The Arrector Pili Muscle (aka the Goosebump Muscle)

Several weeks ago, the arrector pili muscle got significant coverage in a few newspapers around the world. Famed Australian dermatologist Dr. Rodney Sinclair co-authored an important paper titled “The arrector pili muscle, the bridge between the follicular stem cell niche and the interfollicular epidermis” that was just published this month. Dr. Sinclair has been involved in this area of hair loss research for a few years and already published similar findings several times in the past decade. Nevertheless, Australian newspapers were especially interested in this latest study and its findings as evidenced by articles such as this one and this one.

The Arrector Pili Muscle

Arrector pili muscles are small muscles attached to individual human hair follicles on both the scalp as well as body (so we have millions of these muscles throughout our bodies). Contraction of these muscles causes hairs to stand up, a phenomenon that is known as goosebumps. Therefore, the arrector pili muscle is often referred to as the goosebump muscle.

The Arrector Pili Muscle Degeneration and Hair Loss

In recent years, a few studies have come out that suggest a possible connection between the arrector pili muscle degenerating (where it gets replaced by fat) and hair loss due to the subsequent disconnection between various hair follicle stem cell populations. It is possible that an intact arrector pili muscle plays a crucial role in the maintenance of follicular integrity and stability.

However, there are many uncertainties about this theory that I discuss in the next section. It seems that while in alopecia areata (AA) patients the arrector pili muscle remains intact, this is not true in androgenetic alopecia (AGA) patients (and unfortunately over 95 percent of balding men suffer from AGA). So this could be why it is much easier to grow back hair for people with AA compared to people with AGA. However, the rate at which the arrector pili muscle degenerates and gets replaced by fat varies substantially between patients and between individual hairs on the same scalp. Some miniaturized vellus hairs in balding regions might even never lose most of their arrector pili muscle connection.

Points of Contention

  • According to Dr. Sinclair’s own quote from a past paper, “It remains unclear whether arrector pili muscle regression is a cause or effect of permanent follicle miniaturization“. I think this is the crux of the issue surrounding this theory/hypothesis.
  • It seems like hair that is transplanted from the back of a person’s scalp to the front of that same person’s scalp regenerates the arrector pili muscle. A very important related study from 2012  comes to us from Japan. So it might be very possible to regenerate this muscle.
  • There are 100s of anecdotal reports on the internet of people regrowing long-lost hair. On the internet, you can find many reports of old men who recently started taking Dutasteride for enlarged prostate issues and have subsequently noticed hair regrowth on areas of their scalps that have been totally devoid of any visible hair for decades. Maybe they regenerated their arrector pili muscles; or maybe their arrector pili muscles always remained intact in spite of severe AGA; or maybe one can regenerate hair without the need for having any intact arrector pili muscles?
  • It is unclear when exactly we have a point of no return where the arrector pili muscle is largely or entirely replaced by fat and hair stops growing (either as a cause or effect — see first bullet point above). It seems like there is significant variation depending on person and hair follicle. In many cases, the arrector pili might not be entirely degenerated and replaced by adipose tissue even in areas of the scalp where one has been bald for several decades. In such cases, subsequent hair and muscle regeneration could perhaps even be possible via something like Dutasteride or Finasteride?

It would seems that just as with out bodies, muscle beats fat yet again. However, as is always the case in the world of hair, nothing is ever straightforward! Even fat has significant hair growth benefits in the correct circumstances.

New Study Shows Ten Percent Increase in Hair Count at 24 Weeks on Dutasteride 0.5 mg

Dutasteride 0.5 mg per day Significantly Superior to Finasteride 1 mg per day at Regrowing Hair

Dutasteride 0.5 mg per day Significantly Superior to Finasteride 1 mg per day at Regrowing HairA new study from India shows that Dutasteride 0.5 mg once a day increased hair count in 45 men from 223 at baseline to 246 at 24 weeks (i.e. in 5.5 months of treatment). Full highly detailed study here. This represents just over a 10 percent increase. It should be noted that without any treatment, most people would continue to lose hair, so the real benefit is greater than 10 percent.

In the same study, 45 men were also treated with Finasteride 1 mg once a day and saw about a 2 percent increase in hair count at 24 weeks. Still a good results when considering that virtually all of these balding men would have seen a decline in hair count at 24 weeks without treatment.

Before and after photo of best case result of Indian patient taking Dutasteride 0.5mg daily for 24 weeks (Source):

Dutasteride 0.5 mg Before and After Indian Patient

This is yet one more study among numerous others over the past decade that has concluded Dutasteride to be significantly superior to Finasteride in regrowing hair. The majority of such studies seem to come from the western world (with Japan and South Korea also disproportionately represented) so I am glad that this one comes from India. It seems like Dutasteride is effective at growing hair irrespective of one’s ethnicity and genetics.

I liked the below finding (although it should be noted that Dutasteride is still too recent a drug to make any conclusions about long-term side effects; and most other studies show at least slightly higher side effects from Dutasteride compared to Finasteride):

Both the groups showed a similar side effect profile with sexual dysfunction being the most common and reversible side effect.

Dutasteride Regrows Some Miniaturized Hair

One of the more interesting aspects of this particular study is that the authors also measured how many miniaturized/miniaturizing hairs (i.e., “thin hair count”) became thick once again. This is important because it shows that 5-alpha-reductase inhibitors such as Finasteride and Dutasteride can bring back hair from the dead/almost dead/slowly dying. It is unclear how many if any of the thin hair that became thicker were vellus hair (i.e., barely noticeable and almost transparent) that had shrunk many years or even decades earlier. In this particular study, Dutasteride 0.5 mg led to a reduction in the “thin hair count” by 12 percent at 24 weeks, while Finasteride 1 mg led to a reduction in the thin hair count by 1.5 percent at 24 weeks (just a 1 unit reduction in actual thin hair count so not too meaningful a percentage figure interpretation here).

Over the years, I have read many online anecdotal reports (see links at end of this post) of older men who were taking Dutasteride to treat enlarged prostates and saw hair growth in areas of the scalp that were totally bald (i.e., covered in vellus hair) for decades. Dutasteride likely helps bring back long-lost hair in many patients, although usually just some of it.

***If you ever do decide to take Dutasteride, I would highly recommend consulting an experienced dermatologist first and being aware of the worst case potential side effects.

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