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HAIR GROWTH AND LOSS
Randall, V.A.
Department of Biomedical Sciences, Richmond Building, University
of Bradford, Richmond Road, Bradford, BD7 1DP
Hair is a characteristic of mammals with important roles in insulation,
camouflage, communication and sensory reception. Many
of these functions have been lost in human beings,
but protection and social and sexual communication
roles are still important. Increases in easily visible
(terminal) hair distribution signal adulthood in both
sexes (axillary and pubic hair), while beard growth
etc. advertises the mature male. This explains why
hair disorders such as failure to develop a beard
or premature balding can cause psychological distress.
Androgens are the main regulator of these changes.
Interestingly, androgens have paradoxically different
effects on hair follicles depending on which part
of the body they are located. Most follicles are stimulated
by androgens e.g. beard and axilla, while androgens
gradually inhibit hair growth on the head leading
to male pattern baldness. Another paradox in androgen
action is the variable need for testosterone to be
metabolised to 5a-dihydrotestosterone for androgen
stimulation. Men with 5a-reductase deficiency show
only female patterns of axillary and pubic hair growth,
with no significant beard etc. The hair follicle is
a mainly epithelial tissue which projects down from
the epidermis. At its base the follicle expands into
the hair bulb, enclosing the mesenchyme-derived dermal
papilla which regulates many aspects of hair function.
The current hypothesis is that androgens act on the
hair follicle via the dermal papilla cells, altering
their production of paracrine growth factors such
as growth factors and/or extracellular matrix components
which regulate the function of the follicular keratinocytes
and melanocytes. Dermal papilla cells offer a useful
model to study androgen action in vitro as cultured
cells can initiate new hair growth when re-implanted
in vivo. Investigations into androgen action in cells
derived from hair follicles with different responses
to androgens in vivo have supported the proposed mechanism
of action. Dermal papilla cells from androgen-dependent
follicles such as beard and balding scalp contain
specific, saturable receptors with higher levels than
less sensitive non-balding scalp cells. Beard cells
also metabolise testosterone to 5a-dihydrotestosterone
in vitro in contrast to axillary or pubic cells, mirroring
hair growth in 5a-reductase deficiency. Dermal papilla
cells also secrete soluble mitogenic factors with
testosterone in vitro altering the amount in parallel
to its effects on hair growth of the parent follicle
in vivo. Current studies are focussed on identifying
specific factors. This could lead to new treatments
for hair growth disorders.
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