Hair transplantation is the procedure of transferring follicular units from areas of the scalp with hair adequacy to areas with hair deficiency. As analyzed at Androgenetic Alopecia and its causes, a person΄s genetic predisposition determines his hair follicles΄ reaction to androgens:
Hair follicles of specific areas on the scalp resist to the action of androgens, while others are affected and progressively become inactive.
At this point we should distinguish between 2 important areas at the scalp: The stable hair zone and the unstable hair zone.
STABLE HAIR ZONE
This area lies on the sides and back of the scalp. Hair follicles in this area are not affected by androgens. That means that hair loss in the stable hair growth area is negligible for almost all men. Increased hair loss in the stable hair growth zone is observed in very rare cases.
The area at the back and sides of the scalp is also called donor area, as its follicular units are extracted and transferred by the doctors in hair transplantation.
Transplanted follicular units preserve their genetic resistance to androgens even in the thinning area and continue to grow normally throughout a life time.
This unique characteristic (Donor Dominance) has laid the foundations for the further development of modern hair transplantation.
Donor area density is defined a) by the number of follicular units per cm2 and b) by the number of hairs per follicular unit.
The question at hand in hair transplantation is not to create the initial density - impossible for the time being, as hair transplantation redistributes the existing hairs- but to create a satisfactory sense of density.
In order to create a satisfactory sense of fullness, the lower limit is 45 fu/ cm2.
Donor area can provide even 50% of its hair growth without any signs of thinning.
VARIABLE HAIR ZONE
This area of the scalp includes the forehead, temples and vertex. These parts of the scalp are vulnerable to hair loss. Due to their genetic predisposition, hair follicles in the variable hair growth zone are susceptible to androgens.
The long lasting action of androgens on them brings about their gradual miniaturization. As a result, they produce smaller and thinner hairs which are finally shed.