Biology of the Eyelash Hair Follicle: An Enigma in Plain Sight

The document highlights the limited understanding of eyelash hair follicle (ELHF) biology, noting that despite their importance for beauty and the cosmetic industry, there is a lack of comprehensive research on their growth and pigmentation. The scarcity of human ELHFs for study, the underappreciation of their sensory and protective functions, and the differences from scalp hair follicles, such as shorter hair shafts, absence of an arrector pili muscle, shorter hair cycle, and later greying, contribute to this knowledge gap. The paper synthesizes what is known about ELHFs, including their role in health and disease, similarities and differences from other hair follicle populations, and interethnic variations. It identifies key questions in ELHF biology and suggests future research directions, such as investigating the mechanisms behind trichomegaly and developing in vitro models to study ELHF growth, cycling, and pigmentation, as well as exploring new methods for therapeutic and cosmetic manipulation beyond prostaglandin receptor stimulation.