Beyond mere aesthetics, lush eyelashes protect our vision by defending eyes against debris. Arranged in two or three rows, between 100 and 150 strands project from each upper eyelid; about half as many from each lower eyelid.

Like other terminal hairs, eyelashes are coarser, longer, and more pigmented than vellus hairs, and all eyelash follicles are present at birth.

Unlike scalp hairs, they do not usually lose pigmentation and turn gray with age. They lack arrector pili muscles, which make other hairs stand up. They are not influenced by androgens, which make some scalp hairs fall out.

Clinical study. A study published in 2010 investigated the biological mechanisms that determine eyelash length, curl, and pigmentation, in order to characterize the eyelash growth cycle and morphology. Twenty-nine Caucasian females between 26 and 60 years of age were enrolled.

Eyelashes were measured using a high-resolution camera, and histological analyses of eyelid hair bulbs were performed via biopsies from 17 volunteers.

Eyelash follicle morphology was found to be similar to scalp follicle morphology, except that epithelial stem cells were spread along the length of eyelash follicles, not concentrated in bulges as in scalp follicles.

Follicular cycle. Normal scalp follicles have a complete cycle much longer than normal eyelash follicles — typically six or seven years — one of the reasons that scalp hairs are so much longer. Eyelashes also grow slower: on average 0.12 ±0.05 mm per day, compared with 0.30–0.40 mm per day for scalp hairs.

The three stages of hair and eyelash growth are anagen (active growth phase), catagen (transitional phase), and telogen (resting phase). Unlike some animals, human hair cycles are asynchronous — some follicles are growing (anagen) while others remain dormant (telogen). The mean duration of a complete eyelash cycle is 90 ±5 days.

During anagen growth phase, 34 ±9 days, the follicle undergoes rapid cell proliferation and differentiation, and melanogenesis creates hair-shaft pigmentation.

After anagen, eyelash follicles enter catagen, a transitional phase of just a few days, during which parts of the follicle undergo apoptosis or programmed cell death and disintegration.

Finally, telogen phase lasts for about half the total cycle, when no cell differentiation, proliferation, or apoptosis occurs. Exogen, the expulsion of old hairs, takes place during late telogen or early anagen.