Miniaturization in Alopecia Areata: Why is it reversible?

Association with Arrector Pili May Determine Reversibility

Miniaturization of hairs can occur in many hair loss conditions including androgenetic alopecia as well as alopecia areata. For many years, physicians have wondered why miniaturized hairs in alopecia areata hold on to the potential to regrow back to thick hairs whereas miniaturized hairs in AGA only have the potential to thicken up slightly . and only with treatment.

In 2016, Professor Rod Sinclair’s group in Australia proposed that alot of this may be due to a muscle known as the arrector pili muscle. The arrector pili muscle (APM) is not as well known as other muscles like the biceps, triceps or quadriceps. The APM is a very small muscle that is attached to every hair. When the APM contracts, the result for the patient is “goose flesh.” In the past, it was thought that the APM did not have any active role in any type of hair loss mechanisms and more or less acted as a ‘bystander.’ However, in 2016, Sinclair and colleagues proposed a new model of balding. They proposed that the APM has a key role in the decision of a hair follicle to ultimately "miniaturize" during the course of AGA.

By observing how miniaturization occurs within follicular units – the authors proposed that by maintaining tight attachment to the APM, some hair follicles are prevented from proceeding down the pathway of permanent miniaturization. The association with the APM was thought to preserve stem cells.

The authors proposed that in other conditions like alopecia areata, hair follicles maintain their close association with the APM and therefore are protected from the possibility of irreversible miniaturization.

The photo here shows strong attachment of the APM to hairs in alopecia areata (right panel) whereas some hairs in androgenetic alopecia lose their attachment and become miniaturized (left panel).

Reference

Sinclair R, Torkamani N and Jones L. Androgenetic alopecia: new insights into the pathogenesis and mechanisms of hair loss. F1000Res. 2015 Aug 19;4 (F1000 Faculty Rev):585 -