Hair loss is caused by a gradual miniaturization of the hair follicles under these influence of multiple complex biochemical pathways in genetically sensitive individuals. We know that males must possess the hormone Dihydrotestosterone (DHT) to have hair loss. We know that castrated males will not loose their hair unless exposed to exogenous sources of testosterone. We know that products which retard the formation of Dihydrotestosterone delay the hair loss process and sometimes temporarily reverse it. These products do not always work and their affects do not appear to be permanent.
Fortunately, only the top of the scalp is affected by androgenic alopecia in typical male pattern baldness and the Ludwig pattern of hair loss in women. The back and side are sparred. The hairs from the back and sides can be moved to any region on the body and continue to grow. The hairs on the top will still fall out no matter where they are move on the body. This is the principal of "donor dominance" and means that the donor hairs retain their characteristics no matter where they are relocated to. This is fortunate to individuals seeking hair restoration surgery. We can transplant these non-affected hairs to any part of the scalp and they will grow for the remainder of a person life unaffected by the DHT.
Individuals inherit the gene for hair loss from either their mother or father. Sometimes one must trace the gene far back in their family history to identify the "culprit". Often times the gene may have come from one's mother's, mother's, mothers, father. It is difficult to trace this sort of inheritance because the gene for typical male pattern baldness does not express itself in women. We tend to see typical male pattern baldness in women who are exposed to exogenous sources of testosterone or those who have a hyper-androgen state such as an adrenal tumor.
Hair loss does occur in women, however, but he pattern is different. We typically call this pattern the Ludwig pattern of hair loss. There are other patterns of hair loss in women particularly. One common pattern is the generalized thinning pattern, which we call DUPA (diffuse unpatented alopecia). Women who present with a Ludwig pattern or DUPA often have a family history of hair loss from their mother.
While hair loss in men is stalled by Propcia (finasteride) it does not appear to work in women. Rogaine (minoxidil) works in both men and women so it must have a slightly different mechanism of action. All of these slightly different patterns make the study of hair loss and its treatment rather complex. There is still much we need to uncover in the science of hair loss for both men and women.
Age of Onset
The age of onset varies from one individual to another. Men who begin to loose their hair at a younger age will typically have a much greater degree of hair loss than men who begin loosing their hair later in life. For this reason it is best for men to avoid hair restoration surgery at a minimum until their 23rd birthday. Even then surgical solutions are far more risky than with much more mature men. Well informed men less than 23 years of age are sometimes candidates for hair restoration surgery, but the overall plan should be much more conservative for these men.
In general, we insist the younger male knows that he has a limited donor supply and that each transplant reduces that supply based on the size of each procedure. We also insist that they know we may be able to cover only the frontal region of the scalp and that a bare or very thin spot may be left in the vertex and sides of the scalp. Some men first begin to notice hair loss in their late 20s, 30s, 40s, or even 50s. With each decade the amount of anticipated loss is less. Therefore, the older the onset, the more likely one is to be an excellent candidate for hair restoration surgery.
Women may first begin to notice hair loss after a pregnancy. In fact, it is common for a woman to notice hair loss three months after her delivery. This loss is typically temporary and the hair may return to its original state about one year later. If it does not return to normal, this is an indication that the loss may be permanent. Even worse, the loss may become progressive. The majority of women begin to loose their hair in the 5th decade of life. It is generally progressive. Typically the hairline is retained with women and the top thins. With time the amount of thinning progresses until more and more bare skin is noticed.
Pathophysiology of hair loss
DHT in sensitive males causes a premature exfoliation of hair. We call this exodus effluvium. There are four cycles of hair growth. The first is anagen or the actively growing phase. This phase is about 2 to 5 years in duration. The second is catagen or the programmed rise of the hair follicle into the upper layers of the dermis along with the cesstation in growth. The third phase is telogen or a dormant time lasting about 3 months. In this phase the hair just sits. The fourth phase is exogen and is marked by the falling out of the the hair. We believe that exogen is the result of a new anagen cycle hair growing out of the scalp and pushing the dormant telogen hair out of its path of growth. In androgenic alopecia a number of hairs seem to go into telogen all at once.
Not all hairs in the follicular unit (typciall 2 to 4 terminal hairs per follicular unit) are affected at the same time, but many hairs in different follicular units are affected all at once. The hairs fall out in unison with the exogen cycle. This results in a marked degree of thinning all at once. This mass exogen cycle of hair tends to occur in waves. In other words, you hair loss may appear stable for years, then all of a sudden it gets far worse. If this were the only physiogic event, hair loss would not be extreme. Unfortunately, the next event is the far more significant to the appearance of "baldness". When the affected hairs resume growth with a new anagen cycle, they do not regrow with the same hair diameter.
They appear finer, grow slower, have a longer telogen phase, a shorter anagen phase, and have less pigment. Because hairs are cylinders and the volume of a cylinde is dependent on the radius squared, a decrease in radius by 1/2 results in a decrease in hair volume to 1/4th its original volume. A drop in volume of this degree allows far more light to penetrate to the scalp, the scalp burns more easily, and the hair loss process appears far worse. Gradually all the hairs in a follicular unit are affected and the area changes from thin to bald. All hairs remain, but they are fine, non-pigmented, slow growing vellus hairs that give no coverage value and the patient now appears bald.
The difference in men and women appears to be the pattern of loss and the response to various medications. Therefore, it is safe to say that the pathophysiology is different for both men and women. There is much to study here, but we already know that the number of androgen receptors in the frontal scalp of women is less and that the amount of aromatase activity in the frontal scalp of women is greater.