The gradual changes which occur in the inherited hair loss affected scalp is the same whether describing male of female baldness. Alopecia Androgenetica does not develop in all hair follicles at the same time as some are more quickly affected than others. When looking at a punch biopsy under the microscope neighbouring hair follicles can be seen to be variably affected. Some will be normal healthy Terminal hairs with an average diameter of 0.06mm, others will be miniaturized Vellus hair follicles with an average diameter of 0.03mm. So, one parameter of Alopecia Androgenetica is a decrease in the density of Terminal hairs and an increase in the Vellus hair count. In this case hairs in an intermediate state between Terminal and Vellus may also turn into Vellus hair. Over time the Terminal scalp hair follicles undergo progressively shorter and shorter cycles involving reduced Anagen growth periods. This applies regardless of whether the hair follicles grow Terminal, Intermediate, or Vellus hairs. Although periods of Anagen are reduced, Catagen and Telogen time periods remain the same. The net effect is that Alopecia Androgenetica is characterized by a gradual increase in the number of resting Telogen hair follicles present at any one time. In unaffected scalp the percentage of hair follicles in Telogen is up to 10%. In the early stages of Alopecia Androgenetica affected scalp the number of Telogen hair follicles can be up to 20% of the total. As Alopecia Androgenetica progresses the total number of hair follicles can be reduced as the hair follicles are irreversibly destroyed. In inherited hair loss cases the Terminal hair follicles reduce size both in length and diameter. The hair bulb moves upwards in the dermis yielding towards a small Vellus hair follicle. These Vellus hair follicles can be affected by fibrosis. Beneath the miniaturized Vellus hair follicle a fibrous tract, known as a streamer or follicular stela, can be observed marking the Terminal hair follicle bulb’s original position to the base of the current Vellus hair dermal papilla. In many cases of Alopecia Androgenetica an inflammatory infiltrate can be seen around the affected hair follicles especially the upper hair follicles. Normal unaffected scalp hair follicles can also have mild inflammation but the inflammation in Alopecia Androgenetica affected hair follicles is often much more intense with many more cells involved.

The diagram shows Alopecia Androgenetica induced changes in hair growth with a hair follicle passing through Anagen to Telogen phases and then to a new Anagen growth phase. This occurs for all human hair follicles, but as shown below, in pattern baldness the subsequent Anagen hair follicle is smaller than the previous follicle. As pattern baldness progresses, the hair follicles go through several hair cycles and with each one the follicles become shorter, finer, and less pigmented until the initially large Terminal hair follicle has become a small Vellus hair follicle and eventually after certain time simply disappears at all.

 

Phases of hair growth: 1 - Dermal Papilla; 2 - mature stage of active growth (Anagen); 3 - slowing down growth and regression phase (Catagen); 4 - resting phase (Telogen); 5 - resting and fallout phase (Telogen); 6 - an early phase of rapid new hair regrowth (Anagen); 7 - preceding hair fallout phase (Telogen); 8 - the new hair regrowth phase completed (Anagen).