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1. Hair growth cycle
2. Physiology of the hairs.  
- It is important to understand the normal hair growth cycle & physiology of the hairs to
  understand  hair loss.

- Hair loss is a common & distressing symptom.

Hair growth cycle  
- Hair grow is cycle, with phases of growth(androgen), involution (catagen), and rest (telogen)
  ⇒ Fig 1-3 cycle of scalp-hairgrowth

- In a normal scalp, most follicles are growing(90~95%), a few are under growing (1%),and the
  remainder is resting (5~10%)

- At the end of telogen, hair is released,shed,and the next cycle is initiated

- Each day, up to 100 hairs in telogen are shed from the head and about same number of
  follicles enter anagen.

- The duration of anagen determines the length of hair, and volume of the hair bulb determines
  the diameter.

- Follicles can become larger or smaller under systemic and local influence that alter the
  duration of anagen and the volume of the hair matrix.

-  many factors can affect the hair grow cycle and cause temporary or permanent hair loss
  (alopecia) including medication, radiation,chemotheraphy, exposure to chemical,hormonal and nutritional factors, thyroid disease, generalized or local skin disease, and stress.

- Androgens are important in regulating hair growth.

Hair loss in men : "Male Pattern Baldness" (MPB) (androgenic alopecia)  
- Androgenic Alopecia(A/A) is the hereditary thinning of the hair induced by androgens in
  genetically susceptible men and women.

Pathophsiology  
- In susceptible hair follicles of the scalp, dihydrotestosterone (DHT) binds to the androgen
  receptor, and the hormone-receptor complex activates the genes responsible for the gradual
transformation of large, terminal follicles to miniaturized follicles.

- With successive hair cycles, the duration of anagen shortens and follicles become smaller,
  producing shorter,finer hair that cover the scalp poorly

- These miniaturized hair of various lengths and diameters are the hall mark of androgenic
  alopecia (A/A)

- Dihydrotestosterone (DHT) is formed by the peripheral conversion of testosterone by 5α-
  reductase.(two isomers are Type-Ⅰ&Ⅱ)

- men&women with androgenic alopecia have higher levelof 5-reductase, more
  androgen receptor, andlower levels of cytochrome P-450 aromatase which converts testosterone to estradiol, in hair follicles in the frontal region and the occipital region  

- In men, androgenic alopecia ranges from the bitemporal recession of hair, to thining of the
  frontal and vertex regions of the scalp, to complete baldness and loss of all hair, except the occipital and temporal fringes.


Hair loss in women : "Female Pattern Baldness" (F.P.B)  
- Androgenic Alopecia is as common as offer in women as in men but camouflaged by hair
  styling.

- In women,as in men, the most likely cause of scalp hair loss is androgenic alopecia-an
  inherited sensitivity to the effects of androgens on scalp hair follicles.

- women tend to have less obvious pattern of hair loss than men, and non-patterns types of
  hair loss are more frequent.

- Because of differences in the level of 5α-reductuse and cytochrome P-450 aromatase and in
  the number of androgen receptors in the hair follicles of the scalp

- Women typically retain a rim of hair along the fronted hairline, even when the scalp is visible.

- the thinning of hair is diffuse but is most marked on the frontal or parietal areas of scalp
* photo !!


Diagnosis & Analysis of Hair loss  
1.Hair pull
- Used in virtually every patient with a complaint of unexplained Hair loss
- To access the presence or abscence of any abnormalities in the hair growth cycle.
- Normally about less than 5 hair follicles are taken off the scalp, but only a few hairs are
  dislodged with each pull.
- The ends of the hair follicle may be examined under the microscope to evaluate the condition
  of the hair shaft and the bulb.

2. Phototrichogram
- A consentive photographs taken over a period to determine the pattern of the hair growth

3. Scalp biopsy

4. Hair shaft Evaluation

5. Hair Densitometer