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Physical
injury and damage to the hair follicles
Physical injury such as burns or pressure sores can create permanent
patches of hair loss. Burning or long term frostbite, as you
would
expect, destroys regions of skin and the hair follicles in the
affected skin. Our hair follicles first develop when we are embryos
and we have
this
limited number
for the rest of our lives. We cannot spontaneously create new
hair follicle organs in the same way as we cannot create new
internal
organs although we can produce new skin without hair during the
healing process.
Some burns and wounds to skin are very severe.
So much so that the hair follicles are completely destroyed.
If this happens the hair follicles are simply not there in the skin anymore
so there is no possibility of hair growth. Somewhat less severe
damage to the skin may not be enough to completely destroy the
hair follicles, but it is enough to extensively disrupt their
structure. Hair follicles can regenerate themselves if some of
the keratinocyte
cells and mesenchymal cells from the dermal sheath survive
remain in close contact with each other. These cells have the
ability to regenerate and reform a hair follicle even after quite
extensive damage to a hair follicle. If however, one of both of
the cell populations is completely destroyed then there is no
chance that the hair follicles can reform and regrow.
Hair loss from burns due to fires and chemicals can lead to skin
damage of differing severity. Mild burns to the skin
may not destroy the hair follicles or even disrupt their structure
very much, but the injury may still be enough to push the hair
follicles into a prolonged dormant
state. Hair follicles are sensitive to what is going on around them
in the skin. When the hair follicles sense that there is injury,
they may reduce their hair growing activity. This is usually because
the injured skin sends out chemical signals which recruit cells
from the immune system and to help promote wound healing. A low
level of these chemicals actually helps encourage hair growth,
but when the concentration of chemicals gets above a certain level
it's all too much for the hair follicles and they stop growing.
The hair follicles may stay in a dormant state until the wound
healing process is well underway.
Hair follicles are
actually known to help with wound healing. When a mild to moderate
injury occurs in hair bearing skin, the hair follicles next to
the injury
will
change their activity and focus on helping to heal the injury.
Cells from the hair follicles stream out into the adjacent skin
to help
heal the wound.
It may be that when hair follicles switch to helping to heal skin wounds, they
are so focused on this role that less hair is made.
Injury to hair follicle can also occur in other ways. For example,
long term pressure applied to the skin can stop the blood
supply to the skin and the cells of hair follicles "starve" to
death. Hair loss in this way can occur when people are unconscious
for
a long
time as when under surgery on the operating table or after a drug
overdose. This form of hair loss is a type of telogen effluvium.
See the telogen effluvium section of this web site for more details.
Physical
injury and damage to hair follicles references
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