HAIRBALL
H
THE STRUCTURE OF A HAIR
The hair shaft consists of dead cells
and keratin. It has three layers: the
medulla (core), the cortex, and the
cuticle (covering).
Cross-section through a hair
In this light micrograph, the medulla, the
cortex, and the outer cuticle of the hair can
all be clearly differentiated.
p u b e r t y , t e r m i n a l h a i r r e p l a c e s v e l l u s i n
t h e p u b i c a r e a a n d t h e a r m p i t s . I n m o s t
m e n a n d s o m e w o m e n t h e p r o c e s s c o n -
t i n u e s o n t h e f a c e , l i m b s , a n d t r u n k .
COLOUR AND TEXTURE
H a i r
c o l o u r
i s
d e t e r m i n e d
b y
t h e
a m o u n t o f p i g m e n t c a l l e d m e l a n i n t h a t
i s p r e s e n t i n t h e h a i r s h a f t . M e l a n i n i s
p r o d u c e d b y c e l l s c a l l e d m e l a n o c y t e s a t
t h e b a s e o f t h e h a i r f o l l i c l e . R e d m
e l a -
n i n i s r e s p o n s i b l e f o r r e d a n d a u b u r n
h a i r , b l a c k m e l a n i n f o r a l l o t h e r c o l o u r s .
I f c e l l s r e c e i v e n o p i g m e n t , t h e c o r t e x
o f e a c h h a i r
b e c o m e s t r a n s p a r e n t a n d
t h e h a i r
a p p e a r s w h i t e . T h e
d e g r e e o f
c u r l i n e s s o f a h a i r d e p e n d s o n t h e c r o s s -
s e c t i o n s h a p e o f i t s f o l l i c l e .
DISORDERS
B r i t t l e
h a i r
m a y
b e
d u e
t o
e x c e s s i v e
s t y l i n g ,
h y p o t h y r o i d i s m
( u n d e r a c t i v i t y o f
t h e t h y r o i d g l a n d ) , o r s e v e r e v i t a m i n o r
m i n e r a l d e f i c i e n c y . V e r y d r y h a i r c a n b e
c a u s e d b y m a l n u t r i t i o n . I n g r o w n h a i r s
o c c u r
w h e n
t h e
f r e e - g r o w i n g
e n d
o f
t h e h a i r p e n e t r a t e s t h e s k i n n e a r t h e f o l -
l i c l e ,
w h i c h
m a y
c a u s e
i n f l a m m a t i o n .
( S e e a l s o
h i r s u t i s m ; h y p e r t r i c h o s i s
.)
hairball
A b a l l o f h a i r i n t h e s t o m a c h , f o u n d i n
p e o p l e
w h o
n e r v o u s l y
s u c k
o r
c h e w
t h e i r h a i r ( s e e
b e z o a r
).
hair cycle
T h e a l t e r n a t i n g p h a s e s o f a c t i v i t y
i n a
h a i r
f o l l i c l e . T h e r e a r e t w o m a i n p h a s e s :
a n a c t i v e p h a s e , w h e n n e w h a i r f o r m s i n
t h e f o l l i c l e , a n d a r e s t i n g p h a s e , w h e n
c e l l a c t i v i t y s l o w s d o w n a n d s t o p s . T h e
a c t i v e p h a s e l a s t s f o r s e v e r a l y e a r s , u n t i l
a h a i r h a s r e a c h e d i t s m a x i m u m l e n g t h .
D u r i n g t h e r e s t i n g p h a s e , t h a t h a i r d i e s .
D u r i n g t h e n e x t g r o w t h p h a s e , a n e w
h a i r i s f o r m e d a n d p u s h e s t h e d e a d o n e
o u t o f t h e f o l l i c l e .
T h e
g r o w t h
c y c l e s
o f h a i r s m a y
b e
d i s r u p t e d b y v a r i o u s f a c t o r s , i n c l u d i n g
h o r m o n a l o r i m m u n e s y s t e m d i s o r d e r s ;
p r e g n a n c y ; a d v e r s e r e a c t i o n s t o d r u g s ;
a n d
r a d i o t h e r a p y
.
hairiness, excessive
S e e
h i r s u t i s m
;
h y p e r t r i c h o s i s
.
hair removal
H a i r i s u s u a l l y r e m o v e d f r o m t h e b o d y
p u r e l y f o r c o s m e t i c r e a s o n s . I t m a y a l s o
b e s h a v e d f r o m a r o u n d a n i n c i s i o n s i t e
b e f o r e s u r g e r y . T e m p o r a r y
m e t h o d s
o f
h a i r r e m o v a l i n c l u d e s h a v i n g , w a x i n g ,
d e p i l a t o r y
c r e a m s ,
a n d
w a x i n g ;
e l e c t r o -
l y s i s
,
h o w e v e r ,
i s
t h e
o n l y
p e r m a n e n t
m e t h o d o f r e m o v a l .
hair transplant
A
c o s m e t i c
o p e r a t i o n
i n w h i c h
h a i r y
s e c t i o n s o f s c a l p a r e r e m o v e d a n d t r a n s -
p l a n t e d t o h a i r l e s s a r e a s t o t r e a t
a l o p e c i a
( b a l d n e s s ) . T h e r e
a r e
s e v e r a l
d i f f e r e n t
t e c h n i q u e s a v a i l a b l e .
HOW IT IS DONE
I n s t r i p g r a f t i n g , a s t r i p o f s k i n a n d h a i r
i s t a k e n f r o m a d o n o r s i t e , u s u a l l y a t t h e
b a c k o f t h e s c a l p o r b e h i n d t h e e a r s . T h e
r e m o v e d
h a i r s
a n d
t h e i r
f o l l i c l e s
a r e
t h e n i n s e r t e d i n t o n u m e r o u s i n c i s i o n s
m a d e i n a b a l d a r e a , k n o w n a s t h e r e c i p i -
e n t
s i t e .
T h e
p r o c e d u r e
u s u a l l y
t a k e s
6 0 - 9 0
m i n u t e s . T h e p a t i e n t i s
g i v e n a
m i l d s e d a t i v e a n d t h e d o n o r a n d r e c i p i -
e n t s i t e s a r e a n a e s t h e t i z e d . T h e d o n o r s i t e
h e a l s i n a b o u t f i v e d a y s . A l t h o u g h t r a n s -
p l a n t e d h a i r s f a l l o u t s h o r t l y a f t e r w a r d s ,
n e w h a i r g r o w t h a p p e a r s f r o m t h e f o l l i -
c l e s
b e t w e e n
t h r e e
w e e k s
a n d
t h r e e
m o n t h s l a t e r .
O t h e r
t r a n s p l a n t t e c h n i q u e s
i n c l u d e
p u n c h
g r a f t i n g ,
i n
w h i c h
a
p u n c h
i s
u s e d
t o
r e m o v e
s m a l l
a r e a s
o f
b a l d
s c a l p , w h i c h a r e r e p l a c e d w i t h a r e a s o f
h a i r y s c a l p ; f l a p g r a f t i n g , i n w h i c h f l a p s
o f h a i r y
s k i n
a r e
l i f t e d ,
r o t a t e d ,
a n d
s t i t c h e d t o r e p l a c e b a l d a r e a s ; a n d m a l e
p a t t e r n
b a l d n e s s
r e d u c t i o n ,
w h i c h
i n -
v o l v e s c u t t i n g o u t a r e a s o f b a l d s k i n a n d
s t r e t c h i n g
s u r r o u n d i n g
a r e a s
o f
h a i r -
b e a r i n g s c a l p t o r e p l a c e t h e m .
SCALP HAIR GROWTH
There are about 100,000 hairs on the
scalp, though there is considerable
individual variation. The exact number
depends on the number of hair
follicles, which is established before
birth. Scalp hair grows about one
centimetre per month. Each hair goes
through alternating periods of growth
and rest. On average, a person sheds
about 100 scalp hairs a day.
Growth phase
At the start of a
growth phase
(which, on the
scalp, lasts about
three years for
each hair),
the hair root
stimulates the
growth of a bulb
and then a shaft.
B u lb
R o o t
O ld h a i r
Rest phase
f a l l s o u t
During the rest
phase (which
lasts aboutthree
months on the
scalp), the bulb
retracts from the
root and
eventually the
hair falls out. A
new hair begin
to grow in the
same follicle.
N e w h a i r f o r m s
H a i r s h a f t
358
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