Do hair follicles operate as primitive, multifocal kidney‐like excretory (mini‐) organs?

Abstract

Besides their many other functions, hair shafts (HS) also are a repository for potentially noxious compounds. These are neutralized by their deposition within terminally differentiated, avital epithelial cells (trichocytes) that also facilitate the interaction of potential toxins with melanin, a toxin‐adsorbent biopolymer. Trichocytes are completely extruded via HS shedding during exogen, an actively controlled process. This underappreciated functional property of the human hair follicle (HF) makes it a bona fide excretory (mini‐) organ. Here, we ask whether the ca. 2 million HFs of the human integument operate in part as primitive, spatially dispersed kidney‐like excretory organs. Despite the many obvious differences between kidneys and HFs, this provocative hypothesis is also supported by other underappreciated renal‐follicular similarities such as anatomical parallels between Bowman's capsule and the anagen hair bulb, renal podocytes and HF winged cells ["Fuegelzellen"], and hypoxia‐dependent production of erythropoietin and extensive prostaglandin synthesis by human scalp HFs—just as in the kidney. The proposed kidney‐like excretory function of HFs may have constituted a major selection advantage of mammals during evolution and could be clinically relevant. We explain how the many open questions (eg, how are molecules destined to be excreted by hair shaft entrapment recognized, taken up and deposited into hair matrix cells?) can be tested experimentally. Finally, we explore how the therapeutic targeting of kidney‐like excretory HF functions may usefully complement classical nephrological therapy (dialysis) and ask whether stimulation of intrafollicular erythropoietin synthesis might become exploitable for the benefit of patients with renal anaemia.

Following historical “tracks” of hair follicle miniaturisation in patterned hair loss: Are elastin bodies the forgotten aetiology?

“…Techniques used in renal disease pathology include elastin imaging 58 so comparison of the follicle in AGA and kidney would be a worthy exercise, not least as these two organs have already been compared recently in this journal. 59 In future studies, particular attention needs to be made to an examination of the changes in stelae under follicles that are undergoing miniaturisation 60 to seek out the "Arao-Perkins" bodies and to characterise them using modern molecular methods and characterising this particular type of fibrosis. Strategies such as expanding the size of the DP to reverse miniaturisation 61 also should consider whether the Arao-Perkins elastin body may remain an impediment to efficacy.…”

“…Techniques used in renal disease pathology include elastin imaging 58 so comparison of the follicle in AGA and kidney would be a worthy exercise, not least as these two organs have already been compared recently in this journal. 59 …”

New developments in hair research

“…While hair fibre is well recognized for its role in skin protection and social communication, they ask whether hair follicles can be viewed as waste processing plants, assisting in the neutralization and removal of toxins from the body. [53] As such, hair fiber could be described as an excreted waste product, an idea that was first put forward by Aristotle. He claimed that "gross humors" purged and evacuated from the brain, pass through pores in the skin, dry and harden into hair.…”

“…Carré et al consider the role of hair follicles and their production of hair fibre in a rather different light. While hair fibre is well recognized for its role in skin protection and social communication, they ask whether hair follicles can be viewed as waste processing plants, assisting in the neutralization and removal of toxins from the body . As such, hair fiber could be described as an excreted waste product, an idea that was first put forward by Aristotle.…”

Proteins observed in scalp hair from preschool children

“…The hair follicle includes most cellular processes, including cell growth, cell death, interactions of histologically different cell types, cell migration and differentiation, and it responds to multiple hormones, microenvironmental and systemic changes(36). Structural and functional characteristics of hair follicles also mimic the renal tubules, potentially performing unique, distributed excretory functions(37).…”