Hair Follicle Aging

What is Senescent Alopecia?

Hair follicle aging appears to be a real thing, like any tissue in the body. Traditionally, a form of hair loss known as senescent alopecia ("SA") has been defined as a very specific type of age related thinning that is distinct from androgenetic alopecia ("AGA"). Androgenetic alopecia tends to start somewhere between age 8 and age 50 - at least that has been the traditional view. Hair thinning that occurs after age 60, with no thinning prior to this, has a high likelihood of representing senescent alopecia. (Of course other types of hair loss may also occur after age 60). A study by Karnik and colleagues in 2013 confirmed that these two conditions (AGA and SA) are truly unique. The authors studies 1200 genes in AGA and 1360 in SA and compared these to controls. Of these, 442 genes were unique to AGA, 602 genes were unique to SA and 758 genes were common to both AGA and SA.

The genes that were unique to AGA included those that contribute to hair follicle development, morphology and cycling.

In contrast to androgenetic alopecia, many of the genes expressed in senescent alopecia have a role in skin and epidermal development, keratinocyte proliferation, differentiation and cell cycle regulation. In addition, the authors showed that a number of transcription factors and growth factors are significantly decreased in SA.

Conclusion
The concept of senescent alopecia is still open to some debate amongst experts. The studies by Karnik give credence to the unique position of these two conditions. But studies by Whiting suggested that it is not so simple as to say anyone with new thinning after age 60 has SA - many of these are also more in keeping with androgenetic alopecia. As one ages into the 70's, 80's and 90's - hair loss in the form of true senescent alopecia becomes more likely.

Reference

Karnik et al. Microarray analysis of androgenetic and senescent alopecia: comparison of gene expression shows two distinct profiles. J Dermatol Sci. 2013.

Whiting DA. How real is senescent alopecia? A histopathologic approach.
Clin Dermatol. 2011 Jan-Feb.