Biology of the eyelash hair follicle: an enigma in plain sight

Abstract

Because of their crucial impact on our perception of beauty, eyelashes constitute a prime target for the cosmetic industry. However, when compared with other hair shafts and the mini-organs that produce them [eyelash hair follicles (ELHFs)], knowledge on the biology underlying growth and pigmentation of eyelashes is still rudimentary. This is due in part to the extremely restricted availability of human ELHFs for experimental study, underappreciation of their important sensory and protective functions and insufficient interest in understanding why they are distinct from scalp hair follicles (HFs) (e.g. ELHFs produce shorter hair shafts, do not possess an arrector pili muscle, have a shorter hair cycle and undergo greying significantly later than scalp HFs). Here we synthesize the limited current knowledge on the biology of ELHFs, in humans and other species, their role in health and disease, the known similarities with and differences from other HF populations, and their intrinsic interethnic variations. We define major open questions in the biology of these intriguing mini-organs and conclude by proposing future research directions. These include dissecting the molecular and cellular mechanisms that underlie trichomegaly and the development of in vitro models in order to interrogate the distinct molecular controls of ELHF growth, cycling and pigmentation and to probe novel strategies for the therapeutic and cosmetic manipulation of ELHFs beyond prostaglandin receptor stimulation.

Buschke-Ollendorff syndrome

“…Paus et al 2 have also highlighted other interesting EHF characteristics. EHFs represent pigmented hairs in humans possibly because they contain both active and persistent melanocytes.…”

“…3 Do EHFs share the same machinery of stem cell maintenance with other types of hair follicle? The presence of cell subsets expressing stem cell markers, cytokeratin 15 and 19, 4 around the similar anatomical location within the outer root sheath (ORS) in both porcine and human EHFs 2,5 suggests that the answer to this question is 'yes'. The absence of APMs also hampers defining the border between the 'permanent' and cycling portion of hair follicles.…”

“…The latter could be detected by the clear cells in the ORS. 2 The fact that EHFs are independent of sex hormones is not unexpected as they remain relatively stable throughout human life.…”

“…It is of interest that some drugs (analogues of prostaglandin F2a, bimatoprost and latanoprost; epidermal growth factor receptor antagonists; ciclosporin) included in the list of pharmacological agents causing EHF trichomegaly have been shown to prolong anagen. 2 Further dissection of drug-induced trichomegaly may provide insights that result in the development of more effective medications for EHF hypotrichosis. For this to occur, Paus et al 2 have highlighted the need for new and better experimental approaches.…”

Essential trichomegaly of the eyelashes

“…[1] In spite of what is commonly considered, eyelash trichomegaly may disturb significantly the quality of life and be the cause of corneal abrasions and eye disorders such as amblyopia. Eyelash hair has a growth cycle of 5–6 months with a short anagen phase.…”