Eyelashes: An Exploration of their Formation, Structure, and Function

Eyelashes, while often seen as a mere aesthetic feature, play a significant role in the biological scheme of mammals. These hair follicles are a distinct mammalian feature with ancient roots that extend to lower vertebrates like reptiles and birds. This article delves into the fascinating world of eyelashes, their development, structure, and the critical functions they perform.

Evolutionary Roots of Eyelashes: While eyelashes are a uniquely mammalian development, there are instances of eyelash-like protective skin appendages found in other vertebrate species such as reptiles and birds. For example, periorbital scales can be seen in the eyelash viper (Bothriechis schlegelii), and periorbital feathers are particularly noticeable in the ground hornbill (Bucorvus leadbeateri). These features highlight the evolutionary importance of eyelash precursors in vertebrate evolution.

Development of Eyelashes: Eyelashes start developing between the 7th and 8th week of human embryonic development. They grow in multiple rows, five to six in the upper lid and three to four in the lower lid. The mean number of eyelashes varies between 90-160 in the upper lid and 75-80 in the lower lid. The hairs grow to lengths ranging from 8 to 12 mm, and 6 to 8 mm, in the upper and lower lids respectively. Compared to scalp hair, eyelash fiber is much shorter due to the shorter hair growth cycle and slower hair growth rate (see below). Eyes are dynamic systems that blink, tear and move. As such, these actions probably help compensate for the observed variability in eyelash length and density seen in different people.

Hair Structure: The structure of eyelash hair is similar to human scalp hair, consisting of three main layers: the cuticle (the outside layer of the hair), cortex (the bulk of the hair), and medulla (the central core of the hair). Eyelash hairs are characterized by regular curved shapes that vary based on the individual’s ethnic origin. Transmission electron microscopy (TEM) reveals the three layers mentioned above, with the cuticle having seven overlapping cell layers on average. Mature eyelashes are rooted approximately 2 mm deep into the eyelid skin dermis in hair follicles that look similar to those of the scalp, albeit smaller in size. There is one significant difference though, they lack the arrector pili muscles that are usually associated with other hair follicles. Consequently, eyelash hair doesn’t stand on end when the skin is cold.

Growth Cycle: Hair and eyelash follicles follow a cycle of growth (anagen), transition (catagen), and resting (telogen) phases. The anagen and telogen phases for eyelash hair follicles lasts 1-4 and 3-9 months, respectively, based a number of small scale studies. At any given time, 59–85% of eyelash follicles are in the telogen phase, depending on whether you are looking at upper or lower lid. The most detailed study on growth cycles was based on a study of 29 Caucasian women. The calculated durations of the anagen phase and the complete cycle in this study were 34 ± 9 and 90 ± 5 days, respectively, though the growth cycle varied significantly between different women in the study.

Growth rate: The anagen phase of the eyelash follicle, which is the growth phase, has a notably shorter duration and a lower growth rate compared to scalp hair follicles. In addition, the average daily growth rate of an eyelash, measured at 0.12 ± 0.05 mm, is substantially lower than the 0.3 mm daily growth rate observed in scalp hair. Consequently, both of these features combine to ensure that the length of an eyelash typically does not surpass 12 mm (unless there is some disease or drug action to promote a longer anagen phase and/or faster growth rate).

Associated Glands: Eyelash follicles may be free of arrector pili muscles, but they are closely associated with specific glands such as the Zeis, Moll, and Meibomian glands. Zeis glands produce an oily substance, while Moll glands are modified apocrine sweat glands, these contribute to maintaining eyelash and eyelid skin health. The Meibomian glands secrete lipids, forming a tear film which is vital for maintaining eye health.

Differences by Ethnicity: An intriguing study on the ethnic characteristics of eyelashes, specifically focusing on comparative analysis in Asian and Caucasian females, offers valuable insights into the differences between these two groups. The study involved twenty Asian and ten white Caucasian female volunteers, aged between 20 and 29 years, and employed various methods to obtain detailed information examining a total of 4661 eyelashes (2946 for Asians and 1715 for Caucasians).

The results of the study unveiled remarkable ethnic differences between Asian and Caucasian females in eyelash morphology and growth characteristics. Asian eyelashes were found to have lower lift-up and curl-up angles, fewer numbers overall, and a thicker transverse diameter in comparison to Caucasian eyelashes. There was no significant statistical difference in the length or growth rate between the two groups, and the duration of anagen was similar. The number of cuticular layers was greater in Asian eyelashes, but no statistical difference was found in single cuticle layer thickness between the two groups.

Functions of Eyelashes: Eyelashes perform important functions, including protecting the eye. Their density and organization have been hypothesized to influence the aerodynamic flow of air around the eye, aiding in the protection of the cornea. The distribution among different rows ensures that eyelashes can be shed without leaving any gaps and consequently leaving parts of the eye unprotected.

A ground-breaking study on eyelash aerodynamics (so far the only one published) has unveiled the vital role that eyelashes play in protecting the eye. Using an in vitro model of an adult human eye, the research team created a wind tunnel to study air flow and particle dynamics around eyelashes, employing a mesh to represent the lashes. By measuring water evaporation and particle deposition with various mesh lengths, they determined that the optimal lash length was one-third the eye’s width, reducing tear evaporation and particle deposition by half. Shorter lashes were found to create a zone of airflow stagnation, while longer lashes pushed airflow towards the ocular surface.

The study also underlined the possible consequences of modifying lash length through pharmaceuticals or cosmetic procedures, though the authors emphasize the need for further research to confirm these effects. In principle, the data suggests that longer lashes might increase tear evaporation, though whether this would be enough to cause noticeable dry eye problems is unknown.

Conclusion: The fascinating world of eyelashes offers a window into evolutionary biology, human development, and the intricate structures that work in harmony to perform essential functions. From their early appearance in embryonic development to their complex cycle of growth, eyelashes are more than a mere decorative feature. Understanding their structure and function has medical implications as well, especially concerning eyelash and eye health. Their presence in other species also highlights the adaptive and protective roles they have played throughout vertebrate evolution. Further studies may reveal even more about these remarkable structures, and perhaps elucidate undiscovered functions and characteristics that further connect us with the broader biological world.

Bibliography