An eyelash (also called lash) (Latin: Cilia) is one of the hairs that grows at the edge of the eyelids. It grows in one layer on the edge of the upper and lower eyelids. Eyelashes protect the eye from debris, dust, and small particles and perform some of the same functions as whiskers do on a cat or a mouse in the sense that they are sensitive to being touched, thus providing a warning that an object (such as an insect) is near the eye (which then closes reflexively). The Ancient Greek word for eyelash is βλέφαρον (transliterated as blepharon), which is seen as a root in biological terms like Blephara. Structure Development The eyelashes of the human embryo develop from the ectoderm between the 22nd and 26th week of pregnancy. Natural eyelashes do not grow beyond a certain length, and fall off by themselves without any need for trimming. Eyelashes take about seven to eight weeks to grow back if pulled out, but constant pulling may lead to permanent damage. Their color may differ from that of the hair, although they tend to be dark on someone with dark hair and lighter on someone with light hair. Eyelash hair is not androgenic and is therefore not affected by puberty. Glands The follicles of eyelashes are associated with a number of glands known as the glands of Zeis and the glands of Moll. Clinical significance There are a number of diseases or disorders involving the eyelashes: Madarosis is the loss of eyelashes. Blepharitis is the irritation of the lid margin, where eyelashes join the eyelid. The eyelids are red and itching, the skin often becomes flaky, and the eyelashes may fall out. Distichiasis is the abnormal growth of lashes from certain areas of the eyelid. Trichiasis refers to ingrown eyelashes. Eyelashes may become infested with parasitic crab louse. An external hordeolum, or stye, is a purulent inflammation of infected eyelash follicles and surrounding sebaceous (Gland of Zeis) and apocrine (Moll's gland) glands of the lid margin. Trichotillomania is a disorder that urges the sufferer to pull out scalp hair, eyelashes, etc. Demodex folliculorum (or the demodicid) is a small mite that lives harmlessly in eyelash and other hair follicles, and about 20% of people have these mites living on them. Occasionally they may cause blepharitis. Eyelash and eyebrow transplant surgeries may help to reconstruct or thicken lashes or eyebrow hair.