Normal Hair Growth Cycles
Undisturbed, each terminal scalp hair usually grows continuously for about approximately three to five years. Then, the hair transitions into a resting state where the visible portion above the skin is shed. No hair grows from the follicle for 90 days. Once this time has passed, a new hair begins growing through the skin and continues for another three to five years at a rate of approximately 1/2 inch per month.
It is thought that as many as 100 genes are involved in regulating the creation, construction and cycling of scalp hair. To date, very few of these genes have been identified.
Common Pattern Hair Loss
Hamilton-Norwood Hair Loss Scale
For those concerned about hair loss, many myths and half-truths abound, but useful information can be difficult to obtain. Therefore, an objective overview of pattern hair loss is presented herein.
In healthy well-nourished individuals of both genders, the most common form of hair loss is androgenetic alopecia (AGA), also known as pattern hair loss. The disorder affects approximately 40 million American men. Perhaps surprisingly, the same disorder affects about 20 million American women. The difference between men and women is that a woman suffering hair loss usually retains her feminine hairline and experiences thinning behind this leading edge. In men, a distinct “pattern” of loss manifests where the frontal edge recedes at the same time that a thinning zone expands from the posterior crown. In more pronounced cases, these zones meet and the person is said to be clinically bald.
Importantly, three things need to occur in order for one to be affected by AGA. First, one must inherit the genetic predisposition. This means that the problem comes from one or both sides of the family. Second, one needs to attain a certain age. Nine year old children do not experience pattern hair loss. And third, one needs to have the circulating hormones that precipitate onset and progression of the disorder.
Typically, the earliest onset of AGA occurs in late puberty or one’s early 20’s. As a general rule, the earlier hair loss begins, the more pronounced it is likely to become.
Hormones, Enzymes & Other Factors
Crystallography of DHT molecule
From a susceptibility standpoint, the principle hormonal trigger linked to pattern hair loss is 5-alpha dihydrotestosterone, commonly referred to as DHT. Intriguingly, it has been shown that in persons genetically insensitive to DHT, pattern hair loss does not occur. DHT is synthesized from the androgen hormone testosterone and is useful early in life and during puberty.
In adults, DHT is thought to cause significant harm, but very little good. Disorders as disparate as benign prostatic hyperplasia and pattern hair loss are both triggered by DHT. The synthesis of DHT occurs via two closely related forms of the enzyme 5-alpha reductase. Hair loss treatment options that efficiently interfere with the interaction between 5-alpha reductase and androgen hormones like testosterone have been shown to offer clinical benefit in treating pattern hair loss.
Because hair growth is regulated by multiple genes and attendant biochemical pathways, the underlying factors are extremely complex. Another challenge to understanding hair loss has been the fact that humans, alone among mammals, suffer from androgenetic alopecia. Thus, no efficient animal model exists that would otherwise tend to shed light upon the key factors at work.