Eyelash

Eyelash
Human eyelashes
Human lashes grow in up to six distinct layers on each upper and lower eyelids, in some cases resulting in sets of over 200 lashes surrounding each eye
Details
System	Sensory
Function	Lines the edges of the eyelids, heightening protection of the eye from dust and debris and triggers the blink reflex
Identifiers
Latin	cilium
Greek	Bλέφαρον (blépharon)
MeSH	D005140
TA98	A15.2.07.037
TA2	7057
FMA	53669
Anatomical terminology [edit on Wikidata]

An eyelash (also called lash) (Latin: cilia) is one of the hairs that grows at the edges of the top and bottom eyelids, spanning outwards and away from the eyes. The lashes grow in up to six layers on each of the upper and lower eyelids.^[1] Eyelashes serve to protect the eye from debris, dust, and small particles, and are highly sensitive to touch, thus providing a warning that an object (such as an insect or lint) is near the eye (which then closes reflexively). The eyelid margin from which lashes grow is among the most sensitive parts of the human body, with many nerve endings enveloping the roots of the lashes, giving it sensitivity to very light tactile input even at the tips of the lashes,^[2] enabling it to trigger the blink reflex when touched.^[3] Eyelashes are also an important component of physical attractiveness, with prominent lashes drawing attention to the eyes.

Etymology

The word eyelash is a

In humans, the eyelashes serve three main functions:

1. Protect the eye from dust and debris by catching them before they can enter the eye
2. Trigger the blink reflex in the case of a foreign body approaching the eye and touching the lashes
3. Regulate or reduce evaporation of the tear film on the cornea^[4]

Structure

Development

The eyelashes of the human embryo develop from the ectoderm^[5] between the 22nd and 26th week of pregnancy.^[6] 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^[7] and is therefore not affected by puberty. Lash follicles do not have an arrector pili muscle associated with them, making the lashes static.^[8]

Length and thickness

While there is considerable variation in the length of lashes between individuals (3 mm to 10 mm for upper lashes), lashes do not vary in length by sex^[9] or ethnicity,^[10] with the upper lashes of humans typically 7 to 8 mm in length regardless of sex,^[9] and generally do not exceed 10 mm in length.^[11] Thus, lashes of humans are considered to be remarkably long when they are 10 mm or greater in length.^[12] Lower lashes average between 5 and 6 mm in length.^[9] In trichomegaly, the lashes may grow exceptionally long^[13] (occasionally 15 mm for the upper lashes). The longest lashes usually grow from the centre of the eyelid.^[10] Human eyelash length, thickness, and darkness decrease significantly with age,^[14] hence long and thick lashes are sometimes seen as a mark of fertility and youthfulness.

Eyelash length is closely linked to the width of the eye, with the lashes typically growing to one-third the width of the eye as an evolutionary adaptation to reduce tear film evaporation and dust deposition. Lashes longer or shorter than one-third the width of the eye have been shown to have reduced efficacy in serving their function.^[4] The typical width of the human eye is 24.2 mm,^[15] resulting in the average human eyelash length of 7 to 8 mm. This evolutionary trait is perhaps why lashes in humans rarely grow beyond 10 mm in length.^[11]

Lashes grow thick at the roots, and have tapered ends. People of Asian ethnicities have significantly thicker lashes, with an average lash diameter of 71.7 μm, than people of Caucasian ethnicities, with have an average lash diameter of 61.0 μm.^[10] However, the number of lashes on the eyelids is fewer in Asians, who typically have 150 lash hairs on the upper lids, than in Caucasians, who typically have 170 lash hairs on the upper lid.^[10]

Curvature

Eyelashes grow outwards from both upper and lower eyelids, with varying degrees of curl. People of Caucasian ethnicities have lashes that are more curled than those of Asian ethnicities.^[10] However, the degree of curling varies significantly even within a population or ethnicity, where people of Asian ethnicities can also exhibit naturally highly curled lashes.^[16]

Density

The number of individual lashes on the upper eyelid is typically 90 to 160, and on the lower eyelid 75 to 80.^[2] The number of follicles and hence lashes cannot be increased after birth because all follicles develop during embryogenesis.^[8]

Glands

The

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
  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.^[17] Occasionally they may cause blepharitis.
• People with vernal keratoconjunctivitis have longer eyelashes,^[18] with the increased growth likely a result of the occular inflammation accompanying the condition.
• Trichomegaly is the condition of abnormally long and/or lush lashes (the objective criteria being lashes of 12 mm or greater in length on the upper eyelids).^[19]

Eyelash transplant surgeries may help to reconstruct lost or damaged lashes. The procedure may also be employed to increase the number of lash follicles on the eyelid to enhance the prominence and fullness of the lashes. The transplanted hairs usually do not have the tapered structure of real lashes, and continue to grow at the same rate as hairs from the area the follicle was extracted from. As such, they require maintenance by means of trimming.

Society and culture

Long and full eyelashes have long been considered a sign of beauty in many cultures, almost universally.^[20] Despite not varying by sex and not being a secondary sex characteristic, long, prominent lashes are frequently considered a feminine trait, with studies showing that women possessing longer lashes are perceived as healthier and more feminine.^[21] Nevertheless, long lashes are regarded as an attractive facial feature in both males and females.^[22] On the other hand, Hadza women are known to trim their own eyelashes.^[23]

Because of the role long lashes play in contributing to overall facial attractiveness, they are a prized and desired trait. A study found that a group of Japanese women in their 20s and 30s desired upper eyelashes as long as 14.1 mm, a length outside the range of typical human lash lengths and over twice longer than the average length of 6.8 mm of the same population.^[24]

Ancient Romans considered long eyelashes a component of aesthetically ideal eyes. Pliny the Elder wrote that eyelashes fell out from sexual excess, so women desired long lashes as a symbol of chastity.^[25] This may have had medical basis as a symptom of syphilis is often alopecia of the lashes,^[26] suggesting that individuals with short or sparse lashes were sexually promiscuous.

The traditional

Nonverbal communication

The lashes and lids play a role in eye contact and nonverbal communication. Voluntarily blinking slowly or a few times in succession quickly is a feminine flirtatious behaviour commonly referred to as to "bat an eyelash", "bat/batting eyelashes", or "flutter/fluttering eyelashes". Fluttering one's lashes against another's skin by rapidly blinking the eyelids in close proximity to another such that the lashes brush the skin is known as a butterfly kiss.

Cosmetics

Because long eyelashes are aesthetically pleasing and seen as a desirable trait that adds to physical attractiveness, some people seek to enhance their eyelash length artificially, by means of

Permanent eyelash tints and eyelash extensions have also become popular procedures, even in fairly basic salons. It is also possible to get eyelash transplants, which are similar in nature to hair transplantation often done on the head. Since the hair is transplanted from the hair on the head, the new eyelashes will continue to grow like head hair and will need to be trimmed regularly.^[30]

Latisse was introduced in the first quarter of 2009 by Allergan as the first drug to receive FDA approval for eyelash growth. Latisse is a solution of bimatoprost, a prostaglandin analog and the active component of the glaucoma medication Lumigan. According to Allergan, noticeable eyelash growth occurs within 16 weeks. Growth is reported to occur primarily on the upper eyelashes, with as much as a 25% increase in length.^[31] In addition, the past decade has seen the rapid increase in the development of eyelash conditioners. These conditioners are designed to increase the health and length of lashes. Many utilize seed extract, minerals, and other chemicals to achieve these results.^[32]

Cosmetic companies have recently relied on scientific research of prostaglandins and the

Lashes, being hair, are found in

Eyelashes are an uncommon but not unknown feature in birds.

• Eyelash extensions
  , used to enhance the appearance of length, curliness, fullness, and thickness of natural eyelashes
• Eyebrows, an area of short hairs above the eyes on the brow ridges

References

Wikimedia Commons has media related to Eyelashes.

External links

• Media related to Eyelashes at Wikimedia Commons