By
observing the skin of mice, it has been found that the resting
follicles near the wound, if they are not involved in the epithelium
formation, enter into anagen.
Detailed investigation on skin of mice has shed further light
in the understanding of the relation between hair growth and skin.
In these mice, keratinization of the hair follicles stops after
the catagen stage, so the club hair is not formed. The epithelial
cells form a strand that is constricted at various places, forming
separate beads of cells. The degeneration of cells finally causes
the beads to separate and become disconnected from each other.
This is a result of an abnormality of the connective tissue sheath,
which fails to produce an anchoring club.
Moreover, when the immunology of the skin is affected by the
nervous system, it also affects hair growth. The anagen and catagen
follicular stages in mice are dependent on mast cell degranulation.
Any irregularity in this disturbs the rhythm of the hair follicle
cycle.
Influence of hair follicle on skin biology
Chase first emphasized the fact that the hair follicle cycling
also has an impact on the surrounding skin environment. In rodents,
where whole groups of follicles cycle together, the entire skin
architecture tends to change. In early anagen stage, there is
increased mitotic activity and the epidermis becomes thicker.
The corium and the adipose layers also increase in thickness,
along with an increase in dermal vascularity and stromal content.
The hair cycle also changes the size and function of the sebaceous
glands-which are larger in the telogen stage and show changes
in the gene patterns and neuropeptide expressions through the
duration of the cycle.
There is evidence of a morphological continuity in all parts
of the germinal epithelium that is the basal layer of the epidermis,
the external sheath, the germ and the bulb and the peripheral
cells of the sebaceous glands. These cells also share a common
functional role, possessing the ability to divide, keratinize,
and to accumulate lipids and glycogen. When the hair germ goes
into anagen, it does not do so in isolation, but all four parts
of the germinal epithelium are stimulated in a similar fashion
demonstrating continuity between the two tissues.
The hair follicles also have a reparative and regenerative property,
which also plays a role in skin repair. Ulcerated wounds are covered
by the epithelical cells formed by the outer root sheath. When
Langerhans cells are destroyed by irradiation, the epidermis is
provided with new cells that were stored in a premature form in
the distal outer root sheath.
The hair follicle regenerates itself with each cycle. Even after
intense chemotherapy, which damages the follicles, it retains
the property and regenerates itself. When sebaceous glands were
destroyed experimentally, they have been regenerated by the follicle
In murine transplants, the total skin organ has been regenerated,
together with the hair follicles by the cells that were disassociated
from perifollicular buds or papilla. This regenerative property
of the follicular cells indicates that the connective tissue
sheath plays a role in skin repair. The fibroblasts of the connective
tissues can regenerate a new papilla. This regenerative property
extends beyond the embryonic stage into the mature stage, thus
affirming their role in dermal repair.
Angiogenesis during hair cycling
Dye injected experiments have indicated that there is a change
in the density and spatial arrangement of perifollicular and follicular
blood vessels during synchronized anagen in rodents. During catagen,
on the other hand, there was a degeneration of capillaries within
the papilla of human hair follicles. The skins around the anagen
follicles are also found to be more densely populated with blood
vessels than that around telogen follicles.
Similar studies have proved that papilla fibroblasts and outer
root sheath keratinocytes have angiogenic factors like vascular
endothelial growth factors. This has gone to prove that the follicle
also
controls the changes in the blood vessels of the dermis and not
the other way round.
Recent experiments have also substantiated the premise that during
anagen, not only is there a rearrangement of skin vasculature
but there is also angiogenesis – that is formation of new
blood vessels from existing ones.
Although it is not known what elements of the cell and molecules
controls these changes, the papilla fibroblasts and the hair follicle
epithelium seem to play a significant role as they are the source
of the angiogenesis stimulator VEGF.
Changes in skin immune system due to hair cycling
Synchronized hair follicle cycling has a definite effect on skin
immune parameters. When anagen was induced in mice, the number
of interfollicular Langerhans cells, lymphocytes and perifollicular
macrophages were seen to fluctuate significantly. This had a functional
relevance too as shown in a study conducted on mice. In a standard
mouse model, contact and photo contact sensitization to the allergen
picryl chloride could only be obtained when all the follicles
of the back skin were synchronized in telogen. Sensitization was
not possible through anagen skin. Subsequently, it was observed
that synchronized anagen development over back skin suppressed
contact hypersensitivity in telogen skin of the abdomen. This
proved that the system goes into immunosuppressive activity during
anagen - possibly due to inter follicular secretion of ACTH, alpha-MSH,
TGF-beta-7 and I2-10, which are potent immunosuppressive agents.
It has also been seen that during anagen, the epithelium surrounding
the follicle shows an absence of MHC class1 expression and antigen
producing cells and produces immunosuppressant agents. This surrounding
follicular epithelium thus goes into a state of “immune
privilege”. This characteristic has been significant in
the understanding of the immunopathological processes of alopecia
areata. The fact that each hair follicle entering into anagen
creates such an area of immune privilege where antigen production
is reduced along with suppression of contact hypersensitivity
proves that the hair follicle has a significant impact on the
skin immune system.
It is very definite therefore, that hair production is not only
a concentrated activity of the skin, it is also intimately related
to the structure and physiology of the skin as a whole. A greater
understanding of hair growth in relation to the whole skin therefore
is necessary to find answers to the ways of hair growth control.
Relation of hair growth to the whole skin references
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