Androgenetic Alopecia: Hair Cycle Changes in Men and Women

Introduction: Androgenetic alopecia (AGA), commonly known as pattern hair loss, is a complex condition affecting millions worldwide, characterized by progressive hair thinning and loss in a distinctive pattern. This article delves into the intricate changes occurring in hair follicles during the development of AGA, in both men and women. The focus is on the gradual transformation of healthy terminal hair follicles into miniaturized vellus hairs, a hallmark of AGA. The article provides an in-depth analysis of the altered hair growth cycle and structural changes in the hair follicles, offering insights into the dynamics of this prevalent condition.

Differential Progression in Follicular Miniaturization: The gradual changes that occur in the androgenetic alopecia affected scalp skin is the same whether describing male of female pattern hair loss. Androgenetic alopecia does not develop in all hair follicles at the same time. Some are more quickly affected than others. When looking at a punch biopsy under the microscope neighboring hair follicles can be seen to be variably affected. Some will be normal healthy terminal hairs with a diameter of 60 micrometers or more, others will be miniaturized vellus hair follicles with a diameter of 30 micrometers or less. As such, one parameter of androgenetic alopecia is a decrease in the density of terminal hairs and an increase in the vellus hair count. Hairs in a transitional state between terminal and vellus hairs (called intermediate hairs) will also be observed; these hairs are in the process of miniaturization and will eventually become vellus hairs.

Hair Growth Cycle Alterations in Pattern Hair Loss: The typical life cycle of a healthy human scalp hair follicles is between 3 and 7 years long. With development of pattern hair loss, however, 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 are terminal, intermediate, or vellus hairs. Although the durations of anagen are reduced in affect follicles, the catagen time periods remain much the same and last around 2 weeks. For the telogen time period, in the early stages of pattern hair loss this also remains about the same as seen in a healthy follicle, but as pattern hair loss becomes chronic and more extensive, the duration of anagen becomes much longer and may last several months.

Quantitative Changes in Hair Follicle Types: The net effect of these hair cycle changes is that androgenetic alopecia 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 around 10-15%. In the early stages of androgenetic alopecia affected scalp the number of telogen stage hair follicles can be around 30% of the total. For men, as androgenetic alopecia progresses, the total number of hair follicles can be reduced as the hair follicles are irreversibly destroyed. Herein lies the key difference in male and female pattern hair loss. In men the affected follicles will eventually completely disappear, but this rarely occurs in women unless they have a male hair loss pattern. Very few women develop a male pattern of hair loss and when it does occur it is typically linked to high androgen hormone production.

Structural Changes in Terminal Hair Follicles: In androgenetic alopecia the terminal hair follicles also reduce size both in length and diameter. The hair bulb moves upwards in the skin dermis yielding a small vellus hair follicle. These vellus hair follicles can be affected by fibrosis. Beneath each 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 some cases of androgenetic alopecia an inflammatory infiltrate can also be seen around the affected hair follicles, especially around the upper part of hair follicles. Normal unaffected scalp hair follicles can also have mild inflammation, but the inflammation in androgenetic alopecia is often much more intense with many more cells involved.

The Progressive Nature of Follicular Miniaturization: The diagram shows a hair follicle passing through anagen to telogen and then to a new anagen growth phase. This occurs for all human hair follicles, but as shown, 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 cycle the follicles become shorter, finer, and less pigmented until the initially large terminal hair follicle has become a small vellus hair follicle. How many cycles it takes to shrink down from terminal follicle size to vellus follicle size varies. Some people lose their hair pretty quickly it is likely many of their terminal follicles probably cycle through just one intermediate sized follicle before they become fully vellus sized. For those with a more slowly progressing pattern of hair loss, the follicles can remain relatively stable as intermediate hair follicles and may go through multiple cycles of intermediate hair production before “giving up” and finally entering a vellus hair follicle state.

Conclusion: In summary, the development of androgenetic alopecia is a multifaceted process marked by a progressive transformation of terminal hair follicles into miniaturized vellus ones, accompanied by changes in the hair growth cycle. The decrease in the density of terminal hairs, increase in vellus hair count, and the intermediate hair follicles represent the morphological spectrum of AGA. Additionally, the shortening of the anagen phase and the eventual lengthening of the telogen phase underscore the disrupted hair cycle dynamics. This detailed exploration of follicular changes, including fibrosis and inflammation, enhances our understanding of AGA’s pathophysiology. It also underscores the complexity of hair follicle biology and highlights the need for continued research in this field to develop more effective therapeutic strategies for managing pattern hair loss.

Bibliography